Don't be fooled by 16X CD drives--they may be slower than 12X
In the dim, distant past when we began using CD-ROM, all drives worked the same way and
had the same 1,000-millisecond access times. As faster drives arrived, the originals
became known as single-speed or 1X drives.
1X drives turn at constant linear velocity, or CLV. This means that the drive will
always read the same length of track in the same amount of time, no matter whether it's
reading on the outer edge of the CD or near the center. It's designed this way because the
reflective data marks on the track are equally spaced, so CLV moves the data past the read
head at a constant speed.
But because the distance once around the disk is more than twice as long near the rim
as near the hub, the drive mechanism must speed up from about 200 revolutions per minute
to about 500 rpm to maintain CLV as the data read progresses from the edge to the center
of the CD.
CD drives up to the 12X level are still CLV drives, and the access time will be the
same whether data is located near the center or at the disk rim. These drives spin at
anywhere from 1,580 rpm to 4,280 rpm.
But today's fastest drives, 16X and higher, use constant angular velocity, or CAV,
technology used in write-once optical drives. The rpm of these drives does not vary
depending on where the data is located on the disk. These drives work fine, but they
aren't any faster than CLV drives.
Some vendors label them honestly as 16X maximum-speed drives, but many others just call
them 16X drives, implying considerably faster access than 12X drives deliver.
But the 16X drives I have seen all seem to turn at about 8X near the hub, reaching 16X
only at the rim.
The next twist in the story involves how CD-ROMs are created, which happens to be from
the hub outward. Unless a full 550M of data is stored, the outside of the track remains
Few CD-ROMs are actually full. That means few of them have data placed where a 16X
drive can read it at maximum speed, so you don't even get an average reading of 8X at the
inside and 16X at the outer edge of the disk.
In fact, for many CDs, a 16X drive actually works slower than a 12X drive, which can
read the whole track at full 12X speed.
So don't be swayed to select your office's new computers on the basis of fast CD-ROM
drive claims--unless you can confirm performance.
Now that digital video disk drives are appearing on the market, you'll soon be seeing
commercial DVD disks. They look just like CD-ROMs, but a DVD won't play in a CD-ROM drive.
However, a CD-ROM or PhotoCD disk will play in a DVD drive, though a CD-recordable disk
Second-generation DVD drives will fix this incompatibility by adding a second laser
pickup for CD-R.
DVD's short-wavelength blue laser can read smaller data bumps so the DVD can carry more
data than a CD. But the DVD standard also encompasses five-channel surround sound with
subwoofer signals and spectacular Motion Picture Experts Group-2 video compression.
DVD initially is arriving in kits with the drive, interface card and MPEG-2 decoder.
Later drives may not need this extra hardware if boardmakers start building some of the
chips onto PC motherboards.
Having a SCSI interface is not a priority, because the DVD cards now shipping are all
for Busmaster PCI slots. Early DVD titles are mostly game-oriented. But telephone
directories that take up six or seven CDs are beginning to appear on a single DVD.
Training videos should quickly move to DVD because of its capacity to accommodate
alternate story lines on the same disk.
Although DVDs spin at only 570 rpm to 1,530 rpm, performance is about equivalent to 6X
and 8X CD-ROM drives, with search speed around 200 milliseconds.
I don't suggest jumping on the new product bandwagon, but the only drawback I see to
buying a first-generation DVD drive is that it can't read CD-R disks. Sites that already
have multiple CD and CD-R drives won't find that an important downside.
First-generation DVD drives can read double-layer, double-sided DVD disks holding as
much as 16G, even though no such disk yet exists. There just isn't any common application
that would fill a 16G disk--not even Microsoft Windows 98 or Windows 99 bundled with
Bottom line: Don't invest in DVD now unless you have a use for it, such as working in
published DVD databases. And be cautious about ordering any of the hot new CD-ROM
replacement drives, because DVD is likely to become the standard in PCs as soon as next
Matsushita Electric Industrial Co. Ltd. of Japan has just announced a $1,400 car
navigation and map system on DVD, complete with voice directions--in Japanese, of course.
That makes me wish for a single disk that would hold all U.S. streets and highways, all
U.S. telephone numbers and a bundle of other everyday information we could use at the desk
instead of in the car.
John McCormick, a freelance writer and computer consultant, has been working with
computers since the early 1960s. E-mail him at firstname.lastname@example.org.