DISC TESTING AND VERIFICATION

Is it necessary to verify a writable DVD disc after recording?
Verifying a disc after recording helps to maintain an appropriate quality level. The amount of ongoing integrity checking and data verification that may be prudent is really a question of acceptable risk for any particular application. For example, letting recording software conduct data comparisons immediately after writing is usually sufficient in casual situations but critical data archiving and large-scale duplication may call for more comprehensive testing. This is due to the differences that often exist among recorders, drives and players. Consequently, successfully verifying a written disc on only a recorder does not guarantee broad playing compatibility, especially in cases where a disc is of marginal quality.

How can the quality of a recorded DVD disc be assessed?
Several methods can be used to assess the quality of a written disc. These include measuring its optical signals, examining the integrity of its physical and logical formats, performing interchange testing and conducting data verification. Each method is a piece of the quality testing puzzle. The extent to which a disc needs to be tested depends, of course, upon the imperatives of the application.

At a basic level it is possible to confirm that information has been correctly written to a disc by comparing it against the source material using the verification features found in many off-the-shelf writing software packages. When more detailed analysis is warranted, interchange testing can be performed to provide some practical indication of real-world compatibility. To accomplish this, DVD-Video discs are played back in a number of consumer electronics (CE) DVD video players and computer DVD-ROM drives to check for quality issues while data discs are checked in a variety of DVD-ROM drives to make sure that recorded information is completely recoverable and at speeds established by the manufacturers. Specialized computer software controlling everyday DVD-ROM drives can also be used to read a disc at a lower level of organization to verify that its physical and logical formats conform to industry specifications. In all cases it is assumed that the testing tool used broadly represents the behavior of the general population of reading and playing devices in the market. However, this may or may not be a valid assumption given the wide variety of readout optical systems and the error detection and correction (EDAC) circuitry and strategies in use.

For situations that require appraising more fundamental physical characteristics, a number of commercial analysis tools are available to examine the optical signal characteristics of a recorded disc and thus identify low-level errors. Typically, these devices are standalone or computer-attached and employ DVD-Video or DVD-ROM drives specially modified to measure various disc parameters and provide descriptive reports. As is the case in testing generally, results can vary significantly among inspection systems. To maintain continuity, therefore, discs should always be evaluated on the same piece of equipment. Commercial DVD testing companies offering quality verification services using such devices are also widely available. The accuracy and usefulness of commercially available test platforms (other than those produced by a few semi-officially sanctioned manufacturers) are often debated in technical circles. Consequently, the results such systems generate should be viewed cautiously as it is unclear what they mean in the larger context of reliability and in determining conformance to established specifications.

An important question that has always existed for optical disc testing is the uncertainty of the relationship between the results derived from evaluating discs on low-level analyzers and real world disc performance in the installed population of reading and playing devices. Given the extremely rapid technological evolution of reading and playing devices it is impossible to conclusively establish any definitive link between measured and actual performance, especially for marginal discs.

The DVD Forum has established numerous verification laboratories around the world charged with determining and certifying the conformance of the various types of DVD discs and devices to their respective specifications. Philips Electronics, Ricoh and Sony deal with DVD+R and DVD+RW. While these labs are considered to be the last word in verifying disc conformance to specifications and quality testing they are unable to provide any guidance regarding the reading or writing performance of discs with the already installed population of recorders, drives or players. This is one of the many functions served by several ad hoc industry groups such as the DVD+RW Compatibility and Convergence Group (DCCG), Recordable DVD Council (RDVDC), RW Products Promotion Initiative (RWPPI) and the Optical Storage Technology Association (OSTA).

When assessing disc quality keep in mind the huge number of variables involved. These include such things as discs with their different types, batches and manufacturers, recording software and hardware in their many varieties and versions, diverse recording conditions encountered, different test equipment employed, operators of differing experience and even by the physical handling of the discs themselves. Consequently, judgments should be made on a relative rather than absolute basis.

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