High Speed Risks

Contents of this Document

Quality vs. Cost: Replicators and consumers are challenged.

What Do "X" Ratings Mean? Scanning and spindle velocities.

What Are the Risks? Low rewards do not compensate for high risks.

Risk Sources: High write speeds compromise goth discs and drives.

Summary: High write speeds lead to major quality risks.

Quality vs. Cost

Both duplicators and replicators are challenged as CD and DVD discs become high-tech commodities. Costs of high volume replication plunged as molding cycle times decreased from twenty seconds to three seconds or less, but fixed costs of the mastering-molding-labeling process make short runs costly. Replication turn times may also be long, setting the stage for fast turnaround, short run duplication using affordable recordable media, drives, and color printers. Today, some replicators duplicate short runs in competition with in-house or independent duplicators. In turn, duplicators may outsource replication for their customers. Although this blurs process distinctions, replicators are accustomed to significant capital investments, and are more likely to have a stronger quality program supported by trained personnel and expensive equipment.

Higher recording speeds may appeal to the consumer who may not be concerned about quality, but professionals must understand and manage the quality risks that accompany high speed duplication. High throughput is essential to low duplication cost, but quality may suffer at high write speeds. Replicators often have adequate test equipment, but small duplicators may not because of budget constraints, although moderate cost testers are now available.

1X CD drives reliably recorded 63 minute discs in 1990, and media standards were developed for 1X operation. Drive marketers have convinced buyers that faster is better, but risks appear to outweigh rewards. CD drive speeds increased to 52X while 20X DVD drives are appearing. Progress to 12X CD recording produced exceptional benefits, but advantages of higher speeds may be minimal while associated risks multiply. This article contains an overview of these risks and methods of quality management for the duplicator.

What Do "X" Ratings Mean?

“X” ratings determine the scanning velocity at which data features stream past the optical pickup head of a write or read drive. 1X values range from 1.2 m/s for 74 minute CD's up to 1.4 m/s for 63 minute discs. DVD discs have 1X values of 3.49 m/s for single layer and 3.84 m/s for dual layer discs. Scanning velocities increase in proportion to the “X” value, therefore a 4X SL DVD would have a scanning velocity of 13.96 m/s.

Constant linear velocity (CLV) modes operate with a fixed scanning velocity across the entire surface. Since angular velocity is proportional to scanning velocity and inversely proportional to radius, Tables I and II show that RPM must decrease as radius increases. Highest RPM and maximum risk therefore occurs at the critical lead-in and volume information areas near the center of the disc.

According to Tables I and II, excessive CLV angular velocities occur at the inner diameter when CD speeds exceed 12X or when DVD speeds top 4X. Sealed, well balanced hard drives operate at 7,200 rpm in most 3.5" desktop HDDs, and at 5,400 rpm in many 2.5" notebook drives. CD and DVD drives are not sealed, and discs are not as well balanced as HDD platters. Problems caused by excessive CD and DVD inner diameter speeds are controlled by operating at a constant angular velocity (CAV) where data rates are proportional to radius, as illustrated in Figure 1. Partial-CAV (P-CAV) runs in the CAV mode over the inner part of a disc and then switches to a CLV mode over the outer part. Zoned-CLV (Z-CLV) is often used with high speed rewritable discs.

It is apparent that CAV drives operate at their rated “X” data rate only near the outer diameter, and that implied speed advantages may be an illusion. For example, a 16X CAV DVD writer may start recording at 6.6X, and averages about 13X across the entire surface that is written in six minutes. The same writer at 8X P-CAV records the full surface in 7-8 minutes. The minor advantage of 16X CAV disappears if only a small amount of data is recorded in the inner area. Similar results are observed for CD drives, where CAV 48X operation may start at 21X and averages 36X across a full surface that is written in three minutes. Conservative 12X CLV operation writes a full CD disc in seven minutes.

What Are the Risks?

