CD Rom Construction

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CD-ROM Drive Construction and Operation In terms of construction and basic components, CD-ROMs are rather similar in most regards to other storage devices that use circular, spinning media, which isn't that much of a surprise. The big difference of course is the way the information is recorded on the media, and the way that it is read from the media as well. This section takes a look at the basics of how CD-ROM drives are constructed and how they work. Optical "Head" Assembly The middle two letters in "CD-ROM" stand for "read only", so it shouldn't be any surprise that standard CD-ROM drives are read only devices, and cannot be written to. (Newer variants of CD- ROMs, CD-R and CD-RW drives, break this long-standing rule of this type of device.) Most people know this anyway because CD- ROMs use the same basic technologies that CD audio players have for many years, and everyone knows that these devices can only play back, not record. The reason that the word "head" is in quotes is that CD-ROM drives do not use a read head in the conventional sense the way a floppy disk or hard disk does. It isn't just that the head cannot record, it really isn't a single solid head that moves over the surface of CD-ROM media, reading it. The head is a lens-- sometimes called a pickup-- that moves from the inside to the outside of the surface of the CD-ROM disk, accessing different parts of the disk as it spins. This is just like how a hard disk or floppy disk head works, but the CD-ROM lens is only one part of an assembly of components that together, read the information off the surface of the disk. Here's how the CD-ROM works, in a nutshell (I'm not going to go into the gory details of how the laser beams are manipulated within the drive because that can get complicated--there are in fact several slightly different ways that the internals work): 1

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Transcript of CD Rom Construction

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CD-ROM Drive Construction and Operation

In terms of construction and basic components, CD-ROMs are rather similar in most regards to other storage devices that use circular, spinning media, which isn't that much of a surprise. The big difference of course is the way the information is recorded on the media, and the way that it is read from the media as well. This section takes a look at the basics of how CD-ROM drives are constructed and how they work.

 

Optical "Head" Assembly

The middle two letters in "CD-ROM" stand for "read only", so it shouldn't be any surprise that standard CD-ROM drives are read only devices, and cannot be written to. (Newer variants of CD-ROMs, CD-R and CD-RW drives, break this long-standing rule of this type of device.) Most people know this anyway because CD-ROMs use the same basic technologies that CD audio players have for many years, and everyone knows that these devices can only play back, not record.

The reason that the word "head" is in quotes is that CD-ROM drives do not use a read head in the conventional sense the way a floppy disk or hard disk does. It isn't just that the head cannot record, it really isn't a single solid head that moves over the surface of CD-ROM media, reading it. The head is a lens--sometimes called a pickup-- that moves from the inside to the outside of the surface of the CD-ROM disk, accessing different parts of the disk as it spins. This is just like how a hard disk or floppy disk head works, but the CD-ROM lens is only one part of an assembly of components that together, read the information off the surface of the disk.

Here's how the CD-ROM works, in a nutshell (I'm not going to go into the gory details of how the laser beams are manipulated within the drive because that can get complicated--there are in fact several slightly different ways that the internals work):

1. A beam of light energy is emitted from an infrared laser diode and aimed toward a reflecting mirror. The mirror is part of the head assembly, which moves linearly along the surface of the disk.

2. The light reflects off the mirror and through a focusing lens, and shines onto a specific point on the disk.

3. A certain amount of light is reflected back from the disk. The amount of light reflected depends on which part of the disk the beam strikes: each position on the disk is encoded as a one or a zero based on the presence or absence of "pits" in the surface of the disk. This is discussed in more detail in the section on CD-ROM media.

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4. A series of collectors, mirrors and lenses accumulates and focuses the reflected light from the surface of the disk and sends it toward a photodetector.

5. The photodetector transforms the light energy into electrical energy. The strength of the signal is dependent on how much light was reflected from the disk.

Most of these components are fixed in place; only the head assembly containing the mirror and read lens moves. This makes for a relatively simplified design. CD-ROMs are of course single-sided media, and the drive therefore has only one "head" to go with this single data surface. Details on how data is encoded onto and decoded from the CD-ROM disk are included in the section on CD-ROM media.

Since the read head on a CD-ROM is optical, it avoids many of the problems associated with magnetic heads. There is no contact with the media as with floppy disks so there is no wear or dirt buildup problem. There is no intricate close-to-contact flying height as with a hard disk so there is no concern about head crashes and the like. However, since the mechanism uses light, it is important that the path used by the laser beam be unobstructed. Dirt on the media can cause problems for CD-ROMs, and over time dust can also accumulate on the focus lens of the read head, causing errors as well.

 

Head Actuator Mechanism

Most people don't think of a CD-ROM drive as having a head actuator, in the sense that a hard disk or floppy disk drive does. In fact, however, the lens assembly does move across the CD-ROM media in a similar way to how the heads on a hard disk or floppy disk drive do.

As described in the section on the read head, only part of the whole mechanism used to read the CD-ROM actually moves. This is the lens and mirror assembly that focuses the laser energy onto the surface of the disk. The technology used to move the read head on a CD-ROM drive is in some ways a combination of those used for floppy disk drives and for hard disk drives.

Mechanically, the head moves in and out on a set of rails, much as the head of a floppy disk drive does. At one end of its travel the head is positioned on the outermost edge of the disk, and on the other end it is near the hub of the CD. However, due to the dense way the information is recorded on the CD, CD-ROM drives cannot use the simple stepper motor positioning of a floppy disk. CD-ROM media actually use a tighter density of tracks than even hard disks do!

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Instead, the positioning of the head is controlled by an integrated microcontroller and servo system. This is similar to the way the actuator on a hard disk is positioned. This means that the alignment problems found on floppy drives (and much older hard disks) are not generally a concern for CD-ROM drives, and there is some tolerance for a CD that is slightly off center (but not a lot).

Like a floppy disk, the head actuator on a CD-ROM is relatively slow. The amount of time taken to move the heads from the innermost to the outermost tracks--called a full-stroke seek--is about an order of magnitude higher than it is for hard disks.

 

Spindle Motor, Constant Linear Velocity (CLV) and Constant Angular Velocity (CAV)

Like all spinning-disk media, the CD-ROM drive includes a spindle motor that turns the media containing the data to be read. The spindle motor of a standard CD-ROM is very different from that of a hard disk or floppy drive in one very important way: it does not spin at a constant speed. Rather, the speed of the drive varies depending on what part of the disk (inside vs. outside) is being read.

Standard hard disks and floppy disks spin the disk at a constant speed. Regardless of where the heads are, the same speed is used to turn the media. This is called constant angular velocity (CAV) because it takes the same amount of time for a turn of the 360 degrees of the disk at all times. Since the tracks on the inside of the disk are much smaller than those on the outside of the disk, this constant speed means that when the heads are on the outside of the disk they will traverse a much longer linear path than they do when on the inside. Hence, the linear velocity is not constant. Newer hard disks take advantage of this fact by storing more information on the outer tracks of the disk than they do on the inner tracks, a process called zoned bit recording. They also have higher transfer rates when reading data on the outside of the disk, since more of it spins past the head in each unit of time.

CD-ROMs take a different approach. They adjust the speed of the motor so that the linear velocity of the disk is always constant. When the head is on the outside of the disk, the motor runs slower, and when it is on the inside, it runs faster. This is done to ensure that the same amount of data always goes past the read head in a given period of time. This is called constant linear velocity or CLV.

The reason that CD-ROMs work this way is based on their heritage of being derived from audio CDs. Early CD players did not have the necessary smarts

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or buffer memory to allow them to deal with bits arriving at a different rate depending on what part of the disk they were using. Therefore, the CD standard was designed around CLV to ensure that the same amount of data would be read from the disk each second no matter what part of it was being accessed. CD-ROMs were designed to follow this methodology.

The speed of the spindle motor is controlled by the microcontroller, tied to the positioning of the head actuator. The data signals coming from the disk are used to synchronize the speed of the motor and make sure that the disk is turning at the correct rate.

The first CD-ROMs operated at the same speed as standard audio CD players: roughly 210 to 539 RPM, depending on the location of the heads. This results in a standard transfer rate of 150 KB/s. It was realized fairly quickly that by increasing the speed of the spindle motor, and using sufficiently powerful electronics, it would be possible to increase the transfer rate substantially. There's no advantage to reading a music CD at double the normal speed, but there definitely is for data CDs. Thus the double-speed, or 2X CD-ROM was born. It followed in short order with 3X, 4X and even faster drives. This is discussed more in the performance section.

Virtually all of these drives up to about 12X or so still vary the motor speed to maintain constant linear velocity. As the speed of the drives has increased, many newer drives have come out that actually revert back to the CAV method used for hard disks. In this case, their transfer rate will vary depending on where on the disk they are working, again, just like it does for a hard disk. The "X" rating can be somewhat specious for these drives, since they achieve it only--at best--at the outer edge of the disk. No CAV drive claiming to be 24X actually transfers at that rate over the whole disk. Of course, hard disk drives are the same way and nobody seems to complain about their claims. Some drives actually use a partial CLV or mixed CLV/CAV implementation where the speed of the disk is varied but not as much as in a true CLV drive.

The change back to CAV as the drives get faster and faster is being done due to the tremendous difficulty in changing the speed of the motor when it is going so fast. It is one thing to change a disk spinning at 210 RPM to 539 and back again, but quite another to change it from 5,040 to 12,936 and then back to 5,040! This spin-up and spin-down action is actually one factor contributing to the slow performance of CD-ROMs especially on random accesses.

This table summarizes the differences between CLV and CAV:

Characteristi Constant Constant Angular

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cLinear

Velocity (CLV)

Velocity (CAV)

Drive Speed Variable Fixed

Transfer Rate Fixed Variable

ApplicationConventional

CD-ROM drives

Faster and newer CD-ROM drives, hard disk drives,

floppy disk drives

There are in fact some drives that use a mixture of CLV and CAV. This is a compromise design that uses CAV when reading the outside of the disk, but then speeds up the spin rate of the disk while reading the inside of the disk. This is done to improve the transfer rates at the inside edge of the disk, which can be 60% lower than the rates at the outside of the disk in a regular CAV drive.

 

Loading Mechanism

The loading mechanism refers to the mechanical components that are responsible for loading CDs into the CD-ROM drive. There are two different ways that CD-ROM media are normally loaded into the CD-ROM drive.

The most popular loading mechanism used today is the tray. With this system, a plastic tray, driven by gears, holds the CD. When the eject button is pressed the tray slides out of the drive, and the CD is placed upon it. The tray is then loaded back into the drive when the eject button is pressed a second time. Most drives will also respond to a slight "push" on the drive tray by activating the mechanism and retracting the tray.

Many older CD-ROM drives, and many higher-end drives even today, use caddies. These are small carriers made of plastic. A hinge on one side opens up to let you put a disk within the caddy, and a metal cover on the bottom slides out of the way to allow access to the CD by the drive. The caddy is inserted into the CD-ROM drive as a sort of "virtual cartridge". In fact, the CD inside the caddy is pretty similar to the way a 3.5" floppy disk works within its jacket--a media disk inside a plastic protective carrier with a sliding metal access panel. Of course the CD is still removable. Also, the CD caddies are much more solidly built.

Interestingly, some of the other loading mechanisms found on audio CD players seem to be rarely used for CD-ROMs. For example, many car stereos use direct front-loading of the CD; I've never seen this used for CD-ROMs, but

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I am told that the first Compaq Deskpro 2000 models used this technique. I've also never seen a top-loading CD-ROM drive (but the reason for that is pretty obvious).

