Just since it came up in another mastering thread, here are a couple of screenshots of old reel recordings from my collection. Both have speed errors, probably due to a poorly maintained tape recorder at time of recording.

The first screenshot is of an oratorio live concert recording in the UK made around the early 1950's using the first model consumer tape recorder released to the public in 1948, a Soundmirror. The tape used was a very early 3M paper base tape. The tape had fairly poor performance. Capturing the high frequency bias tone on tape (which can sometimes be used as accurate tape speed indicator) would have been out of the question unless they had used very high tape speeds. But at the bottom of screen are three mains hum interference traces. The tilt upwards towards the end of the reel (lower right side of screen) represents the progressive slowing down of the tape at time of recording. The 50 Hz hum tone could probably be useful at least for correcting the slowly changing speed. For the faster modulation within the trace the 150 Hz harmonic signal might have a chance of correcting or partially correcting that error.

The second screenshot is of a tape recording of an Australian football grand final commentary in 1965, and has the same problem of the tape slowing down as it got to the last minutes of the reel, again a common problem when tape machines werent maintained well. But here the more modern tape was able to capture much of the machine's 55 kHz bias tone ( wiggling line at top of screen) and it may be possible to use this to correct the speed error.

An issue with these sorts of speed errors especially the ones with recoverable bias tones is that as the bias tone is inaudible to the listener, audio personnel may end up throwing away or at least editing out that part of the recording thinking nothing can be done to repair it.

But it's sometimes possible. I know Jamie Howarth and John Chester at Plangent Processes have been doing this specialised work for many years. Maybe others do this work as well. Does anybody know of anyone else in the world also doing this work?