Until qu
ite recently, all CD-ROM drives were based on a technology called constant linear velocity (CLV). Like the selections on a music cassette, the files on a CD-ROM drive are laid down one after another, ultimately forming a spiral. And, like recorded tape, CLV keeps the recorded data moving past the reading head at a constant speed, thus easily maintaining a fixed data-transfer rate.
To make this happen, though, the drive must spin the disc faster as it reads tracks that are located closer to the center, because the inner tracks are short-er and contain less data than the outer tracks -- much as a slice of pie is narrower at the center than at the outer crust. (BYTE readers with long memories will remember floppy disk drives that also used CLV.)
While CLV simplifies data handling, it complicates the physical operation of a drive. As drive speeds push higher, there comes a point where the ever-increasing rotational speed causes its own problems, such as vibration. While it's not a factor at lower rotation
rates, vibration can be problematic at higher speeds with unbalanced CDs and some OEM casings; in some situations, the disc rattles in the drive and can cause undue wear or even drive breakage.
Most CD-ROM drive vendors are opting for a solution that combines CLV with another approach, constant angular velocity (CAV), which uses a steady spin speed analogous to that of a phonograph record. CAV is helpful in achieving high performance at lower spin rates and for minimizing vibration.
Previously adopted by vendors of laserdisc players from hard drive technology, CAV relies on a variable data transfer rate, as opposed to the fixed rate of CLV. Data access can be faster, too, because it isn't necessary for the motor to speed up or slow down to change its rotational rate.
Comparing access times isn't easy, though, because different companies use different measurements. Access times are rated in two different ways: by how long it takes the read head to move a specified distance across the disc, and by
how long it takes the head to actually read data from its location on the disc. The second type of rating, called
access time with latency
, is variously referred to as "random seek time" or "average random access time" by vendors.
Is access time important? The answer depends on your particular application. Random seek time can be a significant factor in applications such as computer games, where the head is moving constantly about the disc. Access time is not important, though, if all you're doing is copying a program or an OS to the hard drive. In this type of application, the head reads data in a steady stream.
Interface Matters
The type of interface used is another differentiator between drives. Despite their advantages for cross-platform and multiperipheral connectivity, SCSI implementations can be more complicated to install (especially on Wintel machines) than drives with IDE interfaces. To ease this process, vendors of SCSI drives are adopting a technology that is
known as SCSI Configuration AutoMagically (SCAM).
Another trait of drives with SCSI connectors: The larger the memory buffer, the better the throughput. Among the contemporary crop of high-speed 12X drives, buffers range from 128 KB up to 512 KB. Implementation, however, can be even more important to drive performance than the type of technology used. Depending on how it's implemented, a 128-KB buffer can be more efficient than a 256-KB buffer, for example.
One important difference between the two types of interfaces (as far as CD-ROM drives are concerned) is that SCSI drives require considerably fewer CPU cycles than IDE drives do, because much of the address processing and data dispatch happens on the SCSI board. This difference is irrelevant for many applications, but it can be significant for multimedia and game CDs, where the data requires lots of processing as it's read in from the drive. Displaying an MPEG-encoded video sequence is an example. With such applications, SCSI drives should show no
ticeably snappier performance.
Other components in the complex CD-ROM drive mechanism that vendors try to keep "up to speed" include the motor, or servo mechanism; the controller chip; laser read-ahead; the pickup read head; the read-ahead buffer; and firmware. CLV drives require a motor that can accommodate variable speeds -- a different type than what's used in CAV units.
Toshiba's first two entries in the 12X arena, the
XM-5701B SCSI
and the XM-5702B ATAPI (enhanced IDE) drives, typify the rising industry trend toward combining CLV and CAV. Under Toshiba's Partial Constant Angular Velocity (PCAV) model for its 12X drives, CAV is used for reading the inner portion of the disc, and CLV is used for the outer tracks.
Like other drives implementing CLV, the Toshiba internal drives attain a sustained data transfer rate up to 1.8 MBps on the outer tracks. Average random-access time is 125 milliseconds. Both drives use a variable-speed playback system designed to reduce random-acc
ess times by allowing the drive to read data upon seek completion, before the disc reaches standard rotational speeds.
