From David Stewart, Sweetwater's Service and Technical Support Manager:

How many generations of DAT machine digital dubs can a signal go through before degradation is heard? This has been a hotly debated topic. Some say that degradation can become noticeable after just a few dubs, while others say they've made hundreds of generations of dubs without any audible change.

Obviously, if each dub is from the original tape, the copies should be identical for all practical purposes. Given that is true, we'll assume the question concerns making dubs of dubs of dubs, where the final tape is many generations removed from the original.

Common sense tells us that the data is digital (ones and zeros), and is simply moved from one DAT tape to the other; there should never be any generation loss. In theory this sounds great, but as usual things are a bit more complex in practice. There are a variety of factors that can cause a digital dub to be slightly different from the original. The two most common are clock anomalies (jitter) and error correction decoding. Both of these can cause a digital copy to vary ever-so-slightly from the original. While it is highly unlikely that any of this would really be audible, some feel that after many generations of copies the cumulative effects can be heard. This also sounds plausible. However, based on our research in Sweetwater's studios, it appears that, in reality, digital signal degradation doesn't accumulate to any major extent.

Your results may vary, but our view is that you can copy all you want and not hear a change in the audio. If you want to be sure of getting the most accurate digital copy, make all of your dubs from the original master, or from a very early generation safety copy.