For iTunes maybe this may help. AFAIK it tracks music files down by their "digital signature" if memory doesn't fail me, that's how they used to call it a few years back when I used iTunes. There also are some other programs for PC which work in a similar fashion but I don't recall any names ATM.
For iTunes maybe this may help. AFAIK it tracks music files down by their "digital signature" (or something like that) checking them up against a database (Gracenote?). There are also some other programs for PCs which work in a similar fashion, but I don't recall any names ATM.Edit: further clarification
ffmpeg -i input.m4a -c:a copy -f s8 output.raw.aac
faad -a output.adts.aac input.raw.aac
many AAC music files
That said, maybe someone can give input on the following: suppose I do ffmpeg -i infile.m4a -acodec copy outfile.m4a, is there any possibility that the infile might contain headers (say, for gapless playback) which will be lost and a player see them as e.g. different lengths? mp3 files might have misleading length information ...
As long as the same decoder is used it should work. Changing decoders will possibly change the hashes for some formats, particularly for 24 bit audio or fixed point arithmetic.