Advantages of high speed operation are low while risks are high. Jitter and other important quality indicators such as error rates are degraded at high write speeds because of media and drive compromises. Because interchange failure is a probability, not a pass/fail certainty, seemingly minor differences in test results for a single indicator become significant when many variances accumulate. Reliable interchange and satisfied customers require the highest level of quality that is economically feasible. This level is not fixed, but depends on the intended market of the duplicator. User problems may be unacceptable for software companies having high technical support and replacement costs, requiring interchange failure rates less than 0.01% and nominal duplicated media characteristics. Failure rates as high as 1% may be acceptable for throwaway samples that have marginal quality characteristics.

Operation of write drives at their maximum speed may offer marginal benefits accompanied by significantly higher risks of interchange failure. High speed recordable disc quality may not equal that of lower speed media. For example, 16X DVDRecordable discs written at 8X often have lower quality than results for 8X media recorded at 8X in the same drive. Test results for two well regarded writers and high quality media were used to demonstrate these effects on jitter as shown in Tables III and IV (note that high speed results can be much worse for lesser quality media and writers).

Risk Sources

High speed recording requires very high laser powers that accelerate degradation of the solid state laser. For example, 1X CD recording may require only 6 mW while over 40 mW is required at 48X. Substrate and dye layer local heating effects are substantial at high laser power levels, and require careful attention by disc designers if the resulting degradation of recorded quality is to be minimized. Pre heating occurs when temperature increases caused by a preceding write propagate into a subsequent write at high speeds, requiring special write strategies designed for the specific disc.

Media pregroove and recording layer designs are often modified for high speed rated discs, but compromises give results that may not equal those of lower speed media. These effects are media and speed sensitive, and drive firmware must incorporate corrective information for each media manufacturer and rated disc speed.

Accurate radial position and focus of the laser spot are essential for acceptable quality, and are maintained by drive servomechanisms that move the objective lens. Characteristics of these servos were originally optimized for 1X operation, and must be accurately adjusted at other speeds. Problems occur because objective lens motion in response to a physical defect requires forces proportional to the square of angular velocity. Therefore a drive requires forces that are 256 times greater at 16X than at 1X in order to correct for radial and axial deviation, eccentricity, and other media issues. Few if any drives can fully respond to these ever present variations at high speeds.

High speed drive operation is degraded by vibration effects. Both drive spindle and media unbalance generate vibrational forces proportional to the square of spindle speed. Some, but not all, vibration is reduced by absorbers that suspend the drive chassis from the outer case. Remaining vibrational forces displace the objective lens and distort the focused laser spot, degrading performance at high angular velocities.

Finally, laser power rise and fall times can be comparable to optical pulse durations at high recording speeds, requiring compensation tailored to the write drive. All of these tweaks for high speed operation can be developed for the average disc and drive, but disc-to-disc and drive-to-drive production variations that cause only minor effects at low speeds can result in major quality variations at high operating speeds.

Summary

Media and drive issues lead to major quality risks at high recording speeds, while shorter recording time rewards may be marginal. Cost savings from slightly higher throughput are often negated by increased scrap and the expense of servicing user problems. High speed, high quality duplication is feasible, but must be supported by a strong quality program.

All recording drives are not the same, even if they are of the same make and model. Every new unit must be carefully evaluated by the duplicator at the designated write speed using a controlled lot of high quality media before it is placed in service. Periodic drive testing is necessary so that degrading drives can be detected and replaced before nonconforming discs are shipped. Incoming media lots must be qualified using a proven writer before being released for production.

Although the above procedures are appropriate at all duplication speeds, they become critically important for high speed duplication. Carefully designed and maintained processes may result in acceptable quality at high speeds, but must be accompanied by regular, comprehensive quality testing. Quality must be maintained according to media standards, not in relation to duplication yields or customer returns. These approaches are familiar to the replicator, but may be foreign to the duplicator who then requires external support. Media Sciences has provided training, and has helped facilities to identify moderate cost testers for daily in-house use supplemented by comprehensive testing in our third-party lab, with the result that high speed duplication quality is maintained and cost is lowered. Duplicators that lack the necessary resources should avoid recording at very high speeds.