width. If you’ve got a hair on your head to spare, pluck it out and take its
measure: It’s about 50 microns wide, or equivalent to about 30 grooves
on a CD.
One major difference between a CD and a hard drive or a floppy disk is that
there is only one long, spiral track. It’s called a serpentine track,coiled in a
continuous groove that runs from the innermost section of the disc to the
outermost. (The outermost edge of a CD is the hardest area to manufacture
accurately and the most easily damaged.)
The CD drive has to deal with the varying data densities of the inside (a small
amount of data wound tight) and the outside (a great deal of data spread
wide). The solution is constant linear velocity (CLV), which is a fancy way of
saying that (in the first design) the motor was made to spin more slowly as
the read/write head gets closer to the outer edge of the disc. More modern
designs may run the motor at a single speed, or vary it only slightly; instead
they may include a large memory buffer and sophisticated electronics that
allow the computer to even out the data flow. Either way, the goal is to main-
tain the flow of data at a nearly constant speed anywhere on the disc.
Ratings for CDs include the following.
Capacity ...............................................................................................
CDs can hold between 600–800MB of data. As with a hard disk drive, some of
that space is taken for overhead— creation of an index of files on the disc.
Not all CD devices are capable of squeezing 800MB of data on a disc. Or they
may be able to record that much information and replay the data on the same
recorder, but the extra-large disc may not function properly on a computer’s
CD player or CD recorder in another machine or in an audio CD player.
The disc capacity is also affected by the nature of the data. If the disc is used
to record audio or video, the information is written in a continuous stream —
in the same order in which it will be played back. Thus, audio and video take
up a bit less space on disc than word processing or spreadsheet files, which
may be scattered around the track in smaller chunks.
The groove width is defined precisely, but disc manufacturers have a bit of
leeway in deciding how tightly spaced the groove is scribed. A 650MB CD has
about 333,000 sectors and can play about 74 minutes of audio; a supposed
700MB CD boosts the sector count to about 360,000 and 80 minutes.
And just for the record, if you will pardon the outdated pun, an audio disc can
hold a bit more data because audio sectors have a capacity of 2,352 bytes,
while data sectors can hold only 2,048 bytes (with the remainder held aside
for error detection and correction). Therefore, a CD officially rated 650MB is
actually equivalent to 747MB when holding music or speech.
Chapter 9: Going Round and Around: CD and DVD Drives 151