[vorbis] Vorbis decoding from scratch and the old MP3->.OGG story (new thread, was: Please confirm your message)

[gtgbr at gmx.net]

Hi,

<p>> I have a question for you.  If I want to write a vorbis client from scratch,
> should it attempt to read the input file from start to finish, or should it
> attempt to start at the end first
> in order to use statistical analysis to optimize the conversion process?  I have
> been playing
> around with oggenc and ogg123 and enjoying the way my music sounds, but I can't
> help thinking about how nice it would be to have a portable device that plays my
> ogg vorbis files.  :)

Uh, well .. getting Vorbis on a portable, non-laptop/-PDA player is a
different matter, I guess. It should be happening not so long from now
on the Neuros Audio Computer, though.

If you want to look at something to start with, check out the Tremor
sources from CVS - this is an integerized decoder, optimized for devices
with low memory and low CPU power.

> In other words, should I "top-post" my input file to the algorithm, or should the
> algorithm
> "bottom-post" it in order to optimize the decompression algorithm?

I'm not a developer, but I know that much: To start decoding a Vorbis
stream, you don't need to know about the end of the stream (only to
calculate some statistics, like stream length and such). All the
necessary information to start decoding is in the three header packets,
that have to be read and processed in order first. If this wasn't the
case, streaming Vorbis would be troublesome, if not impossible. Your
questions should be answered completely in

http://www.xiph.org/ogg/vorbis/doc/Vorbis_I_spec.html

If something in the specs is very unclear or even wrong, documentation
bug reports to http://bugs.xiph.org/ are very appreciated. (Vorbis Core
-> Documentation) People implementing Vorbis from the specs only are
ideal to iron out those things and make Vorbis better!

> Also, does anyone have a recommendation for an existing *nix client for
> "streaming ogg-vorbis" files other than
> xmms, which I like? 

ogg123 is able to play .ogg streams coming via http on the command line,
for example.

> I just used oggasm to convert my entire .MP3 collection to ogg-vorbis.

I hope you are aware that by doing so, you created suboptimal .ogg files
- they won't sound better than your MP3s, maybe even worse. Vorbis can't
magically recreate the audio information that got lost during the
encoding to MP3. Vorbis will even waste bits on trying hard to sound as
close to the source MP3 as possible, with all possible problems, quirks,
blurbs, swirls, etc.

Good advice would be to leave the MP3s as they are and re-rip your music
to Ogg Vorbis, i.e. "simply don't make MP3s anymore".

<p>Moritz
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