Toshiba's XM-5701B is the first 12X drive to be outfitted with a SCSI-2 connector. For easier SCSI connection to a PC or a Mac, the XM-5701B comes with SCAM Level 2 software.
The Toshiba drives can be mounted either horizontally or vertically within a PC or a Mac. Both 1.98-pound internal drives come standard with a tray-loading disc mechanism; an emergency eject button; and a "media-removal prevention" feature for making sure the disc does not eject during read operations. The XM-5701B SCSI unit is priced at $195; the XM-5702B ATAPI costs $165. Toshiba is also planning to offer an external SCSI-2 model, the TXM-5701.
Pioneer was the first company to combine CLV and CAV technologies in its earlier 8X drive. With its new
DR-444
and DR-466 SCSI 12X internal drives, the company adds two new twists: a vibration sensor attached to the on-board embedded controller,
and a new CLV/CAV switching mode, controlled by the drive's firmware.
When the vibration sensor detects a vibration level of over 4 g's, the drive switches from CLV to CLV/CAV mode, slowing the disc's rotational speed. The drive then gradually speeds up until it reaches the CLV threshold, where it maintains a constant rotational rate unless it encounters high vibration again. OEMs can set the vibration level to 2.2 g's. Also new is a redesigned VLSI chip set, intended to provide true CLV throughput in CLV mode.
Both drives transfer data at 1800 KBps under CLV, with a stated average random-seek time of 80 ms in "combination mode," and a data-access time of 100 ms. Pricing was not available at press time.
With its new
12PleX drive
, Plextor is sticking with traditional end-to-end CLV. The theory is that it's better to offer a sustained data transfer rate for high-throughput networked and video streaming applications -- since most CD applications don't contain enough data to fil
l the disc anyway and, consequently, to benefit from combined CLV/CAV technology.
Available in both internal and external tray-loading and caddy-loading versions, Plextor's 12X CD-ROM drive also includes a feature called automatic audio alignment, for no-skip audio playback.
Despite its CLV design, the 12PleX provides a fast random-access time of 105 ms. Software utilities bundled with the drive let you adjust the default 2-minute spin-down time for flexible data-access times.
The utilities also include audio controls plus an audio-capture tool, which allows you to quickly save standard audio CDs to the hard disk as WAV files. For more rapid audio/video playback, Plextor's 12PleX drive uses a SCAM-compliant fast SCSI connector, plus a roomy 512-KB disc buffer.
Priced at $399 for the caddy version and $349 for the new internal tray version, the 12PleX kits include a bus-mastering SCSI host adapter.
Mitsumi's new
FX-120 Series
of 12X CD-ROMs also use CLV technology from
end to end. But that will change when its next drive generation arrives later this year, with rated speeds of 16X.
The Mitsumi 12X internal ATAPI IDE drive operates at a sustained data transfer rate of 1800 KBps, with a reported data-access time of 130 ms. The internal memory buffer has a capacity of 256 KB.
The FX-120 Series also features a patented tray design that holds the CD while permitting the drive to operate either horizontally or vertically. The $249 drive is bundled with two CD-ROM discs from Microsoft: the Internet Explorer Web browser and Games for Windows 95.
For its 16X drives, Mitsumi plans to use a combined CLV/CAV approach. Much like Toshiba's 12X design, Mitsumi's 16X strategy calls for CAV on the inner tracks and CLV on the outer tracks.
Our BYTE Best pick is the Pioneer DR-444. It has the highest throughput and lowest random-access time, giving it the best application score on the
CD Tach test
. Plus, we were impressed with its switched solution to th
e CLV vibration problem, and we expect its high CPU utilization to drop in the SCSI version.
Product Information
DR-444................................Price not established
Pioneer New Media Technologies, Inc.
Long Beach, CA
Phone: (800) 444-6784
Phone: (310) 952-2111
Fax: (310) 952-2990
Internet:
http://www.pioneerusa.com
Circle 1059 on Inquiry Card.
All 12X CD-ROM Drives Are Not Equal
