Does correcting the speed of audio captured from vinyl at the wrong RPM degrade its quality?

Question

I sent a record of mine to an analog -> digital conversion service, and the CD that came back sounded like chipmunks. They had played the record at 45 RPM on their capturing equipment instead of the proper 33 1/3.

I was happy to discover that Audacity has a "change speed" filter with presets for vinyl RPM, so I can fix this on my own in software. It sounds okay on first listen, but I'm worried that this may cause a subtle degradation of quality.

Should I demand that the A->D service re-convert from the source vinyl? Or should I use some other tool (preferably one that runs on Linux) to get a better digital conversion?

Answer 1

It is definitely possible that the quality is degraded. All parts of the recording chain have some upper limit on the frequency range they can handle properly. On high-end turntables, this may well exceed the audible range by quite a lot and therefore not matter, but if it was digitalised at 44.1 kHz (which it almost certainly was, if it came back as a normal audio CD), you now have an inevitable brickwall nyqvist cutoff at 22.05 kHz * (33⅓) / 45 ≈ 16 kHz. Now, in a vinyl record, usually not too much happens above that frequency, but because it's analogue you don't have such a steep cutoff. So that might be audible, as well as any effects from otherwise nonlinear responses in the analogue parts of the conversion equipment.

As for digital resampling in itself, that can be done well using the right algorithms. For such a factor of ≈2/3 simple cubic or even quadratic interpolation is already fine, "perfect" sinc isn't really necessary. Not sure what Audacity uses, but I'd expect it to be fine.

But if they got something as obvious as 45 vs. 33⅓ wrong in a professional format conversion service, you should certainly demand this to be re-done properly.