OTHER TECH

Q. How effective is the "error correction" used in modern-day disc technology? Beyond the large scratches that sometimes ruin my enjoyment of a DVD, do smaller scratches have a cumulative effect on what I see that might be more subtle? As for a CD: With the exception of obvious skipping, can scratches affect the audio quality? Peter Lassig II Centerville, UT

A. David Ranada, Technical Editor, says: Provided you don't exceed its abilities, an error-correction system will completely reconstruct the original data - right down to the last bit. But visible or audible problems can arise when a system is confronted with so many errors that it cannot perform its mathematical magic. What happens then depends on the type of signal that has been corrupted. With PCM audio in CDs, these "uncorrectable errors" are cured by interpolating the gaps created in the signal by the bad data - in essence, the system can guess what data should be there. This is usually inaudible unless the disc has severe damage (long scratches that follow the curved laser path) or dirt or is badly manufactured. Sometimes the interpolations are so frequent that you may hear low-level clicking or swishing sounds synchronized with the damaged area passing through the laser beam.

With compressed-signal media like the standard DVD and the new HD DVD and Blu-ray systems, bad data must be handled differently. Since the damaged data is information about the audio and video signals, not the original signals themselves as in PCM, simple interpolation will not work. And what the various Dolby, DTS, and MPEG decoders do when they receive bad data can vary, depending on precisely what data on the disc has been damaged ("metadata," timing and synchronization data, compressed audio and video data, and so on). So the results can range from invisible and inaudible to the blocking effects you mention, to frame stuttering, or even the loss of sound-picture sync.

The worst case for any optical disc is to have badly damaged table-of-contents data, which accounts for many a disc not even loading. If you have an unplayable disc, look for dust, debris, scratches, or fingerprints in the innermost couple of millimeters of the recorded area.

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