[wplug] MP3 or OGG-Vorbis recovery tool?

[Benjamin Slavin]

I understand what you're saying, but still believe that it would be
possible (if computationally infeasible) to reverse the process.

Your JPEG example is one I had considered before my message, and I had
come to the conclusion that the file, when saved (even at 100%) is not
identical to the original image (which could be, say, PNG or TIFF). I
just ran a test, and even the color was changed there. The problem I see
is that it's identical to re-encoding a decoded MP3.

It is highly possible (and somewhat probable) that there's header
information in an MP3 file which would be difficult to recreate the
second time around. If not, perhaps a "watermarking" feature could be
used to add the information to the waveform in a way that would not
affect the audio quality, but would allow for a fast re-compression.
This would require tools to be specifically written, but in light of my
current situation, they'd be highly valuable.

--Ben

On Mon, 2003-10-27 at 14:26, Bill Moran wrote:
[...]
> Not really.  .wav files are made by sampling the waveform at fixed
> intervals (44khz, right?), whereas mp3s use more complex mathematics
> to store the data.  I don't have a great understanding of how the mp3
> math works, but I've been lead to believe that it's similar in concept
> to how jpeg files store images.  If that is correct, here is what your
> circumstance is:
> 
> You start with a .wav (whether it's straigh from a CD, or in a .wav file,
> it's in the same format) and you convert it to mp3 by changing the data
> format and discarding some details that you'd never hear anyway, this
> results in excellent compression.  Now you convert the mp3 back to .wav
> format.  The .wav is exactly the same waveform representation as the
> mp3, but is not the same as the original .wav.
> 
> At this point, logic would seem to tell you that if you saved the second
> .wav as an mp3, but set the quality to maximum, you would get an exact
> duplicate with the same compression ratio ... however, in the case of
> jpeg files, this is not the case.  If you follow the same process (going
> from bmp to jpg and back) you get an image of equal quality, but much
> poorer compression.  If you compress the image any further, you will
> lose _some_ quality.
> 
> Don't ask me why this is.  It seems to go against all the mathematics I
> understand, but I would assume the case is similar with mp3s, since they
> are basically jpegs for sound.
> 
> I think what happens is this:  In the case of an mp3, the compressed
> waveform itself is not the only data stored, also stored is information
> about what was done to make the waveform compress so well (which
> information was discarded) but the .wav is not able to store that second
> bit of information, so it's lost when you repeat the process.