Components of CD Drives•Drive head•ActuatorIt is the unit that holds the main components of head together.It is responsible for movement of head.It uses an integrated microcontroller and servo system to move the head.•Spindle motorIt is used to spin the drive.It is connected to the spindle that goes through the 15mm hole in the center of disk.•Connectors

Constant Linear Velocity(CLV)• It is developed for standard audio players and

then extended to CD ROM’s.• More data is stored on outer tracks than inner

ones.• To maintain a steady transfer rate of

150KB/sec,the drive adjusts the spindle motor speed from approximately 210 to 539 rpm.This is called CLV.

• Early CD players did not have the necessary buffer memory to allow them to deal with bits arriving at a different rate depending on what part of the disk they were using.

Constant Angular Velocity(CAV)• As the speed increases it is difficult to spin up and

spin down to a desired speed.• Regardless of where the head is same speed is

used to turn the disk.• It is called so bcoz it takes same amount of time

for a turn of the 360° of the disk at all times.• Some CD ROM drives use combined CLV and CAV

technologies. 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 inner tracks.

Characteristic Constant Linear Velocity (CLV)

Constant Angular Velocity (CAV)

Drive Speed Variable Fixed

Transfer Rate Fixed Variable

Application Conventional CD-ROM drives

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

Data transfer rate• Two types- internal and external data transfer.• Internal data transfer is the speed at which

data is collected from the disk.• External data transfer is the rate at which data

is transferred from disk to computer over the interface.

• X is a base multiplier that expresses the speed with which data could be read.

• 1X stands for 150KB/sec.52X means 7800KB/sec.

TrueX technology• Designed by Zen research and used in

Kenwood drives.• It works by spinning the disk at a lower speed

and reading seven tracks at once, instead of one at a time.

• Diffraction grating splits the beam into a row of seven discrete beams.

• Three beams on either side are focused onto the adjacent tracks at the same time.

• Seven beams are reflected into a multibeam photodetector & converts the seven parellel streams to a single stream.

CD ROM Drive specificationsAccess time-time between start of a random read

operation and the moment at which drive starts reading data. It depends on the following three factors.

1) Speed change time-amount of time it takes to spin up or spin down to the proper speed for reading data.

2) Seek time-amount of time needed to move the head to the desired location.

3) Latency-amount of time needed for the disc to spin to the point where the data read is to begin.

CD-R• Stands for CD-recordable• Consists of a layer of photosensitive dye, a

layer of reflective metal(such as gold alloy) and a layer of protective plastic.

• Reflective metal layer forms a big land on the blank disc.

• For writing the drive uses powerful laser that causes the dye layer to react with metallic layer forming non-reflective areas. These areas are called pits.

CD-R Drive• It can read and write CD’s.• CD-R generally has two speed ratings- for

reading and writing disks.• We can’t stop writing until a session is

completed.CD-R software

• It provides an interface for selecting files,writing speed etc.

• Eg:nero,roxio,deep burner etc.

Single session and multi-session disks• Session is a continuously written collection of

data written to a CD.• Commercially produced CDs are all single

session discs.• We can write multi sessions on a CD.

CD-RW• It consists of a polycarbonate substrate, dye

layer sandwiched between two dielectric layers, reflective layer and a protective layer.

• Dye layer is a crystalline compound with special properties.

• During writing, the temperature is about 700 ° Celsius and form non reflective pits.

• During erasing the laser heats the dye layer between 200 & 500 degree celsius for a sufficient period to convert amorphous surface to crystalline state and once again reflective. After this process the disc is ready to receive new data.

• Laser in a CD-RW can run at three power levels as follows.

Read power: lowest power generated by laser.

Erase power: Lasers middle power. To return the pits in an already written disk to a reflective state.

White power: Lasers highest power. Used to create the pits.

SCSI• Stands for Small Computer System Interface

• It was derived from “SASI”, Shugart Associates System Interface.

• It is a set of standards for connecting and transferring data between computers and peripheral devices.

• It is used as an interface for hard disks,CDdrives,scanners etc.

SCSI-1• Transfer rate is 5MB/sec.• Supports 8 bit bus.• Only single ended transmission was supported

with passive termination.SCSI-2• It is an enhancement to very limited original

SCSI.• Transfer rate is 10MB/sec• Supports 16 bits bus.• 16 devices are supported.

• Improved cables and connectors for higher density connection.

• Active termination• Differential signaling• Command queuing• Additional command setsSCSI-3Introduces ultra SCSI that increases the bus

speed.

Name Alias Bus speed(MHz)

Bus width Transfer rate(MB/sec)

Ultra Fast 20 20 8 bits 20

Ultra/wide Fast20/wide 20 16 bits 40

Ultra 2 Fast 40 40 8 bits 40

Ultra 2/wide Fast 40/wide 40 16 bits 80

Ultra 3 Fast 80 80 8 bits 80

Ultra 3/wide Fast 80/wide 80 16 bits 160

IDE Vs SCSI• Usually IDE drive outperforms SCSI drive.

• IDE drive costs less than SCSI drive.

• IDE drives have less command overhead than SCSI drives.

• Both have drive controllers.

• In IDE command queuing is not supported.

SCSI Termination• Passive termination: it uses resistors to

terminate the bus.• Active termination: voltage regulators are

added to the resistors.• Forced perfect termination: it uses diodes for

termination.

Magneto optical drives• It employs both magnetic and optical

technologies to obtain ultra-high data density• It achieves a high density by using a laser and

a magnetic read/write head in combination.• Data can be erased or overwritten unlimited

no: of times.• 2 form factors avaliable:5.5”&3.5”• Eg:Fujisto Dynamo, a 230 MB drive &Pinnacle

Micro Vertex, a 2.6GB drive.

DVD• Stands for Digital Versatile Disk• Developed by Sony & Philips.• Different DVD formats available are DVD-

R,DVD+R,DVD-RW,DVD+RW,DVD-RAM• Same size as CD but can hold more data.• Pitsize:0.4 micron• Track density:0.74micron• Two data layers• Double sided disks.• Improved error checking and correction• Uses red laser

Format Size Layers per side Sides Capacity(in GB)

DVD-1 80mm 1 1 1.36

DVD-2 80mm 2 1 2.48

DVD-3 80mm 1 2 2.72

DVD-4 80mm 2 2 4.95

DVD-5 120mm 1 1 4.38

DVD-9 120mm 2 1 7.95

DVD-10 120mm 1 2 8.75

DVD-14 120mm ½ 2 12.33

DVD-18 120mm 2 2 15.90

Blu-ray disc

• Officially announced in Feb 2002.• Appeared globally in 2006.• Blu-ray refers to the blue laser used to read and

write.• It comes in 3 variants:pre- recorded, recordable

and rewritable.• Single & double layered disks are available.