Of the two mechanisms, the tray is far more common because it makes for a cheaper drive and also for cheaper use of the media. Most consumer-grade drives use trays for this reason. There are problems with these tray drives however:

Fragile Mechanism: One problem is that the mechanism for moving the tray in and out of the drive is really not hard to break if it is mishandled. The CD, when placed in the tray, just sort of "sits there" loose, and if you put it in the tray off-center it is possible for the disk to get stuck in the tray when it retracts, potentially damaging both disk and drive. This problem makes these drives harder for children to use, as they may misalign the disk and cause it to jam in the drive.

Increased Handling: Trays mean each disk must be handled a fair bit, which can increase the chances of wear, dirt accumulation and scratches on the media. Caddies eliminate virtually all handling of the individual disk.

No Vertical Orientation: These drives cannot be side-mounted, as the CD would fall right out of the tray. This isn't a concern for most people but it is for some. There are in fact some CD-ROMs that have four tabs around the perimeter of the tray for holding the disk in place. This might work mounted vertically, but I personally still would not do it.

Caddies are used on many high-end drives and are a much better mechanism, if you can afford to use them properly. This means that you basically need a caddy for each CD you use on a regular basis. Unfortunately, this can be an expensive proposition, because the stupid things still cost a lot of money--like $5 or more a piece! Frankly, it's kind of ridiculous that they cost this much considering that similar cartridges cost a fraction of this price. It must be just due to the fact that they are sold in such low volume. At any rate, if you have twenty CDs that you use on a regular basis then you are looking at an extra $100 for caddies, which turns most people off to these types of drives in rather short order.

Another possibility is to simply use one or two caddies and then switch the CDs into them as you need them. This is of course an option, but in doing this you lose one of the biggest advantages of the caddy: the reduced handling. You also make using your drive a pain in the rear end, because each time you use a CD you have the added step of swapping the disk into the caddy. (In fact, you have more media handling than you would with a tray drive.) If you can't afford the caddies you are usually better off with a

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tray drive, unless you especially need the ability to mount the drive vertically.

 

Single and Multiple Drives

Most CD-ROM drives allow the use of only a single disk at a time. There are available, however, some drives that will let you use four or even more disks at once. This can give you great flexibility, especially if there are one or two disks you use very frequently. You can leave this in the system semi-permanently, and use another slot for disks that you use only infrequently.

The use of a multiple disk CD-ROM drives normally requires special software driver support. The extra disks are normally mapped to additional drive letters by the driver, giving you access to all of the drives independently.

For whatever reason, multiple CD-ROM drives have never really become very popular. They tend to be more expensive than equivalent single drives, of course. They also tend to lag the leading edge of CD-ROM technology; the fastest multiple disk drive is quite a bit slower than the fastest single disk drive.

 

Connectors and Jumpers

The connectors and jumpers on a CD-ROM are similar in most ways to what you will find on a hard disk drive. Mercifully, CD-ROM drive manufacturers have done a much better job of being at least somewhat standardized in the use of jumpers and connectors, and even in where they are located on the drive. All CD-ROMs that I have seen have their jumpers and connectors located at the back of the drive.

You will find a standard 4-pin power connector on the back of a regular internal CD-ROM drive, the same kind that is used for hard disk drives and most other internal devices. This is pretty universal and is found on most every drive. The other connections and jumpers depend on the interface that the drive is using; an IDE/ATAPI drive will use different ones than a SCSI drive for example. For ATAPI, you will find the standard 40-pin data connector, along with jumpers to select the drive as a master or slave device. For SCSI, you will find a 50-pin connector and jumpers to set the device ID and termination.

One connector that is found on a CD-ROM and not on a hard disk drive is the audio connector that goes to the sound card. This three- or four-wire cable is

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used to send CD audio output directly to the sound card so it can be recorded or played back on the computer's speakers.

 

Logic Board

Every CD-ROM drive contains a logic board. As with the hard disk drive, the job of this board is to both control the drive and interface to the PC either using IDE/ATAPI or SCSI (in most cases).

 

Audio Output and Controls

Most CD-ROM drives come with convenient features to allow you to use the drive to play and listen to audio CDs. Drives seem to vary drastically in terms of what they provide on their front panels, but you will usually see some combination of the following:

Stereo Headphone Output: A mini headphone jack that allows you to plug headphones directly into the drive to listen to the CD audio being played back. This is found on virtually every CD-ROM drive.

Volume Control Dial: Most drives include a dial control to allow you to select the volume of the CD audio output.

Start and Stop Buttons: Many drives include control buttons to start and stop the play of the CD. On some drives these are the only controls found on the front panel.

Next Track and Previous Track Buttons: The addition of these buttons makes listening to CD audio on the CD-ROM drive much more practical. Unfortunately many drives save 79 cents or whatever by leaving these off.

It's important to note that software audio CD programs are readily available (and for the mostpart, totally free--there's one included in Windows 95) and they will let you do basically anything you want (change tracks, see time remaining on the disk, even set up catalogs with the names of the CDs and tracks) with an audio CD from the comfort of your desktop. This software will work regardless of what controls you have on the front of the CD-ROM drive itself. For this reason, the presence of these buttons is a convenience only, and not a necessity (which may be why some drives omit them, I suppose.)

The amplifier included in most CD-ROMs to allow the use of headphones for direct playing is pretty wimpy. The sound quality produced directly from the CD-ROM is adequate; it is digital CD audio quality. Still, though, you will get much better quality (particularly deep bass and high treble response) from

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feeding CD audio through a soundcard to a home stereo system with a real amplifier.

 

External Packaging and Mounting

CD-ROM drives were developed after the drive bays in the PC were standardized, and are designed to fit into a standard 5.25" drive bay. The height of a CD-ROM drive is about 1.75", the height of a standard "half-height" drive bay.

Most newer drives have sheet metal enclosures that surround the drive itself. There are screw holes tapped into the sides of the drive to allow for simple mounting directly into a standard drive bay on a modern case. Some older PCs require the use of mounting rails that are screwed into the drive, and then slide into the drive bay.

CD-ROM drives that use a tray loading mechanism must be mounted horizontally; if mounted vertically the CD can fall right out of the drive. Caddy-based drives can technically be mounted horizontally or vertically, although it is generally better to mount them horizontally (I've never seen an internal CD-ROM mounted sideways.)

CD-ROM Drives

In a few short years, the Compact Disk - Read Only Memory (CD-ROM) drive has gone from pricey luxury to inexpensive necessity on the modern PC. The CD-ROM has opened up new computing vistas that were never possible before, due to its high capacity and broad applicability. In many ways, the CD-ROM has replaced the floppy disk drive, but in many ways it has allowed us to use our computers in ways that we never used them before. In fact, the "multimedia revolution" was largely a result of the availability of cheap CD-ROM drives.

As the name implies, CD-ROMs use compact disks, in fact, the same physical disk format as the ones we use for music. Special formatting is used to allow these disks to hold data. As CD-ROMs have come down in price they have become almost as common in a new PC as the hard disk or floppy disk, and they are now the method of choice for the distribution of software and data due to their combination of high capacity and cheap and easy manufacturing. Recent advances in technology have also improved their performance to levels approaching those of hard disks in many respects.

CD-ROM drives play a significant role in the following essential aspects of your computer system:

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Software Support: The number one reason why a PC today basically must have a CD-ROM drive is the large number of software titles that are only available on CD-ROM. At one time there were a few titles that came on CD-ROM, and they generally came on floppy disks as well. Today, not having a CD-ROM means losing out on a large segment of the PC software market. Also, some CD-ROMs require a drive that meets certain minimum performance requirements.

Performance: Since so much software uses the CD-ROM drive today, the performance level of the drive is important. It usually isn't as important as the performance of the hard drive or system components such as the processor or system memory, but it is still important, depending on what you use the drive for. Obviously, the more you use the CD-ROM, the more essential it is that it perform well.

This chapter examines CD-ROM operation in detail. It discusses how CD-ROMs work, the basics of CD-ROM media, and the various formats used for storing data and other information such as sound. A discussion on CD-ROM performance, reliability and interfacing is provided, along with a brief look at the newer, recordable CD formats.

CD-ROMs are a huge topic in the computer world today, so large that I cannot spend as much time as I would like discussing every aspect. This is especially true of the recordable formats such as CD-R and CD-RW, which are evolving quickly and represent a new use of the CD-ROM that most people still are not taking advantage of. My concern here is from the perspective of the PC user and home builder/upgrader, so I am sticking with basic CD-ROM topics, though I will expand coverage to the other technologies at the appropriate time. I do provide a basic description of the newer CD-ROM variants and provide references to where more information can be found. For additional generic information, try the http://www.cd-info.com.

 

Compact Disk Media

CD-ROM drives of course use compact disk media. These are the same disks that are used by audio CD players; different formatting methods are applied to the disk to allow them to hold different types of data. These formatting methods are discussed in detail in the section on data formats.

As for the media itself, at the lowest level it is identical to the compact disks you play in your home stereo. The exception to this is the special media used for CD-R and CD-RW drives; the media here is different because the drive is able to write to the disk, something not possible with standard CDs. This section looks at compact disk media and how it works.

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Note: Most of the descriptions below apply to all different CD formats, including CD audio, CD-ROM data, etc., because they are describing the media at a low level, below the level of data formatting.

 

Media Construction and Manufacture

Compact disks start as round wafers made from a polycarbonate substrate, measuring 120 mm (about 4.75 inches) in diameter and about 1.2 mm in thickness, which is less than 1/20th of an inch. These blanks are made into production CDs using a process not dissimilar to how old vinyl records were made (which is somewhat fitting, if you think about it.)

The first step in the creation of a CD is the production of a master. The data to be recorded on the disk (either audio or computer data, there are many different formats) is created as an image of ones and zeros. The image is etched into the master CD using a relatively high-power laser (much more powerful than the one you would find in a regular CD player) using special data encoding techniques that use microscopic pits to represent the data. The master CD is then used to create duplicate master stamps.

The actual CDs are produced by pressing them with the master stamp. This creates a duplicate of the original master, with pits in the correct places to represent the data (see the encoding section for details on this). After stamping, the entire disk is coated with a thin layer of aluminum (which is what makes the disk shine, and is what the laser reflects off when the disk is read) and then another thin layer of plastic. Then, the printed label is applied to the disk.

Many people don't realize that the data surface of the CD is actually the top of the disk. The media layer is directly under the CD label, and the player reads the CD from the bottom by focusing the laser through the 1.2 mm thickness of the CD's substrate. This is one reason why the bottom of the disk can have small scratches without impeding the use of the disk; they create an obstacle that the laser must look through, but they don't actually damage the data layer. On the other hand, scratches on the top of the disk can actually remove strips of the reflective aluminum coating, leaving the disk immediately unusable.

CDs are fairly hardy but are far from indestructible. They are reasonably solid but overly flexing them can make them unreadable. They are not too sensitive to heat but will warp if left to bake in direct sunlight on a hot summer's day. CD media should always be cared for properly. The use of

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caddies or jewel cases will protect them; in general, the less handling, the better.

 

Data Encoding and Decoding

Like hard disks and floppy disks, the compact disk is a digital storage medium. At the very lowest level, only two different values can be recorded on a disk: a one, or a zero. Magnetic disks record data using tiny magnetic fields, and the flux reversals that are detected by the read head as the disk moves from one type of field to another. Compact disks use a physical recording technique instead of a magnetic one.

