Information Technology Reference
In-Depth Information
1.2 mm. They can store up to 650 MB of data which gives around 74 minutes of compressed
video (MPEG format with near VCR quality) or uncompressed hi-fi audio. The reflective
coating (normally aluminium) on the disk is approximately 30
µ
m and the pits are approxi-
mately 0.1
m long and deep. A protective transparent coating is applied on top of the reflec-
tive coating with a depth of 1.2 mm (the approximate thickness of the disk). The protective
coating also helps to focus the light beam from about 0.7 mm on the surface of the coating to
the 0.1
µ
m pit. Data is stored on the disk as a spiral starting from the inside and ending at the
outside (which is opposite to hard disk. The thickness of the track is 1.6
µ
µ
m, which gives a
total spiral length of 5.7 km.
6.9.2 WORM drives
WORM (write once read many) disks allow data to be written to the optical disk once. The
data is then permanent and thus cannot be altered. They are typically used in data logging
applications and in making small volumes of CD-ROMs. A 350 mm (14 inch) WORM disk
can store up to 10 GBs of data (5 GB per side). This gives around 15 hours of compressed
video (MPEG format with near VCR quality).
WORM disks consist of two pieces of transparent material (normally glass) with a layer
of metal (typically tellurium) sandwiched in between. Initially the metal recording surface is
clear. A high intensity laser beam then writes information to the disk by burning small pits
into the surface.
6.9.3 CD-R and CD-RW disks
CD-R (CD-recordable) disks are write-once disks that can store up to 650 MB of data or 74
minutes of audio. For a disk to be read by any CD-ROM drive they must comply with ISO
9660 format. A CD-R disk can also be made multisession where a new file system is written
each time the disk is written to. Unfortunately, this takes up around 14 MB of header data for
each session. Typical parameters for sessions are:
No. of sessions
Header information Data for each session
1
approx. 14MB
636MB one session
5
approx. 70MB
116MB each session
10
approx. 140MB
51MB each session
30
approx. 420MB
7.7MB each session
Typically CD recorders write at two (or even four) times the standard writing/playback speed
of 150 KB (75 sectors) per second.
A CD-RW (CD-rewriteable) disk allows a disk to be written-to many times, but the file
format is incompatible with standard CD-ROM systems (IS0 9660). The formatting of the
CD-RW disk (which can take a few hours) takes up about 157 MB of disk space, which only
leaves about 493 MB for data.
New CD-R and CD-RW writing systems incorporate a smart laser system that eradicates
the problem of dirt on the disk. It does this by adjusting the write power of the laser using
Automatic Power Control. This allows the unit to continue to write when it encounters minor
media errors such as dirt, smudges, small scratches, and so on.
6.9.4 CD-ROM disk format
The two main standards for writing a CD-ROM are ISO 9660 and UDF (universal disk for-
mat). The ISO 9660 disk unfortunately uses 14 MB for each write to the disk.
In 1980, Philips NV and Sony Corporation first announced the CD-DA (digital audio)
