DISC TESTING AND VERIFICATION
Is it necessary to verify a writable DVD disc
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
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
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.
CONTINUE TO DISC CONSTRUCTION