The disk starts out totally flat. At each data-holding position on the disk, the CD is either left flat (these areas are called "lands") or is imprinted with a "pit", which is burned by a laser into the CD master, and then stamped into production CDs using a metal stamp made from the master. So as the disk spins, the laser traverses from lands to pits, many thousands per second. When the laser hits a land, it reflects cleanly off the aluminum coating, but when it hits a pit much of the light is diffused. The photodetector in the read head senses the difference and this is how it knows if the bit was a one or a zero.

While CDs are often referred to as having "tracks", this is actually imprecise. In fact, the entire CD is one very long, tightly-packed spiral. This is just like the single track on a phonograph record in concept, but there is a huge difference in scale. A standard CD has a spiral comprised of about 20,000 "tracks", so the spiral is in fact about three miles long! The tracks of the spiral are spaced about 1.6 microns apart. This is equivalent to a track density of about 16,000 tracks per inch, which exceeds that of even high-end hard disks today.

 

Error Correcting Code (ECC) and Data Recovery

Most people know that compact disk audio is remarkably resilient to data loss; bits of dust or dirt on the surface of the disk, or even small scratches, will often not impede the performance of the CD player at all, and usually not even a CD-ROM. There are in fact several techniques that are used to create this robustness.

The first and most important is the use of error correcting code or ECC. ECC is a special data encoding protocol that uses a combination of redundant information and special data positioning, to make it possible to detect and

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recover from missing bits of data. Through special algorithms, the CD player can reconstruct missing data from the ECC information and allow the disk to play right through most errors seamlessly. ECC is described in more detail in the section on hard disks.

When ECC fails to provide the correct information, there is another tool available: interpolation. This technique is used to fill in missing bits of data on audio CDs. Let's suppose that you have a series of values that is read from a CD audio track, which go like this: 400, 525, 650, 825, 1100. Now let's suppose that due to an error, the fourth value cannot be read, so we in fact see 400, 525, 650, ___, 1100. Using linear interpolation, we can estimate the missing value as half-way between 650 and 1100, or 875. This isn't the correct value, but it's close enough for audio. Since it is only one value of thousands being played to the ear each second, no human can possibly detect the mistake. In fact, humans won't notice much larger quantities of missing numbers.

With computer data on a CD-ROM however, interpolation is useless; there is no way to make a guess at the correct value in a series of program bytes. An error of even one single bit in a 1 MB file can be the difference between an application that works properly and one that wipes out data on your hard drive. (Seriously. It would be really bad luck but it is possible.) This was a big problem when the original CD-ROM formats were developed.

For this reason, data formats on CD employ additional ECC data. In addition to the ECC defined at the bit level as part of the "red book" audio CD format, data CDs devote over 10% of the theoretical capacity of the disk to additional error detection and correction codes at the byte level. For each 2,048 bytes of data, 280 bytes are used for error detection and correction codes. This sacrifice in capacity results in extremely high reliability for modern data CD-ROM media; amazingly large errors can be recovered due to this redundancy.

 

CD Capacity

A standard CD has a capacity of about 74 minutes of standard CD audio music. There are extended CDs that can actually exceed this limit and pack more than 80 minutes on a disk, but these are non-standard. Regular CD-ROM media hold about 650 MB of data, but the actual storage capacity depends on the particular CD format used.

Compact Disk Formats

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While at the lowest level all CDs record information in the same way, on a spiral track using lands and pits, there are many different types of data that can be placed on a CD. For example, an audio CD contains bits and bytes just like a CD-ROM data CD, but the information is laid out in a totally different manner. These different ways of organizing the ones and zeros on the disk are called CD formats. There are several different formats in use today, with new ones being invented all the time. Some are more popular than others; some require special drives to access them, while others are compatible with each other to some degree.

These formats are basically equivalent to the logical structures and file systems used on hard disks or floppy disks. Unlike those media, CD formats are fixed in terms of how the data is structured, storage capacity, block size, etc. There is no way to "format" a compact disk the way a hard disk or floppy disk is formatted, and there is no concept of partitioning either. The structures are basically the same for each CD that uses that particular format.

The sheer number of different formats, and the fact that some drives will handle certain formats while others will not, makes all of this a rather confusing issue for most people (including myself, at first.) This section describes the different formats and tries to make some sense of them, including looking at cross-format compatibility.

 

Single Session vs. Multi-session

The number of sessions a compact disk has refers to the number of different continuously-written chunks of data that are placed on the disk. Traditional CD formats such as standard CD audio and CD data, are said to be single session; everything that is ever going to be on the disk is placed there at once when the disk is manufactured. Some newer CD formats however use more than one session, and are called multi-session drives. Using multiple sessions means that information can be written to the first part of the disk, and then later more information can be appended to it in the unused space left after the first session.

Multi-session capabilities are provided by many newer drives, and are required for some of the fancier CD formats. Some formats have versions that use single sessions or multiple sessions. For example, Photo CDs that contain images from a single roll of film are single-session, but to append additional images to a disk that has already been recorded requires a multi-session-capable drive. CD-Recordable disks can be either single-session or multi-session as well.

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Since standard audio CDs and data CDs (CD-ROM disks) are stamped with a predefined image, they are single session. For this reason, support for multi-session has not traditionally been considered an essential feature, and the vast majority of legacy drives will not support it. The creation of CD-Rewriteable (CD-RW) has brought multi-session support more to the foreground, because disks written in CD-RW format can only be properly read by a drive that supports multi-session disks.

The reason that specific support is required for multi-session disks is that standard disks have only a single place for storing the table of contents of the disk. This makes sense, because traditionally disks always had their entire contents recorded at once. When a disk is written a piece at a time, it is necessary to "update" the table of contents each time a new section is written, to reflect the new contents of the disk. Drives that support multi-session disks are programmed to seek out these multiple tables of contents that can occur in various places on the disk.

 

A Rainbow of "Books"

There are many different CD formats used on compact disk media today. Each one of these has a formal name, but also has a somewhat odd name that refers to the color of a book. (Huh? ) This little naming convention is a sort of slang that historically refers to the color of the initial paper manual describing the specification. I am not sure to what extent this is universally true of all of the different "books". It may have started out with some booklets that really had colored pamphlets and then later specs may have just had colors given to them to be "cute", sorta how the original "whetstone" benchmark led to "dhrystones" and "winstones" and all manner of other silliness (actually, dhrystones may have been first, I dunno. :) )

These book colors are sometimes colloquially used to refer to the various standards; they can be a bit confusing, so I have included them in a table below for easy reference:

"Book" Color

CD Format Referenced

Red Digital Audio (CD-DA)

YellowDigital Data (CD-ROM): ISO 9660 / High Sierra,

and extensions such as CD-ROM Extended Architecture (CD-ROM XA)

Green CD-Interactive (CD-I)

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OrangeMagneto-Optical (MO), CD-Recordable (CD-R),

CD-Rewriteable (CD-RW)

White "Bridge" CDs (Photo CD, Video CD and others)

Some of these books, especially yellow, refer to multiple formats, and also have format extensions associated with them. These are described in more detail in their individual sections; there can sometimes be some confusion in terminology because so many formats are extensions to and derivations of earlier ones.

 

Compact Disk Digital Audio (CD-DA)

The first CD format was of course that which defined the audio CD used in all regular CD players, called CD Digital Audio or CD-DA for short. The specifications for this format were codified in the first CD standard, the so-called "red book" that was developed by Philips and Sony, the creators of the original compact disk technology. The "red book" was published in 1980, and actually specifies not just the data format for digital audio but also the physical specifications for compact disks: the size of the media, the spacing of the tracks, etc. In a sense, then, all of the subsequent standards that came after CD-DA build on the "red book" specification, since they use the same specifications for the media and how it is read. They also base their structure on the original structure created for CD audio.

Data in the CD digital audio format is encoded by starting with a source sound file, and sampling it to convert it to digital format. CD-DA audio uses a sample rate of 44.1 kHz, which is roughly double the highest frequency audible by humans (around 22 kHz.) Each sample is 16 bits in size, and the sampling is done in stereo. Therefore, each second of sound takes (44,100 * 2 * 2) bytes of data, which is 176,400 bytes.

Audio data is stored on the disk in blocks, which are also sometimes called sectors. Each block holds 2,352 bytes of data, with an additional number of bytes used for error detection and correction, as well as control structures. Therefore, 75 blocks are required for each second of sound. On a standard 74-minute CD then, the total amount of storage is (2,352 * 75 * 74 * 60), which is 783,216,000 bytes or about 747 MB. From this derives the handy rule of thumb that a minute of CD audio takes about 10 MB, uncompressed.

Using special software, it is possible to actually read the digitally-encoded audio data directly from the CD itself, and store it in a computer sound format such as a WAV file. However, while every CD-ROM drive will play standard "red book" digital audio, not every drive will allow you to read the

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digital audio data directly, which is sometimes called CD-DA extraction. Sometimes the reasons are technical, but more often they are political; some drive manufacturers intentionally program their drives not to allow data extraction, in order to avoid possible copyright infringement by their owners

 

 

CD-ROM Digital Data (CD-ROM, ISO 9660, "High Sierra")

The standard that describes how digital data are to be recorded on compact disk media went through several different iterations before the format was finalized. The first step was the creation of the original data format standard, called the "yellow book", by Philips and Sony in 1983. This specification was based on the original "red book" format that was the basis for CD digital audio disks.

The "yellow book" specification was unfortunately general enough that it was feared that many different companies would implement proprietary data storage formats using this spec, resulting in many different incompatible data CDs. To try to prevent this, representatives of major manufacturers met at the High Sierra Hotel and Casino in Lake Tahoe, NV, in 1985, to come together on a common standard for data CDs. This format was nicknamed High Sierra Format. It was later modified slightly and adopted as ISO standard 9660.

Today, the terms "yellow book", High Sierra and ISO 9660 are used somewhat interchangeably to refer to standard data CDs, although the most common name is simply: "CD-ROM". This isn't technically precise, but the important thing is that virtually all data CDs that are in use today are standardized and will work in all standard CD-ROM drives, which was the main objective of all of this, of course. I will call this format simply "data CD" for the rest of this section, for simplicity.

Under the data CD standard, there are two modes defined:

Mode 1: This is the standard data storage mode used by virtually all standard data CDs. The data is laid out in basically the same way as it is in standard audio CD format, except that the 2,352 bytes of data in each block are broken down further. 2,048 of these bytes are for "real" data. The other 304 bytes are used for an additional level of error detecting and correcting code. This is necessary because data CDs cannot tolerate the loss of a handful of bits now and then, the way audio CDs can.

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Mode 2: Mode 2 data CDs are the same as mode 1 CDs except that the error detecting and correcting codes are omitted. Hmm. Why would they define this then, when it is so similar to standard audio CDs? The reason is that mode 2 format provides a more flexible vehicle for storing types of data that do not require high data integrity: for example, graphics and video can use this format. Furthermore, different kinds can be mixed together; this is the basis for the extensions to the original data CD standards known as CD-ROM Extended Architecture, or CD-ROM XA.

