How Fast, at What Cost? High-Speed CD-ROM and Readability Issues

[Sidebar to the article, CD-ROM's New 8X, 10X, 12X Drives by Brad Thompson.]

Stephen Nathans

EMedia Professional, February 1997
Copyright © Online Inc.

The pressure to be first-to-market with high-speed drives has resulted in a push to make existing CD-ROM drive components perform beyond their designed limits.

Here's a question that keeps coming up: Do today's 8X, 10X, and 12X CD-ROM drives threaten to strain the acuity and stability of error correction standards and optical components designed for 1X readers? After all, such high-speed spindling was little more than a pipe dream way back when. But then the world of CD-ROM encoding has never been a perfect one--that's what the standard-specified error-correction algorithms were all about. And have CD-ROMs ever been perfectly proportioned platters, then or now?

In fact, the thickness of CD-ROM media often varies from point to point on the disc, which means the disc's effective center of mass is not always the same as the geometric center of the disc, and CD-ROM drives must be designed to absorb the idiosyncrasies of spinning imperfectly balanced platters, just as the drives must account for the inevitable scratches and such that accrue over time on this removable media. But physical disc imperfections, such as warping, scratches, or an off-center hole, are not a serious problem at 2X or 4X, thanks to over-engineered drive design elements implemented with the first, single-speed generation of CD-ROM drives.

But with the advent of 8X to 12X read speeds, which means CD-ROM drives are spinning discs at 4,200RPM to 6,300RPM, the same drive components required to steady performance at 1X, 2X, or 4X must be significantly more precise and robust in order to read a faster spinning disc consistently. And as Dirk Peters, marketing manager for Sony Electronics' shipping 8X and forthcoming 12X and 16X CD-ROM drives says, "The pressure to be first-to-market with high-speed drives has resulted in a push to make existing CD-ROM drive components perform beyond their designed limits." And components pushed too far can yield frequent read errors and even risk breaking down altogether.

Several specific issues arise in bringing CD-ROM drive components up to speed, so to speak, to ensure safe spinning on the fast drive frontier, and the component enhancements vary by manufacturer. In developing Sony's 8X CSD-880E, Peters says, engineers emphasized refinements to several internal elements to minimize disc and drive vibration problems and heat generation issues that arise with faster spinning to keep read errors to a minimum. Felix Nemirovsky, general engineering manager for Plextor Corporation, which released 8X, 10X, and 12X drive models in 1996, also emphasizes issues of the CPU usage required by fast CD-ROM drives. Speed, Nemirovsky says, can contribute to CPU loads, especially if the bus connection is IDE, which inherently demands more from the computer. As higher speed CD-ROM drives turn to re-reads to overcome initial read error, further demand to the available CPU load can be called upon to handle retries through the IDE bus.

A RUMBLE IN THE BOX: DISC VIBRATION VAGARIES

The most immediately pressing difficulties encountered by a read laser when attempting to retrieve data from a disc as speed increases are created by the increased vibration and motion of the disc being read. A disc's center of mass, according to Plextor's Nemirovsky, can be dislocated from the center of its surface by any number of irregularities, such as a pressed disc's poor plastic molding or thick labeling color or a recordable disc's user-applied thick half-label. A disc's vibration, or wobble, increases in direct proportion a drive's rotational speed, the radial distance of the disc's center of mass from its physical center, and the degree of the physical irregularity itself. Any potential misreads caused by physical imperfections in a disc are intensified at higher speeds, with the force behind a disc's vibration being, simply, that much stronger. A physically uniform disc may read equally well at 4X and 8X with little or no adjustment of a drive's internal components to accommodate increased wobble, but conversely, a physically unbalanced disc might generate vibration or wobble within a manageable range at a 4X CD-ROM drive rotation but may be rendered unreadable at higher speeds in a drive only calibrated to handle predictable 4X wobble levels, but not designed adequately to handle greater wobble at the higher speeds.

First and foremost among CD-ROM drive improvements needed for reading a higher-vibrating disc is the greater agility required of the optical laser pickup to read the data that will dart about due to wobble levels that would otherwise strain existing lower rotational speed focus tracking limits. The optical pickups used in high-speed drives, according to Sony's Peters, must be equipped to move up and down and side to side quickly enough to read a more rapidly vibrating disc.

