Animation & Modeling - Day 11

8/13/2019 Animation & Modeling - Day 11

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CD-Technology


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Introduction

A Compact Disc(also known as a CD) is anoptical disc used to store digital data.

It was originally developed to store soundrecordings exclusively, but later it also allowedthe preservation of other types of data.


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Merits

Ease of use and durability of media, as well asHigh storage volumes.

Random access capability as compared to tapes.

Data is not affected by electrical/ magnetic fields.

Does not require maintenance or special handling

methods.

Permanence of data is ideal for distribution oferror free software as well as music, video etc.


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Working Principles

A CD is made from 1.2 mm thick, almost-purepolycarbonate plastic and weighs approximately1520 grams.

From the center outward components are

at the center (spindle) hole,

the first-transition area (clamping ring),

the clamping area (stacking ring),

the second-transition area (mirror band), the information (data) area,

and the rim.


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Working Principles

A thin layer of aluminum or, more rarely, gold isapplied to the surface to make it reflective.

Protected by a film of lacquer that is normallyspin coated directly on top of the reflective layer,upon which the label print is applied.

CD data are stored as a series of tiny indentations

known as pits, encoded in a spiral track molded into the top of the

polycarbonate layer.


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Working Principles


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Working Principles

The areas between pits are known as lands.

Each pit is approximately 100 nm deep by500 nm wide, and varies from 850 nm to 3.5 m

in length.

The distance between the tracks, the pitch, is1.6 m.

A CD is read by focusing a 780 nm wavelength(near infrared) semiconductor laser through thebottom of the polycarbonate layer.


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CD Physical

Layers

Protective Lacquer Coating

Reflective Aluminum Layer

Polycarbonate Substrate

Land Pit Land Pit Land


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Working Principles

The change in height between pits and landsresults in a difference in intensity in the lightreflected.

By measuring the intensity change with aphotodiode, the data can be read from the disc.

The pits and lands themselves do not directlyrepresent the zeros and ones of binary data.

Instead, Non-return-to-zero, inverted (NRZI)encoding is used: a change from pit to land orland to pit indicates a one, while no changeindicates a zero.


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Working Principles


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Working Principles

CDs are susceptible to damage from both dailyuse and environmental exposure.

Pits are much closer to the label side of a disc, so

that defects and dirt on the clear side can be outof focus during playback.

Consequently, CDs suffer more scratch damageon the label side whereas scratches on the clearside can be repaired.

Initial music CDs were known to suffer from "CDrot", or "laser rot", in which the internal reflectivelayer degrades.


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CAV

Standard hard disks and floppy disks spin the diskat a constant speed.

Regardless of where the heads are, the samespeed is used to turn the media.

This is called constant angular velocity(CAV)because it takes the same amount of time for aturn of the 360 degrees of the disk at all times.

The tracks on the inside of the disk are muchsmaller than those on the outside of the disk.


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CAV


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CAV & CLV

This constant speed means that when the headsare on the outside of the disk they will traverse amuch longer linear path than they do when on theinside.

The linear velocity is not constant.

CD-ROMs take a different approach.

They adjust the speed of the motor so that thelinear velocity of the disk is always constant.


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CLV

When the head is on the outside of the disk, themotor runs slower, and when it is on the inside, itruns faster.

This is done to ensure that the same amount ofdata always goes past the read head in a givenperiod of time.

This is called constant linear velocityor CLV.


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CLV


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X Rated Speed

The speed of the spindle motor is controlled bythe microcontroller.

It tied to the positioning of the head actuator.

The data signals coming from the disk are used tosynchronize the speed of the motor and makesure that the disk is turning at the correct rate.

The first CD-ROMs operated at the same speed asstandard audio CD players: roughly 210 to 539RPM, depending on the location of the heads.


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X Rated Speed

This results in a standard transfer rate of 150KB/s.

By increasing the speed of the spindle motor, and

using sufficiently powerful electronics, it would bepossible to increase the transfer ratesubstantially.

There's no advantage to reading a music CD at

double the normal speed, but there definitely isfor data CDs.

Thus the double-speed, or 2X CD-ROM was born.It followed in short order with 3X, 4X and even

faster drives.


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X Rated Speed

Characteristic Constant Linear Velocity (CLV) Constant Angular Velocity(CAV)

Drive Speed Variable Fixed

Transfer Rate Fixed Variable

Application Conventional CD-ROM drives hard disk drives,floppy disk drives


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X Rated Speed

Speed change time:amount of time required tochange the speed of the spindle motor as thehead moves radically across the disk.

Seek time: time taken to move the heads to aspecific part of the disc for doing a readoperation.

Latency: amount of time taken for the correct

data block to come underneath the head.


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CD Formats


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Compact Disk Digital Audio

(CD-DA)

The first CD format that which defined the audioCD used in all regular CD players, called CDDigital Audioor CD-DAfor short.

The specifications for this format were codified inthe first CD standard, the red book" that wasdeveloped by Philips and Sony.

The "red book" was published in 1980, and

actually specifies not just the data format fordigital audio but also the physical specificationsfor compact disks:

the size of the media, the spacing of the tracks, etc.


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(CD-DA)

CD-DA audio uses a sample rate of 44.1 kHz,which is roughly double the highest frequencyaudible by humans (around 22 kHz.)

Each sample is 16 bits in size, and the sampling isdone in stereo.

Therefore, each second of sound takes (44,100 *

2 * 2) bytes of data, which is 176,400 bytes.


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(CD-DA)

Audio data is stored on the disk in blocks, whichare also sometimes called sectors.

Each block holds 2,352 bytes of data, with an

additional number of bytes used for errordetection and correction, as well as controlstructures.

Therefore, 75 blocks are required for each second

of sound.


