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Digital copies - identical clones, or only nearly perfect?

Why a copy of a digital recording is meant to be a perfect replica of the original, and why in practice it isn't.

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In the old days of analog tape, one of the most significant problem areas was in copying tapes. An original analog recording can sound very good. But a copy doesn't sound quite so good, and a copy of that copy is starting to sound distinctly 'iffy'.

Each generation of analog copies increases the noise level and the distortion percentage. The frequency response suffers too as inaccuracies in the original are further increased by frequency response inaccuracies in the copying process.

When digital recording came of age in the early 1980s, it seemed like a solution to all copying problems. A digital recording consists simply of a list of numbers, stored on tape. It may be a long list, but it is just that.

Numbers can be copied identically, unlike electric or magnetic signals where there is always a degree of inaccuracy. In theory therefore, a digital copy is identical to the original and sounds just as good. It is sometimes called a 'clone'.

However, digital tape recordings are subject to errors, even in the original. Where data has not been recorded correctly, it must be compensated for. 'Error correction' makes use of additional data in the recording intended for just this purpose - if some data is bad, then the error correction data can be used to reconstruct the original. This is OK, the sound of the recording is not affected at all.

Error concealment is another matter. This occurs when the damage is too much for the error correction system to completely reconstruct the original data. What happens now is that the system 'guesses' what the data should have been. This doesn't necessarily sound too bad, but it's not quite the same as the original. This is called 'error concealment'.

Any copy made of this recording will treat the error-concealed data as though it were absolutely correct, and any further copy made from the copy will compound the problem.

In practice therefore, it will be found that a digital copy is not as good as the master, and just like in the old analog days, there will be one 'true' master, which is the original, and copies will always be imperfect to a degree, compared to that.

Hard disk systems are better in this respect, simply because it is in the nature of computers to copy data all the time. Consider the computer systems used by a bank. If there were any errors in the data, then concealment would be tantamount to fraud!

Generally, when all is working well, a disk copy will be 100% identical to the original. This is thanks to the more robust error correction system used, and the 'verification' process where data is checked to be correct immediately after recording.

Even so, glitches do happen, and the original recording should be kept safe just in case.

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By David Mellor Monday January 9, 2006
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