Most of us have dealt with the frustration of trying to read a burned CD or DVD, only to find that it simply won't play in certain kinds of drives at all. What's worse (and harder to understand) is when a professionally pressed DVD can't be read in certain DVD drives. When inserted, the disc spins (sometimes very quickly), but doesn't register as a DVD at all.
Sometimes Windows XP will register the disc as a blank CD. Other times it won't even recognize that a CD or DVD has been inserted. Sometimes the disc will register, but only after a long delay, and then may only be erratically readable. Yet in other drives, the disc may spin up immediately with no issues at all. In most cases the disc is not scratched or dirty (and cleaning it has no effect on its performance).
Why this happens seems to be related to how well some DVD drives handle the variances in manufacturing tolerances that exist in pressed DVDs.
One key factor is the drive's tolerance for jitter, a term used to describe a range of issues that involve differences in the timing of a signal. With DVDs (and CDs), jitter is something every player must take into account and compensate for. . .although some drives deal with it far better than others, depending on how well they're made.
The standards for DVD manufacturing state there can be no more than, on average, 15% jitter in the signal retrieved from the DVD. Some pressing plants may push awfully close to that margin, and some DVD drives may have a much lower threshold of tolerance for jitter than others.
As a result, when you put a disc with a relatively high jitter rate into a drive with a lower tolerance for jitter, the disc may not be recognized. Many of the problem discs I encou
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ntered came from the same pressing plant in Taiwan, according to the inner ring and mastering/mould SID codes, so my guess is that this particular plant may be cutting it a bit too close when it comes to jitter tolerances.
It's not always easy to determine if a given drive has a jitter issue, but there are ways to find out. Some CD/DVD burning suites have tools for jitter calculation, such as Nero's CDspeed tool. However, it does not seem to return information about jitter from all drives.
A senior engineer at Liteon, a major OEM of CD/DVD drives for PCs, has created a utility called DVDScan that can be used to measure the jitter tolerance of a given drive and provides slightly more information; lower numbers in the jitter score appear to be better. This tool is primarly intended for professional use, but I was able to use it to determine the difference in jitter tolerance between two drives: one which had terrible problems spinning up certain discs, and the other which accepted anything I threw at it. The "good" drive returned scores in the 600s; the "bad" drive never returned anything less than 900 and sometimes as high as 1200 or 1300.
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter, which is devoted to hints, tips, tricks, news and goodies for Windows NT, Windows 2000 and Windows XP users and administrators. He has more than 10 years of Windows experience under his belt, and contributes regularly to SearchWinComputing.com and SearchSQLServer.com.
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