The rest of this section is concerned with "plain vanilla" CD-ROM data disks, which are mode 1 under the ISO 9660 standard. Each block contains 2,048 bytes of real data. As with the audio format, there are 75 blocks per "second" of the disk, so on a standard 74 minute compact disk, this yields a total capacity of 681,984,000 bytes, which is the same as the commonly-heard 650 MB (actually 650.39 binary MB). Since the disk is designed to allow the reading of 75 blocks per second, this is the basis for the standard single-speed transfer rate of 75 * 2,048 = 150 KB per second. Of course, faster CD-ROM drives transfer at much higher rates.

Much the way a hard disk or floppy disk has a file allocation table and root directory to identify the place to look in order to find the various directories and files on the disk, a data CD needs this "starting point" as well. At the start of the CD, a table of contents lists what is on the disk and where to find it. Newer CD formats that are said to be "multi-session" can have more than one set of data on the disk, recorded at different times, and therefore use multiple tables of contents, one per session. The table of contents is also sometimes called the index of the disk.

 

CD-ROM Extended Architecture (CD-ROM XA)

Finding the existing CD audio and CD data specifications too restricting, a new format called CD-ROM Extended Architecture or CD-ROM XA was developed by Philips, Sony and Microsoft. This format builds on the existing data CD standard and is considered an extension to the original "yellow book" format.

Disks that use CD-ROM XA can mix standard data CD mode 1 and mode 2 tracks, allowing the mixing of standard data along with other types of data. The mode 2 tracks are further divided into two types: Form 1 and Form 2. Between all these different formats and modes, CD-ROM XA disks can store data, audio, compressed audio, video, compressed video, graphics and others. The mixing together of these different types of information is called

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interleaving (which is a term used in other contexts in the computer world as well, such as memory technology).

Using CD-ROM XA disks usually requires a drive specifically certified to be capable of reading the CD-ROM XA format. In many cases these disks include compressed audio or video and often, drives capable of reading them will include hardware decoders to allow on-the-fly decompression of the data. The drive must also know how to handle the different data formats on the disk, of course.

 

"Bridge" CDs

The term bridge CD is used to refer to disks that use extensions or derivations of the CD-ROM Extended Architecture format. These extended formats are described in the "white book" specification. The reason for the term "bridge" (as far as I can tell) is that these disks are designed to work both in CD-ROMs that support CD-ROM XA, and also in CD-Interactive hardware, thus "bridging" the two types of CD hardware.

It is therefore possible to make CD-I disks that will work in CD-ROM XA drives, and CD-ROM XA disks that will play in CD-I drives. Bridge CDs also include both the Kodak Photo CD format and also the Video CD format.

 

CD-Interactive (CD-I)

In 1986, Philips and Sony again joined forces to create the CD-Interactive or CD-I format. This concept was quite ambitious, with the goal to develop both a format and a special new type of hardware to use it. In some ways this was the first serious attempt at what we now call "multimedia", with authors creating disks including text, graphics, audio, video, and computer programs, and hardware sold to handle all of these and connect to a television screen for output.

CD-I is derived from the original standard CD-ROM "yellow book" format, ISO 9660, much the way that CD-ROM Extended Architecture (CD-ROM XA) is. However, the format used by CD-I is somewhat different. A new class of disks called "bridge disks" has been created that will work in drives subscribing to either format.

CD-I never really took off, for one reason or another. It is still around, but isn't really very popular. CD-I compatibility is found in many PC CD-ROM drives, although many older drives won't support it

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Video CD (VCD)

Support for a special CD format for the storing of compressed video information is defined as part of the "white book" specification. Through the use of MPEG compression it is possible to store 74 minutes of full-motion video in the same space that uncompressed "red book" audio uses! This format is called video CD or sometimes VCD.

Playing video CDs requires either a video CD player or a CD-ROM drive that is video CD compatible. Since the compression algorithm used for video CD, MPEG-1, is rather unsophisticated, the quality of these disks has not been anything to write home about. (In my opinion, a decent 4-head VHS VCR produces much better quality video.) This, combined with the fact that video CD titles are few and far between and are not cheap, has meant that video CD has been far from popular. Since the next big format for consumer video playback (and eventually recording) is expected to be DVD, I believe it likely that the VCD format is doomed to a speedy death over the next couple of years.

Note: Video CD is not the same as CD-V, an alternative video CD standard that only holds a few minutes of uncompressed video, along with uncompressed digital audio. (Isn't this standards stuff fun?)

 

Photo CD

Developed in the early 90s by Kodak and Philips (who seems to have its hand in everything CD-related), photo CD is an implementation of CD-ROM extended architecture designed to hold photographic images. They technically use mode 2 form 1 of the CD-ROM XA architecture. Photo CDs are defined in the "orange book" specification.

When you send in film for processing to photo CD, the film is first developed normally. The developed and printed pictures are then scanned and converted to digital form, encoded into the photo CD format, and written to the CD. Writing the photos to the CD is done using a process that is basically the same as how CD-R works: a laser burns the information into the tracks of the CD.

After you have sent in film and created the first photo CD, it is possible to record additional films to the same disk. However, doing this means that the information is written in multiple sessions, and therefore a player that

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supports multiple sessions is required to access the disk properly. A photo CD can be written so that it is a "bridge" CD, which will allow it to be read by CD-I drives as well.

 

Digital Versatile/Video Disk (DVD)

CD-ROMs have been around in the computer world for more than a decade. As with most technology, eventually everything is replaced with "bigger and better". It may be surprising to some to realize this, but one of the main problems with CD-ROMs today is.. they are too small! There are many programs that are even now shipping on four or even more CD-ROMs, and this trend is only going to continue in the future. To provide for the next generation of software, a new format has been created that is abbreviated DVD. This sometimes stands for digital video disk and sometimes digital versatile disk, depending on whom you ask.

DVD uses the same physical form factor as CD-ROM but the similarities end there. The logical formats are considerably different. It is likely that within the next few years, DVD may revolutionize not only how we store data on the PC, but also home audio and video as well. A future version of DVD called DVD-RAM may in fact even replace your VCR! This is still quite new technology and it is changing rapidly. For more information, check out the http://www.videodiscovery.com/vdyweb/dvd/dvdfaq.html.

 

CD Format Compatibility Reference

The table below is a basic compatibility reference chart for the various CD formats. Each column in the chart is a CD format, and each row in the chart is for a specific type of CD drive. The table should be read by looking at the left-most column for a type of drive, and then looking across the row to see which formats the drive will usually support. This table is a guideline only, since any specific drive can have differing capabilities depending on what the manufacturer decides to allow it to support.

Notes:

CD-R and CD-RW are included in the table, although they technically aren't "formats" of course--they write disks using the other formats listed. CD-R disks are supported by most drives as long as the CD-R created is in a format that the drive would read if it were a pressed CD. Some drives can be finicky about some types of CD-R disks, however. Each drive is rated as either "Single" or "Multi", with the former

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meaning only single-session disks are supported and the latter meaning multi-session disks are supported (which of course implies single-session is also supported).

For CD-DA, each drive is specified as to whether it can play CD-DA or extract it in digital form.

"Some" means that some of these drives support the format, and some don't.

Type of Drive

CD-DA

CD-ROM

CD-ROM XA

Bridge CD

CD-I

Video CD

Photo CD

CD-RCD-RW

Audio CD

PlayerPlay No No No No No No Single No

Standard

(Older) CD-ROM

Drive

Play, some extra

ct

Yes No No No No No Single No

(Newer) CD-ROM

XA Drive

Play, some extra

ct

Yes Yes Yes No Some Some Single No

Multi-session CD-ROM

XA Drive

Play, Extra

ctYes Yes Yes No Yes Yes Multi

Some

CD-I Player

Play No No Yes Yes Yes YesSingle

?No

CD-R Drive

Play, Extra

ctYes Yes Yes Yes Yes Yes Multi

Some

CD-RW Drive

Play, Extra

ctYes Yes Yes Yes Yes Yes Multi Yes

Recordable CD (CD-R)

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While CD-ROMs are an amazingly useful medium, and have revolutionized how the average person uses his or her PC, the sticking point with them has always been that they were a read-only medium. Not everyone has a CD press in their basement, and while it is possible to get commercially-produced CDs today in small volumes, this typically still means 1,000 disks and several thousands dollars.

In 1990, part II of the so-called "orange book" published by Philips (who else), specified the characteristics and format of a recordable CD, or CD-R. CD-R is also sometimes called CD-WORM or CD-WO, where WO means "write once" and WORM "write once read many", both reflecting truisms about the medium. (There are other types of drives that are also WORM however.)

CD-R drives, and the media they use, allow a regular PC user to create audio or data CDs in various formats that can be read by most normal CD players or CD-ROM drives, at a reasonable cost. As "write once" implies, the disks start out blank, can be recorded once, and thereafter are permanent and not re-recordable. Part III of the "orange book" defines rewriteable CDs, which are erasable, unlike CD-R.

Note: Part I of the "orange book" contains the specifications for magneto-optical (MO) drives.

CD-R is more than just another standard CD format. CD-ROM, CD-ROM XA, CD-I, etc. built upon the original CD audio standard by simply changing the interpretation of what the bytes in the original CD audio format meant. CD-R is actually in many ways the opposite: it defines new physical media and ways of recording them, while continuing to use the standard formats defined in other specifications.

Initially, CD-R was prohibitively expensive--well over $1,000 for a drive, and $10 or more for each blank disk. As both of these numbers have dropped in half or less, CD-R has become quite popular for several applications, including archiving, software distribution, backup, custom audio, and a host of others. This section takes a look at CD-R in a fair bit of detail, although certainly not exhaustively; there are enough descriptions and aspects related to CD-R to fill a chapter as big as everything I have written about CD-ROMs in general, easily.

 

CD-R Media and Encoding

The reason that regular CDs have never been able to write CD media is that the ones and zeros are encoded using physical changes to the disk: pits that

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are physically etched into the plastic substrate. CD-R technology was faced with the difficult task of finding a way to conveniently create these pits, without the special equipment required when mastering CDs (which can etch the pits, but costs a lot of money).

CD-R media addresses this issue by doing away with pits and lands entirely and using a different kind of media. CD-R media starts with a polycarbonate substrate, just like regular CDs do. Instead of physical etching this substrate, it is stamped with a spiral pre-groove, similar to the spiral found on a regular CD except that it is intentionally "wobbled". This groove is what the CD-R drive uses to follow the data path of the disk during recording. If the disk were totally blank then writing the spiral tracks would be a very complex process; recall that they are spaced only 1.6 microns apart.

On top of the polycarbonate, a special photosensitive dye layer is deposited; on top of that a metal reflective layer is applied (such as a gold or silver alloy) and then finally, a plastic protective layer. It is these different layers that give CD-R media their different visual appearance from regular CDs.

The key to the media is the dye layer (and the special laser used in the drives.) It is chosen so that it has the property that when light from a specific type and intensity of laser is applied to it, it heats up rapidly and changes its chemical composition. (When folks talk about creating a CD-R as "burning" a disk, they're basically correct.) As a result of this change in chemical composition, the area "burned" reflects less light than the areas that do not have the laser applied. This system is designed to mimic the way light reflects cleanly off a "land" on a regular CD, but is scattered by a "pit", so an entire disk is created from burned and non-burned areas, just like how a regular CD is created from pits and lands. The result is that the created CD media will play in regular CD players as if it were a regular CD, in most cases.

