[Vorbis-dev] Re: Vorbis bistream definition / separation from ogg

[Michael Smith]

Hi Nico,

The vorbis specification has the complete definition of the bitstream
format. It also explains how this bitstream gets embedded in ogg, but
that's a seperate section, you can ignore it if you want.

Note that from the raw vorbis bitstream, you CANNOT isolate all frame
boundaries. You must explicitly provide that information in another
layer (typically, ogg is used for this, but you could use anything
else). The spec will give you the other information you want here.

Within a given stream, vorbis frames are always one of two sizes. What
those two sizes are is information given in the primary vorbis header
(though to figure out which one a particular block uses, you need info
from the setup header as well - I guess that answers that part of your
question).

There are many disadvantages to using only a single block size (number
of samples per frame). You are incorrect in thinking that things like
mp3, aac, etc. have constant frame sizes - like vorbis, they use two
frame sizes (at least mp3 does, the others definitely use more than
one, but I'm not certain that it's two).

Mike



On 4/28/05, Nico Sabbi <nsabbi at tiscali.it> wrote:
> Hi,
> I have some questions:
> 
> 1) in the sources of the encoder_example (and of course in oggenc.c, 
> too) I see a lot of dependance
> on ogg. I want to get totally rid of ogg and use the vorbis bitstream 
> alone.
>  From this reason stems the second question:
> 
> 2) is there a definition of the vorbis bitstream somewhere? I don't need 
> to know every single
> detail, just the informations necessary to isolate frame boundaries, 
> frame size, and frame duration in ms.
> 
> 3) does vorbis always use a variable number of samples per frame? If so, 
> is there a way to know
> from every frame how many samples are used? Is there any disadvantage at 
> using a constant
> number of samples per frame (as in mpeg audio, aac, musepack and so on) ?
> 
> Thanks,
> 
>     Nico
> 
> 
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