[vorbis] Technical MP3->OGG transcoding question

[Ross Vandegrift]

On Thu, Dec 05, 2002 at 12:10:10PM -0600, Seth de l'Isle wrote:
> > > CAN it be done in a better way than Poor Man's Transcoding?
> 
>   "Is there information, relevant to audio that is lost in the MP3 -> WAV
>   step."

Of course there is, as you identify.  Shame that's not relevant.

WAV -> MP3: causes audio loss of type 1.
WAV -> Vorbis: causes audio loss of type 2.
MP3 -> Vorbis: now you have loss of type 1 and 2.  There's simply no way
to avoid this, as the MP3 and Vorbis algorithms aren't equivalent.

> For example, if we know that a sample contains only voice, then we can use a
> voice only format to achieve a better quality/compression ratio than if we
> didn't know that the sample would contain only voice.

Yep, you'll still loose extra data.

> So I'm confident that at least at a silly, pathological level trans-coding could
> be improved over MP3 -> WAV -> Vorbis.

If you were willing to implement a transcoder, for each MP3 encoder
implementation, at each possible quality level, you might be able to
reasonably identify compression artifacts and take steps to avoid
compounding them.  Since each encoder optimizes very specificly in their
own way, at each quality level, you've got one nightmare of a job ahead
of you.

Part of the problem is the type of sounds that lossy audio compression
is bad at compressing - one of those is compression artifacts.  Try
encoding WAV -> MP3 -> WAV -> MP3 and you'll see it.  The time I tried
this to fit some songs onto an old Rio, it sounded exponentially worse
to me.


-- 
Ross Vandegrift
ross at willow.seitz.com

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