Since the media is being physically altered by a process of heat and chemistry, the change is permanent and irreversible. Once any part of the CD has been written, the data is there forever. Some drives allow you to record some information in one sitting, and then more information later on, if the disk is not yet full. This is called multi-session recording, and requires a CD player capable of recognizing multi-session disks in order to use the burned disk.

CD-R media are generally 74 minutes in length. There are some disks with less capacity, and some with more, but all 74-minute disks have the same capacity, about 650 MB. Some media seems to work better than others in some drives. The cost of the media has been on a steady decline as the technology matures, and at under $5 each the disks are now a very reasonable medium for archival and long-term storage. I personally think

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they are not an appropriate avenue for backup, given their write-once nature.

 

CD-R Drives

Just as CD-R requires the use of special media, it also of course requires the use of a special CD-R drive. This drive is very different than a standard CD player because it must include a special laser. The laser is the key component from the drive's perspective, in that it is what burns the image into the CD-R media's dye layer.

CD-R drives are capable of reading disks as well as writing them, of course. Most support a large number of formats, to enable the reading and writing of a wide variety of CDs. Most drives are also faster than single speed; it is quite typical for the speed of the drive to be significantly lower when writing than when reading. For example, a drive might be specified to be 4X when reading, but only 2X when writing a disk.

While standard CD-ROM drives often use a variety of formats, SCSI is the interface of choice for the vast majority of CD-R drives. The main reason is that SCSI is a higher-performance interface that allows the flow of data to the drive to be maintained more easily, independently of what other activities are happening within the PC. This is critical for writing CD-R media because burning a disk requires an uninterrupted flow of data from wherever it is coming from (usually a hard disk) to the drive. CD-ROM drives are now also available for the ATAPI (IDE) interface.

Since the CD-ROM's laser is moving at a constant speed as it writes, it must have the data it needs available in a smooth flow; it cannot wait for the data if it is delayed because the disk cannot be stopped. The faster the drive spins the disk when writing, the more data flow is required. Due to how CDs are written, interrupting the writing of the disk will generally ruin it; burned disks that don't work are (semi-)affectionately referred to as "coasters", since about all they are good for is protecting your coffee table...

Many drives take specific steps to avoid this problem. One of the most common ones is the use of a substantial memory buffer that can supply data to the laser head in the event that the flow of data is interrupted. Another is the use of an image file: instead of copying the data to be recorded from its original locations, an exact image of the disk to be created is stored in a large file on disk first, and then transferred "whole" to the burned disk. This reduces the chances of an interruption while writing, but costs a good chunk of disk space.

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Many users have a hard time when initially setting up their CD-R drives, getting them to write disks properly without destroying too much blank media in the process. Once the drives are set up, however, CD-R can be a fairly reliable format, allowing the user to create a large number of disks with minimal waste

 

CD-R Software

Most CD-R drives ship with special software. This software is used not only to control the creation of CD-R disks, but also to allow the user to "compose" what will go on the disk. For example, the software will allow a user creating a "best of" audio CD to arrange the tracks in his or her preferred order, etc. It will also allow the organization and management of mixed-mode disks containing multiple formats, as well as the creation of an image file for better performance in burning CDs.

The importance of CD-R software should not be underestimated. Good software can make the difference between a good CD-R experience and a nightmare. The software has a direct impact on how easy it is to make disks in the way that you want, and can be the difference between success and failure in many cases. Make sure you find out what software is bundled with your drive

 

CD-R and CD-ROM Compatibility

The disks created by most CD-R drives are compatible with most CD-ROM and CD audio disks. The keyword, of course, is "most". In particular, some older drives can have problems with the output of some CD-R drives. In general though, it isn't a problem. Some players can actually have more problems with some types of media than with others, since they use different dye and/or reflective metal layers. Changing the type of CD-R blanks used can sometimes fix an incompatibility problem. The drive itself also has a big impact on the readability of CD-Rs it creates, and I personally have found that changing the drive can make all the difference in the world in this regard.

CD-R disks that are written in multi-session format (writing part of the disk at one time and another part later on) can only be read in a drive that is multi-session capable. See the CD format compatibility table for more.

 

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Rewriteable CD (CD-RW)

Recordable CDs (CD-R) moved beyond the read-only standard compact disk by allowing the writing of CDs by the average PC user. It however had its limitations as well, chiefly the fact that each disk can only be written once. A still newer technology, called rewriteable CD or CD-RW, ups the ante one more notch by allowing CDs to be both written and rewritten. Truly, at this stage, the use of the term "CD-ROM" becomes a bit questionable, since these disk are definitely not "read only"!

Note: CD-RW is also sometimes referred to as erasable CD or CD-E.

The specifications for CD-RW are codified as part III of the "orange book" published by Philips. This document also defines the characteristics of CD-R drives (part II) and magneto-optical drives (part I). CD-RW uses much more advanced technology in order to accomplish its goal of making compact disks both writeable and rewriteable. There are therefore many issues related to operation of these drives, and also compatibility, that aren't applicable to most other CD formats. This section takes a basic look at CD-RW without getting too deep into the details.

 

CD-RW Media and Encoding

CD-RW media are more similar to CD-R media than they are to regular CD-ROMs. CD-R media replaces the physically molded pits in the surface of the disk with a sensitive dye layer that can be burned to simulate how a standard CD works. CD-R media technology is described here; it makes sense to understand it before continuing here.

CD-RW media are formed in the same basic way that CD-R media are; they start with a polycarbonate base and a molded spiral pre-groove to provide a base for recording. There are several layers applied to the surface of the disk, with one of them being the recording layer where ones and zeroes are encoded. The recording layer for CD-RW is different of course than it is for CD-R. The problem with CD-R is that the dye layer used is permanently changed during the writing process, which prevents rewriting.

CD-RW media replaces this dye with a special phase-change recording layer, comprised of a specific chemical compound that can change states when energy is applied to it, and can also change back again. Much the way water can change to steam, or ice, depending on its temperature, there are some types of chemicals that can not only change their state after having heat or

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other conditions applied, but even retain that state when the heat is removed. They can later be returned to their original state through another, different process.

The material used in CD-RW disks has the property that when it is heated to one temperature and then cooled, it will crystallize, while if it is heated to a higher temperature and then cools, it will form a non-crystalline structure when cooled. (Many metals are like this; in fact, different types of iron are formed by controlled heating and cooling to modify its internal structure).

When the material is crystalline, it reflects more light than when it doesn't; so in the crystalline state it is like a "land" and in the non-crystalline state, a "pit". By using two different laser power settings, it is possible to change the material from one state to another, allowing the rewriting of the disk. The change of phase at each point on the disk's spiral is what encodes ones and zeros into the disk. The spiral and other structures are the same as for CD-R; what changes is how the pits are encoded.

CD-RW media have one very important drawback: they don't emulate the pits and lands of a regular CD as well as the dye layer of a regular CD-R, and therefore, they are not backward compatible to all regular audio CD players and CD-ROM drives. Also, the fact that they are written multiple times means that they are multi-session disks by definition, and so are not compatible with non-multi-session-capable drives.

 

CD-RW Drives and Software

CD-RW drives are similar to CD-R drives except of course that they employ a very different kind of laser, to enable them to write the special CD-RW media. Like CD-R drives, CD-RW drives are capable of writing in multiple different standard formats, and reading those formats as well. CD-RW drives can also write CD-R media, making them extremely flexible.

There is one significant disadvantage to CD-RW drives, however, compared to CD-R drives: they are basically limited to writing at 2X speeds. Unlike CD-R, the actual writing mechanism is tied to a specific speed, which is currently 2X as specified in the "orange book" standard. In order to create 4X CD-RW, for example, a new standard would need to be created. With CD-R it's simply a matter of how efficiently and effectively the drive can write at a higher speed.

 

CD-RW, CD-R and CD-ROM Compatibility

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There are a number of compatibility issues associated with CD-RW. First and foremost is the fact that CD-RW media are not backward-compatible with many regular CD-ROM drives. Due to the lower reflectivity of the CD-RW media, regular drives can have problems reading them. In essence, the CD-RW media just does not emulate the pits and lands of a regular pressed CD well enough to fool a standard reader.

Another problem is that CD-RW media are recorded in a multi-session format. Single-session disks are written an entire disk at a time, which obviously isn't practical for a rewriteable medium. Many regular CD-ROM drives are multi-session compatible, but many are not.

One of the great strengths of CD-R is the fact that once you create a disk, it can be read in basically any reasonably-modern PC that has a CD-ROM player. Since CD-RW media does not have this large advantage of universality, it is, in my opinion, relegated into "the pack" of competing removable mass storage formats, such as removable hard disks, high-capacity floppies, and magneto-optical drives. All of these provide removable, rewriteable storage at different price points, and all share the disadvantage of not being readily usable on PCs that don't have the right type of special drive.

It is possible to make a small change to the way regular CD-ROM drives are made to compensate for the different media used by CD-RW. This is now happening in the market, and the latest regular CD-ROM drives now will read CD-RW media.

It is possible that this change will be incorporated into the standard designs of drive makers in the future. Of course, this will do nothing for the older drives that were manufactured and sold before CD-RW even existed.

Note: CD-R disks made in a CD-RW drive can be read in any drive that can read CD-R media. In other words, the compatibility problem is with the CD-RW media, not the CD-RW drives. If a CD-R disk is made in a single session, it should be readable in any regular CD-ROM drive; if it is written in multiple sessions, a multi-session drive is required. These are the same rules that apply to regular CD-R drives and media as well.

 

 

CD-RW vs. CD-R

At casual glance, it might seem that CD-RW is "better" than CD-R and therefore "replaces it". In fact, I think it likely that CD-RW drives will replace

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CD-R drives as prices come down and the technology matures. The reason is simple: the CD-RW drives don't cost a lot more than CD-R drives, they have the ability to write CD-R disks just like CD-R drives do, but they also can write CD-RW media. Therefore, they have more capabilities and really, no drawbacks compared to CD-R, other than a small amount of cost.

The media is a different matter. I do not think that it is likely that CD-RW disks will replace CD-R disks, for several reasons:

Cost: CD-RW media is significantly more expensive than CD-R media. In time the cost of the two types of media may approach each other, but for now they are far enough apart that this is a factor.

Compatibility: CD-R disks are compatible with most existing audio CD and CD-R players. CD-RW disks are not. This is a major factor, because a big part of the allure of CD-R is compatibility with existing CD players.

Permanence: Many of the applications of CD-R actually have the non-rewriteability of the disk is a requirement. For software distribution, historical archiving, and similar applications, the creator of the disk normally doesn't want it to be erasable.

For these reasons, CD-RW is often said to "complement" CD-R, instead of replacing it. Since the drives support both media, users have a choice of which they wish to use. One common application is the use of a CD-RW disk for testing out the creation of a disk. Since it is rewriteable the disk isn't wasted in the event that the user omits a file by accident or whatever. The disk can be developed using a single CD-RW, and once perfected, burned onto regular CD-R, greatly reducing the number of "coasters" created by accident.

It should also be noted that both CD-R and CD-RW are still just becoming reasonably commonplace on the market, and DVD is gaining quickly behind them. As DVD advances it is quite likely that many buyers will skip both recordable CD formats in favor of DVD-RAM, the rewriteable version of DVD. Only time will tell on this score, of course.