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(CD-DA)

On a standard 74-minute CD then, the totalamount 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 aminute of CD audio takes about 10 MB,uncompressed.


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CD-ROM Digital Data (CD-ROM)

The standard that describes how digital data areto be recorded on compact disk media wentthrough several different iterations before theformat was finalized.

The first step was the creation of the original dataformat standard, called the "yellow book", by

Philips and Sony in 1983.


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Under the data CD standard, there aretwo modesdefined:

Mode 1:

This is the standard data storage mode usedby virtually all standard data CDs.

The data is laid out in basically the same wayas it is in standard audio CD format, exceptthat the 2,352 bytes of data in each block arebroken down further.


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Mode 1: 2,048 of these bytes are for "real" data.

The other 304 bytes are used for anadditional level of error detecting andcorrecting code.

This is necessary because data CDs

cannot tolerate the loss of a handful ofbits now and then, the way audio CDscan.


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Mode 2: Mode 2 data CDs are the same as mode 1 CDs

except that the error detecting and correcting codesare omitted.

The reason is that mode 2 format provides a moreflexible vehicle for storing types of data that do notrequire high data integrity: for example, graphicsand video can use this format.

Furthermore, different kinds can be mixed together;this is the basis for the extensions to the originaldata CD standards known as CD-ROM ExtendedArchitecture, or CD-ROM XA.


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CD-ROM Extended Architecture (CD-

ROM XA)

Finding the existing CD audio and CD dataspecifications too restricting, a new format calledCD-ROM Extended Architectureor CD-ROM XAwas developed by Philips, Sony and Microsoft.

Disks that use CD-ROM XA can mix standard dataCD mode 1 and mode 2 tracks, allowing themixing of standard data along with other types ofdata.

The mode 2 tracks are further divided into twotypes:

Form 1 and Form 2.


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CD-ROM Extended Architecture (CD-

ROM XA)

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 ofinformation is called interleaving


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CD-Interactive (CD-I)

In 1986, Philips and Sony again joined forces tocreate the CD-Interactiveor CD-Iformat.

This concept was quite ambitious, with the goal to

develop both a format and a special new type ofhardware to use it.

In some ways this was the first serious attempt atwhat we now call "multimedia and computer

programs, and hardware sold to handle all ofthese and connect to a television screen foroutput.


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Recordable CD (CD-R)

In 1990, part II of the "orange book" published byPhilips, specified the characteristics and format ofa 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.

CD-R media starts with a polycarbonatesubstrate, just like regular CDs do.

Instead of physical etching this substrate, it isstamped with a spiral pre-groove, similar to thespiral found on a regular CD except that it isintentionally "wobbled".


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This groove is what the CD-R drive uses to followthe data path of the disk during recording.

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 mediatheir different visual appearance from regularCDs.


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The key to the media is the dye layer (and thespecial laser used in the drives.)

When light from a specific type and intensity of

laser is applied to it, it heats up rapidly andchanges its chemical composition.

As a result, the area "burned" reflects less lightthan the areas that do not have the laser applied.

This system is designed to mimic the way lightreflects cleanly off a "land" on a regular CD, but isscattered by a "pit", so an entire disk is createdfrom burned and non-burned areas.


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Since the media is being physically altered by aprocess of heat and chemistry, the change ispermanent and irreversible.

Once any part of the CD has been written, thedata is there forever.

Some drives allow you to record some informationin one sitting, and then more information later on,if the disk is not yet full.

This is called multi-sessionrecording, and requiresa CD player capable of recognizing multi-sessiondisks in order to use the burned disk.


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Rewriteable CD (CD-RW)

The specifications for CD-RW are codified as partIII of the "orange book" published by Philips.

CD-RW uses much more advanced technology in

order to accomplish its goal of making compactdisks both writeable and rewriteable.

CD-RW media are formed in the same basic waythat CD-R media are.

They start with a polycarbonate base and amolded spiral pre-groove to provide a base forrecording.


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The recording layer for CD-RW is different ofcourse than it is for CD-R.

The problem with CD-R is that the dye layer usedis permanently changed during the writingprocess, which prevents rewriting.

CD-RW media replaces this dye with a specialphase-changerecording layer.

Now, this layer comprised of a specific chemicalcompound that can change states when energy isapplied to it, and can also change back again.


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The material used in CD-RW disks has theproperty that when it is heated to onetemperature and then cooled, it will crystallize.

While if it is heated to a higher temperature andthen cools, it will form a non-crystalline structurewhen cooled.

When the material is crystalline, it reflects more

light than when it doesn't; so in the crystallinestate it is like a "land" and in the non-crystallinestate, a "pit".


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By using two different laser power settings, it ispossible to change the material from one state toanother, allowing the rewriting of the disk.

The change of phase at each point on the disk'sspiral is what encodes ones and zeros into thedisk.

The spiral and other structures are the same asfor CD-R; what changes is how the pits are

encoded.


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CD-RW media have one very important drawback:

they don't emulate the pits and lands of aregular CD as well as the dye layer of a regularCD-R,

and therefore, they are not backwardcompatible to all regular audio CD players andCD-ROM drives.

Also, the fact that they are written multiple

times means that they are multi-session disksby definition, and so are not compatible withnon-multi-session-capable drives.


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Other Formats

Photo-CD Developed in the early 90s by Kodak and Philips

(who seems to have its hand in everything CD-related),

an implementation of CD-ROM extendedarchitecture designed to hold photographic images.

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 samespace that uncompressed "red book" audio uses!

This format is called video CDor sometimes VCD.


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Other Formats

CD + Graphics (CD + G)

A special audio compact disc that containsgraphics data in addition to the audio data onthe disc.

Animation & Modeling - Day 11

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