[vorbis-dev] thoughts about MP3 -> OGG convertor

[Dirk Knop]

Ahoy List,

finally, after a long time, a new post from me ;)

Linus Walleij wrote:

>On Tue, 18 Dec 2001, John Zitterkopf wrote:
>
>>Why is this a bad IDEA? I take it it's because of the "lossy" nature of
>>both types.
>>
>Its a bad idea because mp3-compression and vorbis compression affect the
>audio in different ways: artifacts from the mp3compression will be mixed
>with artifacts from the vorbis compression.
>
[...]

This is absolutely correct. No additions to make :)

>
>Cliffhanger: I PRESUME it could be possible to convert the kind of
>mathematical information stored in MP3 files to the kind of
>mathematical information stored inside vorbis files, without
>decompressing the audio and recompressing it inbetween, thus creating a
>minimal loss transfer method. If you love maths, transforms and the
>like, by all means have a look at it.
>
I have to disagree.
The decoder is "fixed" in the way it works. All the psychoacoustic 
castrations are done by the encoder. The decoding process is just a iDCT 
of the data created by the encoder, so there is no difference between 
the mathematical data and the decompressed output - it's absolutely the 
same. The "mathematical data" is already lossy - that's the point in 
doing ogg / mp3 /ac3 / anything-compression.
So the data gets "flawed" during encoding, no need to step in at a 
certain point of decompression.
Maybe i got you wrong here, but i think that's what you wanted to say.

(Erm, don't get me wrong, this is no offense or flame!)

Regards,
Dirk

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