CD-ROM Performance and Reliability

The subject of CD-ROM performance is in some ways, a confusing and maybe even slightly controversial one. Depending on what you are doing with your drive, the importance of its performance can range from very great to virtually nil. As with hard disks, the true performance of CD-ROM drives can be difficult to discern, and too many manufacturers obscure the issue by hiding the important performance factors behind marketing gimmicks and just plain advertising puffery.

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For the most part, the performance of CD-ROM drives is based on the same performance factors that measure hard disk performance. However, CD-ROMs use different "key" performance metrics, you know, the numbers that are thrown around by manufacturers and sellers to try to convince the buyer that the drive is fast. While hard disk manufacturers try to sell you on seek time and drive RPM for example, with CD-ROM drives the "X" speed (2X, 4X, etc.) is the key metric, followed probably by transfer rate and access time.

This section takes a look at CD-ROM performance issues in detail, along with looking at reliability issues. (Reliability is much less of an issue for CD-ROMs than it is for hard disks, of course, since CDs are not used for primary data storage and don't normally contain regular user data.) I will make pretty frequent reference back to the appropriate sections in the chapter on hard disks, so that I don't have to repeat the descriptions of the various performance measures.

 

General Performance Issues - How Important is CD Performance?

The never-ending quest for speed, speed and still more speed has brought us CD-ROM drives that have gone from 1X and 2X to 12X, 16X and even 24X ratings. When it comes to hard disks, I always say "the faster the better, but watch your wallet". However, it is my (controversial?) opinion that in many situations the importance of CD-ROM drive performance is greatly overstated.

Your hard disk is used for booting up your machine, running most of your applications, holding your data, and even holding the swap file your operating system uses for virtual memory. Almost every PC user has applications, data, an operating system and a swap file on their hard disk. Because of all of these important uses, the performance of your hard disk impacts on virtually everything you do with your PC. CD-ROM drives are different. They can be used in many different ways, and how they are used depends a great deal on what the PC user primarily does with the drive.

If you are not doing a great deal with your drive, you really do not need the fastest CD-ROM drive on the market. While getting a faster drive will improve performance when using the drive, you aren't really going to notice it much at all if you aren't using applications that demand greater performance. In contrast, many people can live without a faster hard disk, but almost all will realize a noticeable improvement that will improve their computing experience on a daily basis.

There are also different ways to look at performance. In some applications, a lot of data must be transferred from the CD-ROM on a continuous basis; for

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example, if you are playing a video title from the disk. In this case the sequential read rate (also called the transfer rate) of the drive is more important than the random access performance (sometimes called access time). In other applications you don't need a lot of data streamed continuously, but small bits of data from various areas on the disk. Here, your priorities are reversed, and random access performance is more important than sequential performance, relatively speaking.

The table below shows some of the different uses typically made of CD-ROM drives and the approximate level of performance that they require. This is of course both general and subjective, but I think it makes a better guideline than the usual "get a faster drive!" nonsense that most people are constantly bombarded with:

Activity

Random Access (Access Time) Performance Requirements

Sequential Read (Transfer Rate)

Performance Requirements

Driver Installations,

Software Updates

Low Very Low

Software Installation

Low Low

Referencing Archived

DataLow to Moderate Low to Moderate

Executing Application Software

Moderate to High Moderate

Real-Time Database

WorkHigh Moderate to High

Multimedia (Video,

Audio etc.)Moderate High to Very High

Games Moderate to Very High Moderate to High

There are many factors that play a part in determining performance levels and requirements as well. Caching is one; the use of a good-sized cache can "cover up" the poorer performance of a low-speed CD-ROM drive. Another is

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the fact that in many cases, data from the CD-ROM can be copied from the CD to the hard disk and used there. In this situation, the importance of the performance of the CD-ROM drive is greatly diminished.

For example, many games have many and/or large data files that are required while playing the game. However, most of these have multiple installation options, some of which will copy a significant amount of data to the hard disk. Since the hard disk is much faster than any CD-ROM, a PC with a fast hard disk and a slow CD-ROM drive can outperform one with a middling hard disk and a quick CD-ROM, in many situations. (Of course the drawback to doing a lot of big installations on games is that you lose a lot of disk space this way.)

Finally, remember that even though 24X CD-ROM drives are available cheaply today, there are millions and millions of older, slower CD-ROM drives already out in the market. For this reason, most manufacturers of software will ensure that the performance requirements for their titles are rather conservative. It is possible that a software company will put out a new package that requires a 10X or better CD-ROM drive, but if they do this they will give up a large segment of their potential market. Most software only requires about a 4X player or better, and in fact most software will work with 2X or better devices, albeit slowly.

Of course, there is usually nothing wrong with having a faster drive. Just remember that the super-fast ones cost more money, so make sure what you are looking at isn't overkill. At my office, I have installed about a dozen CD-ROM drives, because most of the PCs are older and did not have them. Most of these drives were 2X models, bought for $25 to $30 each, despite the fact that I could have bought 10X models for around $100 a piece.

Would I recommend a 2X drive to save $70 for the typical home user? Definitely not, because multimedia titles don't work well on a drive that slow. But for the business PC, where the CD-ROM gets very light use primarily for installing software, driver upgrades, mapping applications and the like, a 2X is just fine and I saved my company over $1,000. If you are buying a new drive and can get a 12X drive instead of an 8X drive for $15 more, why not? But buying a new 16X drive to upgrade a 10X drive is, for most people, a waste of upgrade dollars that could be better spent elsewhere.

 

Rated "X" Speed, CLV and CAV

It seems that every computer component has a commonly-used metric that is supposed to provide a universal way of allowing you to compare them, but which in fact can be very misleading if you don't fully explore what it means.

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With processors it is the MHz clock speed; with memory the DRAM chip speed; with monitors it is often the nominal screen size; and with CD-ROM drives it is the "X" speed.

The "X" speed of the drive refers to its nominal spin rate. A 1X drive spins that the speed of a standard audio CD; a 2X drive spins at twice this speed, etc. Since regular CD-ROM drives use constant linear velocity (CLV), this means that the actual spin speed of the disk changes from about 210 to 539 RPM depending on whether the inside or outside of the disk is being read. A 2X drive would double this to a range of 420 to 1078 RPM, etc.

Most drives 12X and down use CLV. Newer and faster drives changed to a fixed spindle speed, which is a system called constant angular velocity or CAV. Here, the spindle speed remains the same and the transfer rate changes depending on where you are on the disk. All of this is explained in more detail in the section describing the spindle motor. I personally find these drives enjoyable to use, because they make much less noise when seeking around the surface of the disk, since they don't have to change speeds.

Some people think that the performance of a drive scales linearly with the "X" number of the drive. So they think that a 12X drive will transfer data at 12 times the speed of a 1X drive, will have 1/12th the access time, etc. Even more, they think that if it takes 15 minutes to install Windows 95 on a 4X drive, that it will take only 5 minutes on a 12 X drive. Of course, it is really never this simple.

With a standard CLV drive, the "X" number refers to the spin rate and therefore the theoretical maximum transfer rate. So a 12X drive will have a theoretical transfer rate of 12*150KB/s = 1.8MB/s. This is only theoretical however, and is affected by command overhead, the interface speed, etc.

The access time of the drive, which refers to how quickly it can move around on the disk when not reading sequentially, gets better with faster drives but definitely not linearly. No 12X drive I know of has an access time 1/12th that of a standard 1X CD player. Most real-world uses of the drive--such as installing software--require the read head to do random accesses over various parts of the surface of the disk. You will only partially get the benefits of the 12 times speed of the drive when doing sequential reads of substantial length.

With newer CAV drives, you can't even use the "X" to figure out the transfer rate! Since the drive spins at the same speed but there is less data in the middle of the disk, the highest transfer rate (the one that they put in the specifications and which corresponds to the "X" speed) only applies to reading data from the outermost part of the disk. A 24X drive is only 24X at

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the edge of the CD; in the middle it will be significantly slower, in fact as much as 60% slower. To make matters worse, on a CD information is recorded on the inside first, and then moves to the outside. If the disk is half-full then, none of the data will be read back at anything even near 24X on such a drive. See this section on transfer rates for more. (On the other hand, these CAV drives also have the advantage of not having to change speeds, which can make them perform more smoothly even if their raw transfer rate is below that implied by "the big number").

It is also important to realize that not all drives with the same "X" rating are created equal. They may all spin at the same speed, but differences in control circuitry, the ability to speed up or slow down the motor, and other quality factors, can mean that one 8X drive performs a lot better than another one. Here, as with most things, spending less money usually means you will get less performance. A $200 8X drive is, all else being equal, going to provide better performance (reliability, etc.) than a $100 8X drive, or nobody would buy the $200 model.

Note: There are now on the market so-called "100X" CD-ROM drives. Be aware that these are not a 100X CD-ROM drive at all--not even close. When you read the specifications up close you find out what is really going on: this is a plain 12X CD-ROM drive--not even close to the fastest available--that uses the hard disk to buffer the contents of the CD and therefore dramatically speed up the CD-ROM. Well, this is far from being anything new. You can do the same thing with any CD-ROM drive if you have 650 MB of free disk space, and there are many different CD-ROM caching programs around that work similarly. Don't be fooled by marketing people doing this sort of thing, which is in my opinion blatant false advertising

 

Speed Change Time

One performance factor that is relevant for (most) CD-ROM drives but not for hard disks is speed change time. Standard CD-ROM drives use constant linear velocity (CLV) reading, which means that the same amount of data is read from the disk in each unit of time. This means the speed of the disk must change as the head is moved from the inner part of the disk to the outer, or vice-versa. This is described in detail here.

It can take a significant amount of time to change the speed of the spindle motor, especially with faster drives. The difference in speed can be several thousand RPM in the faster drives, between the speed at the inside and the outside of the disk. When the disk is doing a lot of random accesses, you can sometimes hear this as the whirring noise from the motor audibly "ramps up" in frequency and "ramps down" as it moves around the disk.

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The delay in changing speeds can slow down random accesses on the disk, sometimes significantly. This metric is not usually specified directly by the manufacturer, but instead is usually lumped in to the access time measurement. Changing the speed of the disk can be done in parallel with (at the same time as) the head is moved during a seek. If the seek is completed, however, and the drive hasn't finished changing speeds yet, there will be a small delay.

Due to the amount of time it takes to change speeds (along with other reasons), many newer drives have changed to a constant angular velocity system (CAV). These drives do not change speeds, and therefore do not incur the speed change penalty as they move over the disk. However, they give up some transfer rate performance as they move to the inside of the disk, since they are covering less linear distance in each unit of time. Since these disks are usually quite fast to begin with, this is often a good tradeoff. A 24X CD-ROM drive is only 24X at the outer edge of the disk, but it is still the equivalent of at least 12X even on the inside of the disk. These drives are often more pleasant to use because there is less noise from the drive changing speeds all the time.

 

Seek Time

The seek time of a drive refers to the amount of time that it takes to move the heads to a specific part of the disk in order to do a read. The amount of time that it takes of course depends on how far away from the destination the heads are; in most cases seek time is measured as an average for a typical random read on the surface of the disk. The seek time metric is discussed in full detail in the section on hard disks.

In actual fact seek time as a metric is used for indicating the performance level of hard disks much more often than it is for CD-ROM drives; for CDs it is much more common to see the access time metric stated, of which seek time is a component. For this reason it is often difficult to discern exactly what the seek time of a CD-ROM drive is, however access time is just as useful a metric (if not more useful) in indicating the true performance level of a drive. See the section on access time for more.

