The PC primer: Hardware: CDROMS

CDROMS and DVD's

(Compact Disc Read Only Memory) The compact disc can hold text, graphics, stereo sound and programs. It uses a different format for recording data than its predecessor, the audio CD (CD-DA). An audio CD player cannot play CD-ROMs, however CD-ROM players can read audio CD's. In a PC, most internal CD-ROM drives (players, readers) connect to the EIDE (ATA) interface on the motherboard. External CDROM drives will connect via USB or SCSI.

CD-ROMs hold either 650MB of data (681,984,000 bytes), which is equivalent to about 250,000 pages of text or 20,000 medium-resolution images. Some CD-ROM discs claim 680MB capacity by dividing the 681 million by one million instead of the binary number 1,048,576.

The earliest CD-ROM's transferred data at 150KB per second. Doubling the spindle speed from 530 to 1,060 RPM doubled the rate to 300KB (2x) and continued upward to 50 times the original (50x). Access times range from 80 to 120ms.

DVD's Are more complex and are also covered in this section.

CD-ROMS are constructed of a reflective layer, sandwiched between two layers of plastic. They are 'stamped' in much the same way that LP's were, then the protective layers are added.

CD-R's or recordable CD's are also constructed of a reflective layer, sandwiched between two layers of plastic, however an additional 'filling' is added to the sandwich in the form of a layer of dye between the polycarbonate base layer and the reflective layer.
This dye is 'burnt' during the write process, reducing its reflectivity and causing the burns to act like the 'pits' on a CD-ROM.

Reading a CD-ROM

Data is etched into the CD-ROM as pits (low spots) and lands (high spots). As the laser beam is aimed into the moving pits and lands, a sensor detects a change in reflection when it encounters a transition from pit to land, or land to pit. The signal is either on, when the beam reaches the sensor, or off when a transition stops the beam reaching the sensor. There is only one laser in a drive, however two are used here to illustrate the difference in reflection.

Reading a CD-R

We saw that the CD-R, the recordable CD-ROM has an extra layer, the 'Dye' Layer. During the write process the equivalent of 'Pits' are burnt onto the dye layer, changing its reflectivity. When a transition to a burn stops the beam reaching the sensor the signal from the sensor switches. As with the CD-ROM there is only one laser in a drive, two are being used here to illustrate the difference in reflection.

Caddy Load and Tray Load

Earlier CD-ROM drives used a caddy. The disc must be inserted into the caddy, and the caddy inserted into the drive. Today, CD-ROM drives are caddyless. The disc is placed into a tray.
Of course we are all familiar with the current standard, the tray load.
To the right we see a CD caddy and its external (SCSI) drive.
The chances of a 'Domestic' user coming across a caddy load CD player are minimal but now you are aware of them.

CD-ROM speeds

So what do all those 'X's mean?

CD-ROM drives are sold with three different speed ratings. The first for write-once operations, secongly re-write operations, and thirdly for read-only operations. The speeds are listed in that order, for example a 12x/10x/32x CD drive can, write to CD-R disks at 12x speed (1.8 megabytes/s), write to CD-RW discs at 10x speed (1.5 megabytes/s), and read from CD discs at 32x speed (4.8 megabytes/s).

While the speed format applies to DVD's the transfer rates are much faster.

When installing a CDROM or DVD you need to be aware of the jumper settings if it's an EIDE device. These settings will be shown on the case.

To the right we see the CDROM audio lead.