Why High Speed?
Computer users are attracted to fast CPU's, video cards, printers, and drives like moths to a flame, but do benefits justify the attendant risks? The first 1X drives reliably recorded 63 minute discs in 1990. Subsequent high speed CD-R media and writers appealed to professional duplicators because higher throughput reduces unit cost by amortizing capital, other fixed expenses, and labor costs over more discs. MP3 collectors and gamers are always fascinated by speed and higher capacity. Progress from 1X to 16X recording produced exceptional benefits, while 16X-to-48X gains are more modest. Advantages of pressing beyond 48X to 52X, or even 56X, may be minimal, while risks multiply.
High recording speeds place severe demands on both writers and media. Fast drives are susceptible to vibration because their rotational velocities may be as high as 12,000 rpm, much greater than 7,200 rpm for hard drives that operate in a sealed environment with balanced platters. High speed drives may be incapable of supplying forces necessary to quickly move pickup head optics. High speed media requires customized dyes and pregroove geometries in order to avoid excessive heat buildup during recording. Lastly, internal stresses can result in disc cracking or even fracture at high rotational speeds.
High Speed Discs
Modification of the dye layer during recording creates marks that simulate the pits of a replicated CD. This process requires the external application of energy as determined by the product of laser power and time. High speed recording requires higher power for a given energy input because recording time is much shorter. Typical laser power increases from about 6 mW at 1X to over 40 mW at 48X. Resultant local heating of the information layer by high power beams can adversely affect electrical parameters, or may even result in delamination.
Thinner dye layers help reduce overheating1 because lower recording energy is required, but low HF signal amplitudes, I3/Itop and I11/Itop, may result. Very high temperatures can be reduced by using thick reflective layers as heat sinks1, but cost pressures often dictate thin metal layers. Thermal isolation of surrounding dye regions can be improved by modifying the pregroove1, but push-pull may be affected. Jitter can increase1 when high temperatures from a previously written mark do not fully subside during transit of a short land. The next mark is then written into a hot area, and jitter increases because its resulting location depends on variable cooling times that are determined by the preceding 3T to 11T land lengths.
Both CD-R substrates and replicas are molded by injecting molten polycarbonate at the center of the disc. Non-laminar flow in this area, or separation of the sprue from the hot disc, can create high stresses near the center hole that are retained in the cooled disc. Short cycle times resulting from cost constraints increase these center hole stresses that can induce cracking or even fracture of the disc.
High Speed Drives
Optical pickup heads contain electromechanical components that maintain proper radial position and focus of the laser beam. These components are controlled by servo circuits that must promptly and accurately compensate for any axial or radial shift of the information layer. To achieve this, servomechanisms generate corrective forces that are proportional both to the mass of the component and to the acceleration of inevitable axial or radial shifts. Disc standards limit these acceleration values, but may be inadequate because they were prepared before high speed drives became popular. Since acceleration is proportional to the square of spindle speed, a given physical shift produces an acceleration that is 356 that is times greater in a 48X CAV drive than in a 1X CLV drive (or 2304 times greater in a 48X CLV drive), increasing the required corrective force by 356 (or 2304) times, assuming constant mass. Not all high speed writers or readers may be able to correct physical shifts fast enough, causing offtrack or out-of-focus conditions that result in write defects and read errors.
Drive components are mounted on a chassis that is suspended from its mounting frame by vibration isolators. Any disc unbalance results in motion of the chassis that is partially buffered by these isolators, with some residual amount directly affecting the pickup head. Pickup head vibration in high speed writers may result in momentary offtrack or out-of-focus conditions that record a permanent defect into the disc. Vibration can be enhanced by unexpected disc or drive resonant modes, and by unbalance of the drive's spindle motor.
Radial forces from unbalance are proportional to the square of spindle speed, being 356 times greater at 48X CAV than at 1X CLV. Disc unbalance limits may be inadequate because they were established before high speed drives became popular. Additional unbalance can be created by disc miscentering. Upon disc loading, the 15.0-15.1 mm diameter center hole is centered on a tapered cone attached to the spindle motor, and is then held in position by a magnetic clamp. Some drive dependent miscentering is unavoidable, resulting in unpredictable unbalance and excessive vibration at high speeds. Since only a small portion of the disc's center hole contacts the tapered centering cone, unbalance forces are highly concentrated at this point, and can cause severe cracking or shattering of the disc at high drive speeds, especially if high stress is present near the center hole.
Prior to writing, all recording drives perform optimum power calibration that adjusts recording laser power to produce a predetermined beta as calculated from HF amplitudes. High speed writers must additionally apply special write strategies during recording to compensate for thermal effects described above for high speed discs. This compensation is normally based upon three parameters that are determined by the media manufacturer and encoded in each disc. Two of these parameters specify the level and duration of an enhanced power region that initiates every mark. The third parameter specifies an additional delay that is used only if the mark is preceded by a short land. Successful recording requires accurate initial determination1 of these parameters by the manufacturer, product consistency to maintain their validity, and proper write strategy implementation by high speed writers.
Conclusions
Increases in CD-R recording speed are accompanied by new risks that can nullify apparent benefits. Mechanical problems can occur because discs are manufactured to limits of acceleration and unbalance that were established when only low speed drives were in use. Low stress disc center holes are required to avoid cracking or fracture, but may not be achievable with short molding cycle times. High write power can result in overheating at high recording speeds unless dye and reflective layers and pregroove geometries are modified, but such changes may degrade recorded parameters. Acceptable jitter requires new write strategies that compensate for high temperatures, but requires effective action by both media and drive manufacturers.
Issues created by high speed DVD trends parallel those for CD-R, except that 1r DVD speeds correspond to 3X CD speeds. Successful high speed recording depends on many subtle measures that must be properly implemented by both disc and drive manufacturers. Confidence in these components, and in coordination between their suppliers, requires careful evaluation of both high speed media and writers if predictable interchange is to be achieved. Only a cautious approach can prevent field failures and unhappy users.
1. High speed recording with organic dyes for CD-R, Dr. Jitka Brynjolffssen, Organic Media Development Manager, Plasmon, published in ONE TO ONE, 01-2003, pp 57-58.