bad or misaligned or dirty heads?
This had crossed my mind but I ran a cleaning tape through before starting to play through the tapes and it does seem to be this particular tape that is solely affected. DATs taped on the same machine around the same time, as well as before and since, seem to play back without problem.
Also, would dirty or misaligned heads allow the analogue out signal to be unaffected while the digital output was skipping? I may not have made it clear before, but I was listening as I transfered this tape and didn't notice any of the blips that subsequently became apparent.
The only thing I have been able to think of is that the tape is past its best and that the error correction on the 55ES is correcting things before I hear them through the hifi but that the optical output is the raw, uncorrected stream.
I have no idea if this is possible but it's the only explanation that I can find thus far. Aside from a dodgy optical cable that has conspired to malfunction only when this particular tape is playing...
Thanks for your input - I'll try another batch of tapes this week and see if any others are affected.