[vorbis] streaming

[Beni Cherniavsky]

Vincent wrote on 2003-09-26:

> you said:
> if it encodes something simpe, it drops the bitrate ?
> the other compression formats like Mp3 or VQF do the same isn't it?
>
mp3 originally didn't - the first encoders were all CBR (actually a
little ABR, MP3 has something called a "bit reservoire" IIRC).
Later, VBR mp3 became popular; I'm not sure whether it was directly
allowed by the mp3 format or was an extension of it.  I think it was
allowed but many decoders weren't ready for it (especially w.r.t.
seeking and length calculation).

No idea about VQF.  I'd guess it did VBR from the beginning.

> This doesn't explain me the story about wich technic is being used by ogg.
> it must have its own technic to suit the streaming needs, and this technic
> must be different from the other codecs, but what is this technic?
>
The isn't any single technique.  It just avoids many mistakes that
other codecs made and has some unique features:

- Vorbis never encouraged anybody to assume it's CBR.  Thus there are
  no broken decoders to cope with (except for plugins for frameworks
  like ACM an QT that are completely or partially broken).

- Vorbis is wrapped with the Ogg container which was disigned with
  streaming in mind.  It handles framing, checksumming and seeking
  elegantly.

- Vorbis files can be simply concatenated and the result is a
  valid stream according to the spec.  This simplifies streaming
  a lot in some cases.

- Vorbis files can be cut with sample precision, so that you can
  concatenate arbitrary pieces without gaps/clicks.

- Vorbis is designed to allow "peeling": if you truncate packets, you
  still get a legal Vorbis stream but with lower quality.  This should
  allow very effecient streaming of multiple bitrates from the same
  file (encoded at the highest bitrate).  This works but
  unfortunately, the current encoder and peelers are not tuned well
  enough - the quality you get from peeling is much lower than if you
  you directly encoded at the same rate.  So usable peeling is
  vapourware so far.


-- 
Beni Cherniavsky <cben at users.sf.net>
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