[wplug] MP3 or OGG-Vorbis recovery tool?

K G kuzman at sccs.swarthmore.edu
Mon Oct 27 15:24:43 EST 2003


On Mon, Oct 27, 2003 at 02:26:22PM -0500, 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.

What I understand is that you do a Fourier transform (i.e. you convert
the data to something more like (x frequency, y amplitude) datapoints.
This results in incredibly good sound, but very little compression. So
then what you do is discard some of the datapoints (like if you have a
high amplitude wave and a low amplitude at a similar frequency, people
are probably not going to hear the quieter wave). Anyway, that's where
you loose data. Then this is compressed using a lossless compression
algorithm. 

To answer the original question, I don't know. If you reencode the wave
files, the question becomes "will the algorithm drop more data?". I think
that the answer is using the same algorithm, it shouldn't drop much (but
might drop some). One of my professors did an experiment with a few
photographs and jpeg and the result was that there were a few extra
pixels wrong on the second encoding (I don't remember the resolution,
but it was more than 1024x1024). 

Kuzman
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