[vorbis] Extracting sample count from a Vorbis packet

[Aaron Colwell]

Thanks for the detailed info. It looks like doing this will be a little 
more involved then I originally thought, but it isn't too bad. I'll give 
this a try.

I'll use vorbis-dev for further questions. From the description of the 
list I figured my question wasn't appropriate there since I'm not 
specifically working on the Xiph code.

Thanks,
Aaron

On Wed, 5 Nov 2003, Ralph Giles wrote:

> On Wed, Nov 05, 2003 at 12:31:30AM -0800, Aaron Colwell wrote:
> 
> > I was wondering if there was a way to determine the number of samples in a 
> > Vorbis packet without having to decode the whole thing? The reason I'm 
> > asking this is because I'm thinking about implementing the current Vorbis 
> > RTP draft for Helix.
> 
> Oh yay!
> 
> >                       The problem is that the Ogg pages only contain a 
> > single timestamp. If I want to split up the Vorbis packets on different 
> > boundries than the Ogg page boundries, I'll need to know the number of 
> > samples associated with each packet so I can generate proper timestamps. 
> > Can anyone help me with this?
> 
> This isn't too bad. Each vorbis packet encodes either a short or long 
> block of audio. The sizes of the two block types are stored in the info 
> header (the first packet in the stream) as two 4-bit power-of-2 
> exponents at byte offset 27. See section 4.2.1 and 4.2.2 of the spec 
> for parsing details.
> 
> You can then look at the header of each audio packet to determine which 
> block type it is. See section 4.3.1 for details. Unfortunately, you also 
> need the mode count from near the end of the the setup header (the third 
> packet in the stream, sections 4.2.4.6 and 4.2.4) to get the bit offset 
> of the blocktype field.
> 
> While that gives you the length (in samples per channel) of each block 
> type, that's not how far decoding advances with each packet because of 
> the window overlaps (block lapping) with the next and previous packets. 
> This is a little complicated to describe, so I'll just refer you to the 
> picture at 1.3.2.3. Each packet advances you from the start of the 
> overlap with the previous block to the start of the overlap with the 
> next block. For convenience, the size of the next and previous blocks 
> are also encoded in each audio packet header. (Again see section 
> 4.3.1.) This means that in seeking you can 'work backwards' from the 
> granulepos of the current page pretty easily without having to look at 
> the previous page.
> 
> Also note that the beginning and end of an ogg-encapsulated vorbis 
> stream can be trimed by adjusting the sample numbers given in the page 
> granulepos. You might want to take this into account as well. See 
> appendix A.2 in the spec for some notes on this.
> 
> Anyway, hope that's enough to get you started. The spec is available 
> online as http://xiph.org/ogg/vorbis/doc/Vorbis_I_spec.html (or .pdf) if 
> you don't have a local copy. And please feel free to ask if you have any 
> further questions. We'd really like to see a vorbis rtp implementation.
> 
>  -r
> 
> BTW, vorbis-dev might be more appropriate for technical questions of 
> this sort.
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