okay doc, I had to do some searching. But first, I suppose that I should have said something like "digital copies MIGHT be corrupt" etc....
AES/EBU and S/PDIF formats transmit digital data up around like 6MHZ. This is in the realm of most video data. So, where the problem can start is when the cable that is used to transmit digital data using one of those two formats is not of the highest quality, or designed to allow an "even" transfer across the whole length of the cable. The result is can be one of two things; jitter and errors. Jitter is taken care of when the recording device plays back the jitterfull recording and reclocks it, about the only time this may make any difference would be if you where using say one of wanna be mastering boxes like
the TC Finalizer Express in between two DAT players using the digital I/O's. If there is enough jitter, it would tend to make you adjust the processing is ways you may not need to because the jitter was causing you to hear the sound differently then it would be played back on the recieving DAT. So, one should always playback the recording to assure that it sounds the way they want it to. You cannot in a lot of cases trust what you are hearing while you are actually recording.
Errors can be a few things. Certain bits can be lost during the transfer. You may or may not notice this without ultra high quality converters for playback. Most digital gear now has "error correction" circuitry that will "fill in the gaps" to a certain degree. Now, this should not be confused with the error corrected recording being "an exact digital copy" of the original because obviously it is not. The error correction circuit basically took the sample before the error and the one after the error and "filled in the gap" with an average between the two. Other errors can be waveforms that are different than the original. If it is bad enough, it will just go onto tape as nothing the D/A converter can detect and would create a dropout in the recording. If this dropout is long enough, it will be audible. DROP OUTS ARE A WAY OF LIFE IN DIGITAL, THEY HAPPEN ALL THE TIME.
So, how does one help to assure that jitter and errors don't mess up the audio?
First, not a lot can be done about a device that has high amounts of jitter on playback. You just need a new unit. Also, some gear has circuitry that corrects jitter problems. These are of course present on higher quality pieces of gear and is one of the reasons for the higher price.
The best way to alleviate jitter and errors during digital transfers is to use quality cable that matches the specifications for the digital transfer standard you are using. This usually means using video coax cable for the transfer. AES/EBU calls for 110ohm cable on high quality XLR-3 connectors. S/PDIF calls for 75ohm cable on either RCA or better yet BNG connectors. By not using cable that meets the ohm load requirements for the used format, the wire will create errors THAT CAN BE AUDIBLE. Better safe then sorry though.
Regular mic and intrument quality cable is not really recommended for digital transfers because it is not generally designed to allow frequencies up to 6MHZ to pass through. So, video coax cable is the desired type of cable. Hell, when I was reading around, the pro's just recommend going to Radio Shack and buying their inexpensive coax cable for use.
The length of the cable obviously has an effect too. It is recommended to keep digital transfer cables as short as possible because longer runs increase the impedence of the load, thus, can create errors. For S/PDIF, up to about 3 meters is supposed to be okay if the cable is good. For AES/EBU, you can extend to maybe 100 meters if the wire is really good. But for runs of this length, it is recommended to use an inline impedence transformer to keep the signal strong.
So, there are many links that state the AES/EBU and S/PDIF standards for digital transfers. Here is one
http://www.hut.fi/Misc/Electronics/docs/audio/spdif.html . There are countless other with the same exact information.
Basically, during a digital transfer, you are writing data to the tape. The integrety of the wire plays a roll in the accuracy of the new copy. The newer digital gear allows for a little error, but, you still get better results with better wire as it tends to allow smooth transmission of the data. To say though that a digital copy is "exact" is sort of misleading. It may not sound any worse depending upon how good your playback system is (D/A converters, amp, monitors, etc....) but to say the none of the bits where dropped and error correction was not applied to make up for this is not true.
Anyway, I still haven't found the technical document I read about what happens during digital transfers. It was obscure for sure and just not something that you will find because it dealt mostly with the supposed "exact" copy idea. Even old Bob Katz agrees that a digital copy will "sound" exact to your ears if minimum standards are kept.
Anyway, when I find it I will post it. But for now, keep cable length's short, and use wire that is rated in ohmage for the standard you are using and all should be well and good with your digital copies that sound exact, but are not....
Ed
