How a Computer Works (2015)

19. CD-ROM Drive

CD-ROM

CD ROM Drive

The optical CD-ROM (compact disk read only memory) disk stores huge amounts of data, up to 650 MB on one small disk. Software programs that are not distributed over the Internet typically come on an optical CD ROM disk. The optical drives huge data storage capacity makes them ideal for storing programs, operating systems, movies and large files. Because the disks are removable the storage potential is huge.

Access Times

CD-R

CD ROM Drives are continually improving so we see faster access times and write speeds for new drives. The access time is the amount of time data takes to be retrieved from the disk.

The write speed is how quickly data can be written to a CD-R or CD-RW disk.

4x (600 Kilobyte/sec). 8x (1200KB/sec). 12X(1800KB/sec). 16X(2400KB/sec). 52X(3,120–7,800KB/sec)

CD-R CD-RW

CD-R

With conventional CD-ROM disks, like the ones your software comes on, data cannot be written to these types of disks. With CD-R and CD-RW data can be written with a suitable CD-ROM drive which incorporates a write head. Older CD-ROM drives are read only and not suitable. Data on a CD-recordable (CD-R) disk cannot be erased. A CD-rewritable (CD-RW) can be re-recorded like a floppy disk. Each CD is capable of holding up to 650MB of information.

Laser

Because the laser beam can be focused so accurately mass storage is possible in such a tight space, which isn't possible with magnetic disks. A motor underneath the disk constantly varies the rate the CD-ROM rotates at.

A laser diode produces a laser beam that travels through a prism and a focusing coil.

This highly focused beam penetrates a translucent protective layer on the disk.

Disk Surface

CD ROM Disk (Side View)

The disk surface contains lands and pits. The pits are below the lands surface. As the disk rotates the laser beam is reflected from the disks surface onto a detector.

Whether the laser beam shines onto the detector is determined by the beam hitting a land section.

When the beam hits a pit the light is scattered so it doesn't fall on the detector.

Detector

As the laser beam light lands on the detector the light sensing diode in the detector produces a small low voltage.

This low voltage is connected to a chip which produces a stream of 1s as it receives the low voltage otherwise the chip produces 0s.

CD-R Disks

CD-R

CD-R disks are made from clear polycarbonate plastic. A dyed green colour material is applied as the first layer, with a thin layer of Gold to reflect the laser beam.

Over the top a layer of lacquer and often a layer of scratch-resistant polymer protects’ the delicate disk surface.

During the write to disk process the write head follows an ever-decreasing circle groove called an atip (absolute timing in pregroove). In the groove a continuous wave frequency enables the write head to read this so the drive can calculate where the head is positioned in relation to the disk surface.

Writing Data

When writing data to the disk the write head follows the atip and uses the position information provided by the waves to control the speed of the motor spinning the disk.

As the Write head moves towards the centre of the disk the motor must turn faster. Conversely as the Write head moves towards the disk edge the motor turns slower.

The layer of dye on the disk surface is designed to absorb the light from the Write heads laser beam.

When the high-powered laser beam from the Write head is switched on a stripe along the atip is made in the dye material on the disk.

Table of Contents

When the laser beam is switched off no strip appears thereby creating a gap between the stripes. The gap lengths as well as the strip lengths vary.

The CD drive uses the varying lengths to write information in a code that compresses data and checks for data errors.

The data is saved in a special format, such as ISO 9096 that creates a table of contents. This is required as there is no file allocation table like a hard drive uses.

This information is also stored on the disk.

Reading Data

NO Data being read. Laser hits a pit

Data being read. Laser hits land

The Read head uses a lower powered laser beam than the Write head. Where a stripe is not in the atip the gold layer reflects the light back to the detector. When the laser beam hits a Stripe the light is scattered and so doesn't reach the detector.

This works in the same way as reading an ordinary CD. Every time the laser beam is reflected onto the detector a low voltage is produced, this low voltage is connected to a chip which produces a stream of 1s as it receives the low voltage otherwise the chip produces 0s.

E-IDE

From this data stream the drive decompresses and error checks the data. The drive is connected to the computer through the E-IDE interface.

The hard drive and floppy drive will also use this interface. Spare channels are available to fit an extra CD ROM drive just by plugging in the device into the ribbon cable connector inside the computer.

CD-RW

CD-RW disks are able to save data over and over again, like a floppy disk. The disk differs from a CD-R disk. The disk has a plastic base and is made up of silver, indium, antimony and tellurium embedded in it. The laser beam heats disk areas to 900-1300F.

When the laser beam is switched on the heat melts the surface crystals to a non-crystalline, or amorphous phase. These areas reflect less light than the unchanged areas. When the read head with a low powered laser beam strikes the non-crystalline area the beam is scattered and not picked up by the detector.

Reading and Writing Data

CD ROM Disk (Side View)

These areas are pits and represent 0s. The other areas, lands represent 1s. When the laser beam hits a land the light is reflected back to the detector. Every time the laser beam is reflected onto the detector a low voltage is produced. This low voltage is connected to a chip which produces a stream of 1s as it receives the low voltage, otherwise the chip produces 0s.

With CD-RW disks the surface can be re-written by changing a pit to a land area. The annealing phase uses a low powered laser to heat pitted areas to 400F; the pit recrystallizes to its original state.

E-IDE Controllers

The I/O Controller chip contains the E-IDE controllers. Data is routed from the I/O Controller to the Graphics and Memory Controller Hub.

From here the data is placed onto the PCI bus to the CPU.

The CD-ROM/DVD drive connected to the motherboard.