Another key drive component that demands refinement is a CD-ROM drive's chucking mechanism, to account for heightened vibration on the component which holds an inserted disc in place. Necessary improvements implemented by Sony and Plextor in their high-speed drives include an extended "chucking yoke." The consequence, according to each company, is more reliable performance brought on by lengthening the yoke to extend through the hole in the disc to hold it more firmly on the drive's spindle table. Of course, the CD-ROM drive's spindle table's tolerance for vibration must be improved as well, but by preventing the disc from slipping at all, the longer yoke can help prevent read-error generation and read delays that result from repeated retries at lower, spun-down speeds.

Other enhancements that counteract vibration problems include enlargements of the press pulley and magnet used to clamp down on the disc to ensure it sits firmly on the spindle. "A larger press pulley with more surface area clamps onto a larger section of the inner hub of a CD-ROM," Peters says. "A stronger magnet assures that once clamped down, the disc will not move."

HIGH-SPEED SHOCK ABSORPTION: COUNTERACTING DRIVE VIBRATION

Another challenge to stable high-speed drive readability is the vibration of the drive itself, which can potentially knock even the most agile optical pickups off a data track and generate untold read errors. Sony's Peters compares the effect to a washing machine that starts jumping around in its high-speed rinse cycle because of an off-balanced load. The spindle motors used in 8X, 10X, and 12X drives must generate significantly higher torque to rotate up to specification, and to accelerate or decelerate a disc in a short time.

One key component of high-speed drive physical stability is the quality of its spindle motor bearings. While the use of a single bearing at the bottom of a drive motor and a bearing collar at the top (just below the disc itself) has worked well for lower speed CD-ROM drives, the torque required of 8X or faster CD-ROM drive mechanisms that must be able to move more quickly from track to track can cause the drive spindle to pivot while rotating, according to Plextor's Nemirovsky. This greater torquing introduces a great deal of drive vibration that can erode the upper collar bearing by wear--from repeated rubbing--against the collar's sides.

High-speed disc spinning requires multiple bearings at the bottom and tighter bearing tolerances overall to reduce motor wobbling, and an extension of the bearing collar to better hold the motor's shaft in place. Another measure is to incorporate heavier-duty shock mounts between internal components and between the internal mechanism assemblies and a drive's casing to counter the vibrations that can be created when the different assemblies and parts come in contact with one another, according to both Peters and Nemirovsky. Simple strategies and better components can significantly contribute both to more stable reading and longer drive life.

HEAT GENERATION AND GENERATION 8X/10X/12X

Another problem that drive components designed for 8X to 12X performance speeds must address is increased heat generation. "Some manufacturers," Sony's Peters says, "add heat sinks which help dissipate heat but also allow dust to enter a drive's mechanism." Convention cooling vents let heat out, but dust on the optical pickup lens, like scratches or dirt on the disc, is one of the major factors that can interfere with a disc's physical readability in the course of its usage life.

Increasing the diameter of a drive's spindle motor also helps keep down heat generation, in addition to improving the drive mechanism's stability as the drive accelerates to higher speeds. Plextor's Nemirovsky qualifies such a strategy by conceding that enlarging spindle motors is not always easily achieved given the built-in size limitations of half-height drives, and particularly tray-loading models. Both he and Peters think that the advantages are nonetheless worth noting. "A large spindle motor," Peters says, "is able to dissipate its heat rather than transfer it to other components." The larger spindle shaft can accommodate a larger motor, and a larger motor can also dissipate heat more effectively than a smaller one.

WHAT'S FASTER THAN CD-ROM DRIVES? THE SPEED OF HYPE, AND QUESTIONS

When it comes to fast CD-ROM drives, perhaps the best strategy for dissipating heat is to cool down the hype. With the advent of faster CD-ROM drives, the very definition of what is a CD-ROM drive has been challenged. Does the contact spinning speed--the so-called CAV/CLV technique--drive a CD-ROM drive? Does the performance cited compare with drives that follow the original CLV approach? And questions about bus strategies and the effect such choices can have on CPU utilization must be better understood too, at least by those who care about the difference between theoretical performance numbers and what they're likely to see in a real-world application.

The real-world trials of the fastest CD-ROM drives have barely begun, while more and faster yet drives come to market. Some basic truths, such as that quality tells, will be put to the test. Another concern, however, is that many fast drive models will be quickly replaced, either by models that perform to the task or by the imminent new generation of DVD-ROM drives that gain their high data transfer rates from having over four times the data pass under the optical pickup assembly compared, rotation for rotation, to CD-ROM. --Stephen Nathans

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