Overall, CD-ROM drives have much poorer seek time performance than hard disks do. They use a much less efficient head actuator mechanism that causes them to take much more time to position to different tracks on the surface of the disk. This is probably because of the legacy of CD-ROM technology, which started out with audio CDs. When listening to an audio disk the only time a random seek is done is when changing music tracks, which is done very rarely.

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Latency

The CD media is a spinning disk of information. When a particular part of the disk needs to be read, the heads must be moved to the correct part of the disk; the speed of this action is measured by seek time. Once the head is in the correct place, it will start reading the track of information, but may have to wait for the correct data to turn around on the disk and reach where the head is, since the CD is spinning. The amount of time it takes, on average, for the correct information to come around to where the head is waiting after a seek is referred to as latency.It is discussed in more detail in the section on hard disks.

Since latency measures the amount of time that it is taking for the disk to spin into the right position, latency is directly correlated to the speed that the disk is spinning. This means that drives with a faster "X" rating will have lower latency, because they are spinning faster. This is one reason why these drives will have better performance.

Measuring latency is more complicated for standard CD-ROM drives than it is for hard disks because hard disks are spinning at a constant speed while conventional CD-ROM drives are not. While newer CD-ROMs use constant angular velocity (CAV) and spin at the same speed all the time, conventional CD-ROMs use constant linear velocity (CLV) and spin faster when reading the outside of the disk than when reading the inside. This means that these drives will have better latency performance when reading the outside of the disk.

As with seek time, latency numbers are rarely seen for CD-ROM drives. Instead, access time is used as an overall indicator of the amount of time to access a random part of a CD in the drive. Latency is only one component of access time, which is really the measure to look at in more detail when considering random-access performance of a CD-ROM drive.

 

Access Time

One of the most commonly quoted performance statistics for CD-ROM drives is access time. As with most commonly-used performance metric, it is abused at least as much as it is used properly. Curiously, access time is used extensively in quoting the specs of CD-ROM drives, but is virtually never mentioned with respect to hard disks. With hard drives it is much more common to see quotes of the other metrics that are combined to make up access time. (This despite how similarly the devices access data...).

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Access time is meant to represent the amount of time it takes from the start of a random read operation until the data starts to be read from the disk. It is a composite metric, really being composed of the following other metrics:

Speed Change Time: For CLV drives, the time for the spindle motor to change to the correct speed.

Seek Time: The time for the drive to move the heads to the right location on the disk.

Latency: The amount of time for the disk to turn so that the right information spins under the read head.

Although access time is made up of the time for these separate operations, this doesn't mean that you can simply add these other measurements together to get access time. The relationship is more complex than this because some of these items can happen in parallel. For example, there is no reason that the speed of the spindle motor couldn't be varied at the same time that the heads are moved (and in fact this is done).

The access time of CD-ROM drives in general depends on the rated "X" speed of the drive, although this can and does vary widely from drive to drive. The oldest 1X drives generally had truly abysmal access times, often exceeding 300 ms; as drives have become faster and faster, access times have dropped, and now are below 100 ms on the top-end drives.

Note that while faster "X" rated drives have lower access times, this is due to improvements that reduce the three metrics listed above that contribute to access time. Some of it (latency for example) is reduced when you spin the disk at 8X instead of 1X. On the other hand, seek time improvement is independent of the spin speed of the disk, which is why some 8X drives will have much better access time performance than other 8X drives, for example.

Even the fastest CD-ROM drives are significantly slower than even the slowest hard disks; access time on a high-end CD-ROM is still going to be four or five times higher than that of a high-end disk drive. This is just the nature of the device; CD-ROM drives are based on technology originally developed for playing audio CDs, where random seek performance is very unimportant. CDs do not have cylinders like a hard disk platter, but rather a long continuous spiral of bits, which makes finding specific pieces of data much more difficult.

Even though access time is important in some ways, its importance is generally vastly overstated by the people that sell CD-ROM drives. Random access performance is one component of overall CD-ROM performance, and how essential it is depends on what you are doing with your drive. However even if high random access performance is important, you must bear in mind

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that there are far fewer random reads done, in general, to a CD than to a hard disk.

Another point is that manufacturers are not always consistent in how they define their averages. Some companies may use different testing methods, and some may even exaggerate in order to make their drives look much better than they actually are. A small difference in quoted access time is not usually going to make any noticeable real-world difference. For most purposes, a drive with a 100 ms access time is going to behave the same as one with a 110 ms access time. It's usually better at that point to differentiate them based on other performance characteristics or features (or price)

 

Internal and External Transfer Rate

The transfer rate of a drive refers to how quickly data can be moved from the surface of the CD and into the computer. This is the primary statistic that measures how efficiently you can actually get your data off the surface of the compact disk when you need it. Since many applications involving CDs require the transfer of large blocks of data, the transfer rate of the drive is an important performance metric.

There are two different factors that make up transfer rate: the internal transfer rate and the external transfer rate. There is some confusion in terminology surrounding transfer rate, because different companies tend to focus on one or the other (the problem is more pronounced with hard disks). In a nutshell, the internal transfer rate measures the speed at which data can be actually read from the disk and into the CD-ROM drive's internal controller. The external transfer rate measures the speed at which data can be moved from the control over the CD-ROM interface and into the rest of the PC.

The external transfer rate is rarely an important factor with CD-ROM drives (unlike hard disks) because their internal transfer rates are so low that even slower interfaces are rarely a limiting factor. The theoretical maximum transfer rate of a 12X drive for example is only 1,800 KB per second, and most any SCSI or IDE/ATAPI interface can handle this with no difficulty. It is only at the very high end drives that the interface becomes more of a concern, and even here it isn't much of a concern compared to high-end hard disks that have much higher transfer rates from the disks themselves.

The internal transfer rate is in most respects the "true" transfer rate of the drive, since this is the rate that data can actually be read from the disk. The external transfer rate is more of a limiting factor; the data cannot be fed to

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the PC any faster than the interface's transfer rate, but having a faster interface does you no good if the drive itself is slow. This is explained in much more detail in the section on hard disk external transfer rate. CD interfaces are explained here.

Unlike hard disks, where far too much focus is on the external or interface transfer rate, most manufacturers of CD-ROMs quote the true internal transfer rate of the drive, although even here there are caveats. As explained in detail in the section on hard disk internal transfer rate, the quoted specs are always for ideal, peak transfers, and will never be achieved over a period of time in the "real world", due to overhead and other imperfections (but there is no zoned bit recording for CDs as there is for hard disks; see below). Also, the real world is a mix of random accesses and sequential transfers, and the transfer rate measures only sequential transfers.

Conventional CD-ROM drives use constant linear velocity (CLV). Since there is more data at the outside of the disk than at the inside, they make the disk spin slower when reading the outside of the disk so that the transfer rate over the entire surface of the disk is identical. This means that the theoretical internal transfer rate of the drive can be calculated easily by looking at the "X" rating of the drive and multiplying it by 150 KB/second, the transfer rate for a standard 1X drive. This will be the transfer rate for an entire CD, no matter what part of it is being used.

The newer constant angular velocity (CAV) CD-ROM drives do not vary the speed of the disk depending on where they are reading. They spin the disk at the same speed all the time (thus the name). This means that when reading the outside of the disk, where there is more data per revolution, they will have a (much) higher transfer rate than when they are reading the inside of the disk. (This is exactly the way it is for hard disks as well). The difference is substantial because the ratio of the data density of the outside of the disk to the inside is about 2.5 to 1.

The manufacturers of course only state in the specifications the "big number", the transfer rate when reading the outside of the disk. Since a CD has its data recorded from the inside out, this means on a disk that is half-full, none of the data will ever be read back at 24X when you are using a CAV 24X drive. The data at the very inside edge will be read at the equivalent of less than a 10X drive. Do not be fooled by this!

This table shows a summary of the transfer rates of different types of drives:

Drive Minimum Transfer

Maximum Transfer Rate

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Rate (binary KB/s)

1X (CLV) 150 KB/s 150 KB/s

2X (CLV) 300 KB/s 300 KB/s

4X (CLV) 600 KB/s 600 KB/s

6X (CLV) 900 KB/s 900 KB/s

8X (CLV) 1,200 KB/s 1,200 KB/s

10X (CLV) 1,500 KB/s 1,500 KB/s

12X (CLV) 1,800 KB/s 1,800 KB/s

16X (CAV) ~ 930 KB/s 2,400 KB/s

20X (CAV) ~ 1,170 KB/s 3,000 KB/s

24X (CAV) ~ 1,400 KB/s 3,600 KB/s

12X/20X (CLV/CAV) 1,800 KB/s 3,000 KB/s

As you can see, in many instances you are far better off with a good 12X CLV drive than with a 16X CAV drive, due to the fact that a 16X drive is really a "6X to 16X drive". Even the 24X drive isn't so impressive when you really understand how it works.

The last entry in the table shows the performance of a mixed CLV/CAV drive such as Plextor's 12/20Plex. This drive uses CAV when reading the outside edge of the disk to get maximum transfer rate as good as that of a standard 20X CAV drive. It then switches to CLV when reading the inside edge of the disk, giving it a transfer rate 50% higher than a regular 20X CAV drive when reading the starting edge of the disk. Overall, this drive will give you transfer rate performance probably exceeding that even of the 24X CAV unit.

 

Processor Utilization

A performance consideration that most people don't think about very much is the impact of CD-ROM transfers on the rest of the PC. This is a factor that is based a great deal on the interface that is used by the CD-ROM drive. While both SCSI and IDE/ATAPI are acceptable interfaces for CD-ROM drives in terms of their capability for dealing with the transfer rates required by the drives, there is a big difference between the two related to how much demand they put on the system processor.

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As described in detail here, the ATAPI protocol is used to connect CD-ROM drives over the standard IDE interface that was once used only for hard disks. Using this interface, however, requires substantial involvement by the processor to perform transfers. When the CD-ROM is being accessed, much of the processor's time will be unavailable to do other work. This is not the case with SCSI.

In the real world, the importance of this distinction depends, as always, on how you are using your PC. For the typical home user who is doing one thing at a time, there will probably not be much difference noticeable at all. For someone doing a lot of CD-ROM accesses in a multitasking environment, or sharing CD-ROM drives over a network, SCSI is almost always the way to go.

 

Internal Buffers or Cache

Virtually all CD-ROM drives today have on-board memory buffers that are used to improve performance. These are also sometimes called cache memory, although using this term is problematic because caching also refers to using a system memory cache to reduce accesses to the CD-ROM drive.

The internal buffer is used to improve performance by holding information that is read from the disk so that it is available to the system when needed in the near future. This improves performance by reducing the number of physical accesses that are required to be done to the actual disk surface itself, just the way the built-in cache on a hard disk controller logic board works.

The larger the buffer, the more performance will be augmented--to a point. There is a matter of diminishing returns as buffers get larger, and you may not see a huge difference in speed in a drive with 1 MB of internal buffer compared to one with 512 KB. Of course, extra buffering certainly doesn't hurt.

 

System CD-ROM Caching

Since the CD-ROM drive is a much slower device than the system memory, there is a major performance hit whenever data must be read from the drive itself. Caching refers generally to the use of a small faster store of recently-used data that can be used to fill subsequent requests for the same information while sparing the system from having to re-read it from a slow device. Caching a CD-ROM drive means using a piece of main system

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memory to hold recently-used data from the CD-ROM in this manner. This is exactly the same as how hard disk caching is done.

