Hardware Reference
In-Depth Information
Spiral groove
Pit
Land
2K block of
user data
Figure 2-25.
Recording structure of a Compact Disc or CD-ROM.
the same linear velocity at the head. A constant-linear-velocity drive is quite dif-
ferent than a magnetic-disk drive, which operates at a constant
angular
velocity, in-
dependent of where the head is currently positioned. Also, 530 RPM is a far cry
from the 3600 to 7200 RPM that most magnetic disks whirl at.
In 1984, Philips and Sony realized the potential for using CDs to store com-
puter data, so they published the
Yellow Book
defining a precise standard for what
are now called
CD-ROM
s(
Compact Disc-Read Only Memory
). To piggyback
on the by-then already substantial audio CD market, CD-ROMs were to be the
same physical size as audio CDs, mechanically and optically compatible with
them, and produced using the same polycarbonate injection molding machines.
The consequences of this decision were that slow variable-speed motors were re-
quired, but also that the manufacturing cost of a CD-ROM would be well under
one dollar in moderate volume.
What the Yellow Book defined was the formatting of the computer data. It
also improved the error-correcting abilities of the system, an essential step because
although music lovers do not mind losing a bit here and there, computer lovers
tend to be Very Picky about that. The basic format of a CD-ROM consists of en-
coding every byte in a 14-bit symbol. As we saw above, 14 bits is enough to Ham-
ming encode an 8-bit byte with 2 bits left over. In fact, a more powerful encoding
system is used. The 14-to-8 mapping for reading is done in hardware by table
lookup.
At the next level up, a group of 42 consecutive symbols forms a 588-bit
frame
.
Each frame holds 192 data bits (24 bytes). The remaining 396 bits are for error
correction and control. This scheme is identical for audio CDs and CD-ROMs.
