Codecs like AAC, MP3 and Vorbis have an added issue, that is the MDCT. The more cosine transforms you apply to a signal, the worse it gets, no matter the bitrate you use. MPC, in this case, has the advantage of being a subband codec, and therefore, doesn't degrade that much with repeated reencodings.
Repeat with me now, "distortion"
Could it not be possible than to design some sort of method to *insert* the decoded wave, back into the encode process, *after* the "throwing away" / alteration point, so that this wave (which has already undergone this process, in it's original encoding) can now undergo the same lossless compression. There by getting it back to it's "original" mp3 encoded state?
I don't know if they managed it. But in theory it's no harder than cracking an encryption algorithm. Forget the audio processing - think of mp3 decoding as a mathematical process for converting one set of numbers into a larger set of numbers - the task is to reverse the transformation.Now, it may be impractically complicated to do so, but to say it's impossible is foolish - especially when there isn't even any encryption key to crack! And remember - we can get a close result just by re-encoding the .wav, so it's not like you have to start from no knowledge.
IIRC FhG looked at this. You could call it an mp3 un-decoder if you like.
This reply is in response to no one in particular...Be careful saying that things are "impossible".50 years ago, there were a lot of old 78rpm discs around that people wanted to issue onto LPs. But they couldn't get the noise out of them. Or if they did, the results sounded horrible. And there's a problem: you have a valid signal, and added noise. And theory tells you that when the two are mixed together, you can no longer separate them. Where both are essentially unknown, you can't reconstruct either. It is not possible. You can't get back that nice clean audio signal from within all that noise.And then Cedar came along, followed by many others, and built products that declick and denoise old records! How? Is theory wrong? No! But it doesn't matter if the result is mathematically identical to what was originally recorded - it just matters that, after cedar processing, it sounds significantly better than the noisy version.Whether or not you can "un-decode" an mp3, I'm not sure. I'm 100% certain that you can't get from the mp3 back to the original .wav without loss. However, we often say "the information has been lost - it can't be put back". Not perfectly, no. But come 2050, when the only source of some recordings are 128kbps mp3s, I bet you some bright spark has a process which makes them sound a lot closer to the original than the surviving mp3 does.So, my advice is to try to do the impossible. (Though please, don't jump out of your upstairs window thinking you can fly!). Because some people succeed, and they usually make a lot of money out of a product that is "good enough" - much more money than the theorists who say "it is theoretically impossible to do this perfectly".Cheers,David.
Do it enough, you can reduce the complexity of the result set. Sure it's not guaranteed to succeed. But what if you had a probability of sucess of 80%. Than statistically you just run it a bunch of times to guarantee sucess, which still comes in as polynomial.
Now, the problem is that I can not find this paper again...