[vorbis] bitrate peeling question

[volsung at asu.edu]

On Tue, 28 Aug 2001, Henry Rearden wrote:

> 1) my understanding is that BP allows you to derive a lower-
> bitrate version of an encoded file (say 128kbps) directly from
> a higher-bitrate version (say 192kbps), without having to re-rip
> from a CD or generate an intermediate WAV file.
> 
> this is potentially a huge win for those who want to archive all
> their CDs into high-quality Oggs, and to then use files from that
> same archive on their Ogg-capable portable players (don't i wish)
> when going to exercise, etc. is my understanding of how BP is
> supposed to work correct, or am i missing something?

Yes.  Bit peeling is just intelligently throwing away data from a file to make
it smaller without destroying the audio.  In the optimal case, BP should be
significantly faster than a decode-encode cycle AND should produce audio that
has no more artifacts than if you had encoded the WAV at the lower bit rate to
begin with.

> 2) is BP currently implemented? if so, how do i use it? if not,
> any idea as to when it will be working?

As I understand it (from hanging around #vorbis), the "smarts" for bit peeling
are divided between two places: the encoder and the bit peeler.  The smarter
the encoder is, the dumber the peeler can be.  This is because the encoder can
order data in each vorbis packet so that it is easier for the bit peeler to
figure out what to throw away.  I believe that Monty has worked to make the
encoder smarter in this department, but don't know how much more work there is
to do.  As for the peeling utility itself, no official tool exists, though
Segher has apparently written a quick and dirty test peeler which doesn't do
so well.  I would imagine that once the encoder has been tweaked to produce
bit peeler-friendly oggs, writing such a tool won't be too hard for someone
who understand the structure of vorbis packets.


---
Stan Seibert

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