[vorbis-dev] