Speed....................................................................................................
CDs are marketed as being capable of a particular speed of use. But which
use are they talking about?For a CD player,the speed rating of the device itself refers to how fast
it can pick up and pass through the information it readsfrom the disc.
And because more information is stored in the disc’s larger outer tracks,
modern CD players are capable of varying their speed of rotation, run-
ning much faster when the laser is reading from the outer tracks and
slowing down when it is accessing the inner tracks.For a CD-R device,you see two speed ratings: one for the readfunction
and a second for the writing function. Most CD-Rs are much faster at
reading than writing.
For CD-RW machines,you see three speed ratings: a readspeed, a writ-
ingspeed, and a rewrite or erase speed.I have been speaking here about the machines; the CDs used for CD-Rs and
CD-RWs state their speed limits, which may be greater or lesser than the
potential speeds of the machines they are used on. Your best bet is to try to
match the ratings between disc and machine as closely as possible; a disc
rated at 52X for reading is not going to perform at that speed in a drive that
promises only 40X playback. The most modern of CD-R and CD-RW drives are
capable of reading identification codes on blank discs and set their recording
speed accordingly.The most important measure of speed for a CD-ROM drive is its data transfer
rate, also referred to as throughputor its readspeed. This tells you how fast the
CD-ROM is able to transfer data to the computer’s data bus. This is especially
important when it comes to playing music from a CD or using a computer game
that pumps a great deal of graphics to the screen; if the throughput is slow or
uneven, playback will be unacceptably poky or choppy.Here’s where I come to the definition of X. The first CD players had a standard
data transfer rate of 153.6K per second (usually referred to in technogeek
shorthand as 150 kilobytes per second), which is pretty slow by modern mea-
sures. But there was a reason: That was the speed used for playback by CD
audio devices, and here is an obvious example of consumer products driving
the computer market.In any case, computer engineers quickly doubled that speed, and the next
round of devices were called double-speed,or 2X devices.As you can see,
X has become a relative number. A 40X device is supposed to be capable of
delivering information at forty times the speed of that original 150 kilobytes
per second drive. As the drive speed increased, so too did the access speed
of the read/write heads. That happens because the faster rotational speed
brings a particular section of the track beneath the heads that much faster.152 Part III: Laying Hands on the Major Parts