audio
I've found that audio files seem to degrade slightly over time (10 years +), and I'm looking
for a tool to check the integrity of those, and say, "yip, it's all there and good!".
I need something that checks:
I've seen MP3Check. It's good, but it only works on MP3s.
The best way to determine if something has been sourced from a lossy source, is creating a spectrogram:
One can clearly see that it goes up to the 22.1kHz a proper CD has.
When transcoded to a lossy MP3 128kbps, you can clearly see the destructive work of the encoder:
More details and examples here: http://blowfish.be/eac/Spectral/spectral.html
More generally, I would recommend to have a look at the FFmpeg Wiki page on AAC encoding. Here you will find tips on how to achieve the best quality, which involves:
libfdk_aac encoder instead of the built-in one (the encoder can make a huge difference in quality!)Thus, the following would work:
ffmpeg -i input.opus -c:a libfdk_aac -vbr 5 -cutoff 18000 output.m4a
This should be absolutely transparent to anyone's ears. You may even use -vbr 4 and check whether you can detect the difference. I guess you won't.
More specifically, you ask:
And finally, youtube-dl also can download an AAC file of 128 kbs. (…) But would transcoding the opus to AAC (…) maintain that quality difference, or would the transcoding (…) induce artefacts so that the audio quality (…) would actually be worse?
Great question that I don't know how to answer without conducting a formal listening test.
However, if you assume that stereo AAC at 128 kbps is at least good quality (but not excellent), you may forgo any conversion and use that directly if it sounds good to you. I think I would listen to the original Opus and AAC samples that you directly downloaded and try to do a blind test — can you reliably detect the difference? If no, then just use AAC and save yourself the conversion.
PS: I listened to a few music clips from YouTube with both the Opus and AAC variants and I personally liked Opus better — it seemed to have a bit more high-frequency spatiality to it. But don't consider that a scientifically sound conclusion!
Finally, there is a thread discussing a similar issue, and here, one user thinks that there is more aggressive psycho-acoustic shaping in VBR encoding, which may enhance already present artifacts when encoding a lossy file again. He concludes that ABR at a high bitrate may be better suited for such tasks.
Best Answer
Foobar with the file integrity verifier might do the trick.
The Flac encoder will check a file with the -t arguement
You might also want to look at comparing the files against the accuraterip or cuetools database.
That being said, digital is digital, there's no reason they should degrade over time unlike analogue media.