Most good operating systems will cache CD-ROM accesses automatically as part of their file system setup, although you can often control how the cache works and how much memory is devoted to it. If you hardly ever use your CD-ROM you may want to reduce the amount of memory set aside for CD-ROM caching, and if you use it a lot you may want to increase the size. Also, slower CD-ROM devices generally need more caching than faster ones, for obvious reasons. Remember that as with hard disk caching, spending too much memory on CD-ROM caching will quickly move you to a point of diminishing returns.

Interestingly, there are products that will cache CD-ROM accesses to the hard disk. This can be advantageous because CD-ROM drives are as much as ten times slower than hard disks (although they are both thousands of times slower than system memory). Caching the CD-ROM to system memory will give much better performance than caching it to the hard disk, but your PC probably has only about 32 MB or so of system memory but gigabytes of hard disk, so you can make the disk-based CD cache much larger. You generally will only want to bother with products like these if you do a lot of CD work.

Tip: A simple and free "caching" procedure for CD-based materials you use a great deal: copy them to a directory on a hard disk volume and use them there instead. Most games do this with some portion of their data for performance reasons.

 

 

Drive Quality and Reliability Issues

The quality of CD-ROM drives in general is important, but is only as important as the quality level of other peripherals in the system. It is not the "big deal" that hard disk quality is, simply because the stakes aren't nearly as high. When your hard disk fails, you have to contend with the double-whammy of a system that is dead in the water, and possible loss of your critical data files. When your CD-ROM drive fails, you just can't use CD-based titles until you get it repaired or replaced.

It is perhaps because of this lack of criticality that there are so many el-cheapo CD-ROM drives on the market, far more than is the case with hard disks--people just won't buy unreliable hard disks. Unfortunately, many CD-ROM drives are so cheap and so cheaply-made that they are practically

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disposable; they are purchased for less than $100 and fail after a short period of time, but repairing them is rarely cost-effective so the user simply buys a new unit.

Here, as in most other places, remember that there ain't no such thing as a free lunch (TANSTAAFL). Some people are surprised to find CD-ROM drives from top vendors selling for $300 when inexpensive no-name brands are available for as little as one-third the price that are "just as fast". Just remember that in a market economy there are usually reasons why things cost more, or the companies trying to sell them would go under. If the performance is similar (usually they actually aren't when you examine the performance characteristics in detail anyway) then the difference is going to be in quality, service, support, documentation, or somewhere else.

One of the biggest problems with quality when it comes to the newer CD-ROM drives is that they are spinning the disk faster and faster, but they aren't engineering the drives properly to do this in an efficient, reliable way. Some of the newer, cheap 12X and higher drives have terrible problems with vibration and noise. Many of them can be extremely loud and can become louder when used over several months' time. The vibration can eventually cause failure of the drive.

Another problem with these faster drives is "ramp up / ramp down" noise as the drives change speeds. A 12X CLV drive has to change its spindle speed from 2,520 RPM to 6,468 RPM very quickly when moving from the inside to the outside of the disk and then back again when reversing direction. This increase and decrease in spindle speed is very noticeable on some drives and can get quite annoying to some users. It also causes wear and tear on the drive. In fact, CAV drives were developed in part to combat this problem.

In general, drives that use caddies tend to have fewer problems than those that use trays. The tray uses a cheap gear-driven mechanism that is prone to failure, especially if it gets jammed when opening or closing the tray. Caddy-based units also seem to be less susceptible to dirt problems that can plague the cheaper units. See this section for more on the two different loading mechanisms.

In many of the faster drives, due to the high spin rate and the stress that this places on the spindle motor, the drive will automatically spin down and stop after a period of inactivity. This can be as short as a few seconds. The problem is that the next time you need to access the drive, you will have to wait while the spindle drive spins the disk back up to speed. For many applications this delay is unacceptable and can cause real problems. If this is the case, you need to make sure to either get a drive that doesn't do this, or one where the spin-down feature can be controlled or disabled.

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CD-ROM Interfaces and Configuration

CD-ROM drives were relatively late additions to the personal computer, at least compared to their older cousins the floppy drive and the hard disk drive. They were also relatively slow in being adopted into the market over a period of years. As a result, they tended at first to have non-standard interfaces to the rest of the PC .

As the CD-ROM has become quite mainstream today, its interfaces have become more standardized as well. The trend has been to make the CD-ROM use the interfaces that have traditionally been employed by hard disk drives, for simplicity and to reduce cost. As a result, today's CD-ROM drives generally use either IDE (actually ATAPI) or SCSI, just as hard disks do.

This section takes a brief look at CD-ROM interfaces. Again, since most CD-ROM drives use hard disk interfaces today, I will tend to refer to the extensive discussions on the IDE and SCSI interfaces that can be found in the Reference Guide section on hard disks, instead of repeating them all here. For a procedure describing how to physically install a CD-ROM drive, look here; for a procedure on connecting the CD-ROM drive to the system, look here; and for instructions on installing CD-ROM drivers, here.

 

Proprietary Interfaces

Most of the early CD-ROM drives were sold in so-called multimedia kits, to be added into existing systems that really had no way of interfacing them in a standard way. A typical multimedia kit came with a sound card, a CD-ROM drive, and a pair of small stereo speakers. These kits are still sold today, but they are much less popular since most new PCs now come pre-equipped with what these kits contain.

Since the kits came with a sound card and a CD-ROM drive, the most natural way to interface the CD-ROM to the PC was to connect it to the sound card, and that is exactly what was done. Sound cards, especially the SoundBlaster series from Creative Labs, were designed to incorporate one or more proprietary CD-ROM interfaces that were used with specific brands of CD-ROM drives.

In the early days there were only a few different types of CD-ROMs, and a sound card would typically be sold with the interface required for the specific drive it came with in a multimedia kit, or with multiple interfaces for two or three types of drives. Some of the drives that came out later on were designed to emulate these types of drives so that they could use the same proprietary interfaces found on the sound card. The whole situation became

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rather confusing and messy, which is part of why these proprietary interfaces were eventually done away with.

These are the main proprietary interfaces that were commonly found on older CD-ROM drives (and sound cards):

Creative / MKE / Panasonic: MKE stands for Matsushita-Kyotobuki Electronics, which is the proper name for the large company more commonly known as Panasonic. These drives were commonly sold in Creative Labs multimedia kits and this therefore is one of the most commonly-encountered of the proprietary interfaces.

Sony: One of the earliest players in the CD-ROM market. Mitsumi: Mitsumi is a large manufacturer of PC peripherals and had a

proprietary interface on many sound cards.

There were several problems with these proprietary interfaces: the first is the simple fact that they are proprietary. If you changed the drive in your PC and your sound card didn't have a connector for the new type of drive, it wouldn't work. The second is that when many more brands of drives entered the market, the situation with compatibility and knowing which proprietary interface to use for a specific drive got very confusing. The third is that the interfaces tend to use the same physical connectors, meaning the same number of pins in the same configuration, so it was easy to connect to the wrong interface on a sound card that had more than one.

Proprietary interfaces quickly went the way of the dodo when the new ATAPI standard was created, which allowed CD-ROM drives to be connected to compatible IDE hard disk controllers. Proprietary interfaces are most commonly seen on 1X and 2X speed drives. If you have an older 2X drive and try to connect it to an ATAPI controller and it doesn't work, it's probably because it really requires a proprietary interface of one sort or another.

 

ATA Packet Interface (ATAPI) / IDE

The most common interface used in modern CD-ROM drives is the AT Attachment Packet Interface, more commonly called just ATAPI. This is a special protocol that was developed to allow devices like CD-ROM drives and tape drives to attach to regular IDE controllers normally used for hard disks. CD-ROM drives that use ATAPI are often called "IDE CD-ROMs" but this terminology is not strictly correct.

The ATAPI interface is a derivative of the standard IDE interface; regular IDE commands cannot be used properly for CD-ROM drives, so a modified command structure was created. A special driver is used to control the CD-

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ROM drive. ATAPI itself is described in more detail here, in the general discussion of the IDE/ATA device interface.

Physically, ATAPI CD-ROM drives connect to the system in about the same way that IDE hard disks do. They are normally configurable to act as master or slave drives, with slave often being the default. See this section for a full discussion of configuring IDE and ATAPI devices.

Warning: One problem that is often encountered when configuring ATAPI devices is accidentally connecting them to a connector on a sound card that appears to be an IDE channel but really is not. Many older sound cards include several connectors for the proprietary interfaces discussed here. These may appear physically identical to an IDE port, but an ATAPI CD-ROM will not function if connected to one.

 

Small Computer Systems Interface (SCSI)

As with hard disks, the second-most popular interface for connecting CD-ROMs is the Small Computer Systems Interface, better known as SCSI (pronounced "skuzzy"). SCSI is a bus that is used for connecting a wide variety of peripherals to the PC. It is most commonly used in higher-end systems and higher-quality CD-ROM drives. It tends to provide better performance at a higher cost than ATAPI drives.

 

Resource Usage

CD-ROM drives really do not use any system resources other than those that are used by their interface. If you require the addition of an interface (such as an IDE controller or a SCSI host adapter) then you will of course "spend" the resources that the interface requires. If you decide to slave an ATAPI CD-ROM drive off an IDE hard disk, then of course you are using the existing interface and there is no resource usage cost.The CD-ROM drive itself doesn't use any resources of the PC, except for a bit of memory for the driver.

 

Software Drivers and File System Extensions

CD-ROM drives generally require two pieces of software in order to function properly. These are a driver, and a file system extension. The driver is

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responsible for controlling access to the CD-ROM drive. The file system extension is what allows the CD-ROM drive to appear to the system as a regular file system volume, with directories and files, etc.

Virtually all CD-ROM drives come new with a floppy disk that includes the software driver designed for the drive. The driver is loaded in the CONFIG.SYS system file when the PC boots up. In most cases these drivers are unique to the drive and cannot be interchanged with a different one; if you install a new CD-ROM you need to install a new driver as well. The floppy disk will normally include an installation program that will copy the driver to the hard disk and insert the "DEVICE=" command into your CONFIG.SYS file for you.

There are two common file system extensions used to enable CD-ROMs to work on the PC. The first is "MSCDEX.EXE" (which stands for Microsoft Compact Disk Extension). This program is used to permit CD-ROM access under DOS and Windows and is normally loaded in the AUTOEXEC.BAT system file. The second common file system extension is the one built into Windows 95. When using Windows 95, it is not necessary to load MSCDEX.EXE in the AUTOEXEC.BAT file, because Windows 95 includes native support for CD-ROMs (in fact, you shouldn't load it there under Windows 95).

One complicating factor under Windows 95 is CD-ROM access when booting into Windows 95 "DOS mode". When you boot Windows 95, its native file system extension will allow the CD-ROM to work. However, if you shut down to DOS mode, that file system extension is unloaded and the CD-ROM drive won't be accessible. The way to fix this problem is to load MSCDEX.EXE after shutting down to DOS mode. This section of the Troubleshooting Expert gives more details on how to do this.

Tip: After setting up a new CD-ROM drive, I strongly encourage that you change the default drive letter that is selected for the drive. By default, the CD-ROM is normally set up to take the next available drive letter after the hard disk volumes. If you leave this at the default, then if you later add a hard disk volume the drive letter for the CD-ROM will shift upwards and your installed CD-ROM software will stop working properly.

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