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Publisher10.2 CD-ROM Drive Performance
Although
CD-ROM drives differ in reliability, the standards they support, and
numerous other respects, the most important issue for most users is
performance. But performance is not accurately described by the
simple transfer rate number that most manufacturers use to
characterize drive performance. There are actually two important
performance metrics:
How fast the drive delivers
sequential data to the interface. Data transfer rate (DTR) is
determined by drive rotation speed, and is specified as a number
followed by an X. All other things equal, a 32X drive delivers data
at twice the speed of a 16X drive. But note that we have used 16X
SCSI drives that transfer data faster than some 32X ATAPI drives, so
it's a mistake to depend solely on X-rating. The
specifications for some drives list only maximum burst transfer rate,
which is always the advertised number, while others list sustained
transfer rate, which is far more important to overall drive
performance. Fast DTR is most important when you use the drive to
transfer large files or many sequential smaller filese.g., for
gaming video.
How fast the drive accesses random files located anywhere on the CD.
Average access time is only loosely tied to DTR, is determined by the
quality of the head actuator mechanism, and is rated in milliseconds
(ms). Some inexpensive drives have very high nominal DTR ratings but
relatively poor average access performance. To make matters more
complicated, different manufacturers calculate average access using
different methods, so you cannot necessarily compare figures from one
manufacturer with those of another.
The following sections describe DTR and average access time in detail.
10.2.1 Data Transfer Rates
CD-DA discs record music as a digital
data stream. The analog music is examined or
sampled 44,100 times per second (the
sampling rate) using 16-bit samples, for a data
rate of 88,200 bytes/s. Multiplied by two channels for stereo, that
means the CD stores 176,400 bytes for each second of music. Each
second's data is stored as 75 physical sectors, each
of 2,352 bytes.Music data need not be completely error-free because an occasional
flipped bit will be inaudible, which means the entire 2,352-byte
capacity of each physical sector can be used to store actual music
data. The same is not true for computer data, for which every bit
must be correct. Accordingly, CDs store computer data using 2,048
bytes/sector, with the remaining 304 bytes in each physical sector
allocated to error detection and correction data. This means that a
computer CD running at the same speed as an audio CD delivers (75
x 2,048 bytes) per second, or 150 KB/s. This data rate is
called 1X, and was the transfer rate supported by early CD-ROM
drives.Later CD-ROM drives transfer data at some integer multiple of this
basic 150 KB/s 1X rate. A 2X drive transfers (2 x 150
KB/s) or 300 KB/s, a 40X drive transfers data at 6000 KB/s, and so
on.
10.2.2 CLV Versus CAV Versus P-CAV Versus Z-CLV
Unlike hard disks, which record data on a series of concentric
tracks, CDs have only one track that spirals from the center of the
CD out to the edge, much like a vinyl LP. Because the portions of the
track toward the center are shorter than those toward the edge,
moving data under the head at a constant rate requires spinning the
disc faster as the head moves from the center, where there is less
data per revolution, to the edge, where there is more. If an audio CD
spun at some compromise constant rate, the audio would sound like the
Addams Family's Lurch when the CD was playing the
inner portion of the track, and like Alvin the chipmunk when it was
playing the outer.
The solution is to change the
disc rotation rate as the heads progress from the inner to the outer
portions of the track. When you play an audio CD in a CD player (or
in your computer's CD-ROM drive), the drive speeds
up and slows down according to what portion of the track the heads
are currently reading. This technology, shown in Figure 10-1, is called Constant Linear
Velocity (CLV ).
Figure 10-1. CLV technology, which spins a disc at a constantly varying speed to keep the data rate identical regardless of what part of the disc is being read (image courtesy of Ahead Software)
All audio CD players use CLV. CLV is a good choice for audio for two
reasons. First, the drive only need spin fast enough to deliver 150
KB/s. Second, music is inherently sequential. That is, a music track
is played from beginning to end, which requires only gradual changes
in rotation speed. Early CD-ROM drives also used CLV, but it soon
became apparent that CLV was not the best choice for data CDs, for
two reasons. First, market demands meant that the speed of CD-ROM
drives had to keep increasing, from 1X to 2X, 4X, 6X, 8X, and
eventually to 12X or 16X. Delivering data at 16X requires spinning
the disc much faster than for 1X audio. Second, data CDs are accessed
randomly, which means that the head may have to move quickly from the
inner to the outer portion of the track, or vice versa. In order to
maintain CLV with such rapid head moves, the drive motor was required
to make radical changes in speed. Motors capable of doing that are
large, power-hungry, loud, and expensive. That meant 12X was the
realistic limit for CLV CD-ROM drives, although a few 16X CLV CD-ROM
drives were made.
The solution to this problem was to
implement Constant Angular Velocity
(CAV), shown in Figure 10-2.
CAV is a fancy term for simply spinning the CD at a constant speed,
allowing the data rate to vary according to which portion of the
track is being read. The advantage to CAV is that the drive can use a
relatively simple, inexpensive motor because that motor runs at a
constant rate. The disadvantage is that the data rate varies
according to what portion of the disc is being read, which is really
no disadvantage at all for data CDs. Actually, CAV drives are also
capable of running in CLV mode, which is why you can play an audio CD
in any CD-ROM drive. But that's slow CLV. For
delivering data, which is their true purpose, CAV drives run at a
much higher, but constant speed.
Figure 10-2. CAV technology, which spins a disc at a constant rate and the data rate varies according to what part of the disc is being read (image courtesy of Ahead Software)
Because the data rate on a CAV drive varies according to which
portion of the track is being read, there's no
single number that describes the drive's transfer
rate. Accordingly, such drives are called variable
speed or *-Max (as in
"48X Max") drives, and are rated
using the fastest DTR (that on the outer portion of the track). The
upshot is that a 40X Max drive may read the longer, outer portions of
the track at 40X, but may read the shorter, inner portions of the
track at only 17X, with an
"average" speed for a full CD of
27X, and a somewhat lower rate for a partially full CD.
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Some drives use Partial
CAV (P-CAV), shown in Figure 10-3, which combines CLV and CAV. For P-CAV, the
transfer rate increases until the drive reaches maximum speed. At
that point, the drive slows as necessary to maintain CLV. P-CAV
drives reach maximum speed quicker than CAV drives, so their average
transfer rate is typically higher.
Figure 10-3. Using P-CAV, the disc reaches maximum CAV speed and then slows
as needed to maintain CLV across the reminder of the disc (image
courtesy of Ahead Software)
Finally, some drives use Zoned
CLV (Z-CLV), shown in Figure 10-4. A Z-CLV drive treats a disc as having a small
number (usually three or four) of arbitrarily specified areas, and
uses a different CLV rate for each area. Z-CLV is used primarily for
CD writers, for which using CLV over the entire surface of a disc
simplifies write parameters while maintaining high performance. Figure 10-4 shows the momentary troughs that occur when
the drive motor changes from one CLV rate to the next as the heads
move from section to section.
Figure 10-4. Z-CLV technology, which spins a disc at a constant rate, with
the data rate varying according to what part of the drive is being
read (image courtesy of Ahead Software)
10.2.3 TrueX Drives
TrueX drives are no longer made, but were
an interesting historical footnote. TrueX drives use CLV with a
difference. Conventional CD-ROM drives read data with one weak LASER
beam. TrueX drives split a stronger LASER beam into seven separate
beams, which read seven sections of the track simultaneously. The
drive combines those signals into one high-speed data stream, which
allows a TrueX drive running at 9.5X CLV to provide DTR similar to a
52X CAV drive.Because they spin discs slowly, TrueX drives are quieter than CAV
drives with similar DTR. But TrueX drives have several drawbacks.
TrueX drives sold for twice the price of comparable CAV drives,
vibrate excessively, and have mediocre random access performance.
They generate so much heat that the drive becomes quite warm during
sustained operations, and discs may become uncomfortably hot to
touch. Finally, Kenwood never released Windows 2000 or XP drivers for
the following TrueX models:
- UCR415 and UCR416 (52X SCSI)UCR04010 (40X, 42X ATAPI)UCR411 and UCR412 (52X ATAPI)UCR420 (62X ATAPI)UCR421 (72X ATAPI)
We don't use TrueX drives at all. If we disassembled
an old system with a TrueX drive, we'd toss it in
the trash. New ATAPI CD-ROM drives cost less than $25, so attempting
to recycle an old TrueX drive simply isn't worth it,
even if the operating system supports it.
10.2.4 Average Access
Although it bears superficial
resemblance to the hard drive rating with the same name, average
access time for a CLV CD-ROM drive is much more complex to calculate,
and is subject to manipulation by drive manufacturers who wish to
boost their performance figures. Hard drives spin at a constant rate,
and average access time is calculated as average seek time (the time
required for the heads to move over the proper track) plus latency
(the time required for the disk to spin the one-half revolution
required on average to move the correct sector under the heads).Average access for CLV CD-ROM drives was originally calculated using
a similar 1/3 stroke method, assuming that the drive would be used
mainly for reading large multimedia files. In about 1993, some
manufacturers began substituting "random
access" for 1/3 stroke testing. This method was
subject to abuse because manufacturers could define the size of the
zone they used for testing. Some chose very small zones to boost
their average access ratings, with the result that some drives were
advertised with average access times of less than 60 ms. Worse still,
some makers began promoting seek time as a performance measure. Seek
time is a useless performance measure for a CLV drive because it
ignores the fact that a CLV drive needs to speed up or slow down the
disc to the speed required for data to be read. The time required for
this steproughly analogous to latency, but subject to much
wider variationis determined by the quality of the motor used.Newer drives, which use CAV, are less subject to these manipulation
methods because the disc spins at a constant rate. However,
manufacturers are still free to define the zone they use for random
access testing, which means that you cannot safely compare different
drives unless you know the testing method used to rate them. The
upshot is that if you have an older CLV drive that has a very good
average access rating, you should suspect that it is artificially
inflated. A newer drive, even one with a substantially slower rated
average access, will likely outperform the older drive by a
significant margin. As a point of reference, the fastest CD-ROM drive
we ever used, the recently discontinued Plextor UltraPlex Wide, is
rated at 85 ms average access. Testing that drive against inexpensive
ATAPI drives with faster published average access times revealed that
the Plextor was in fact much faster at retrieving random data from a
CD. Therefore, take any published average access rating with a grain
of salt.
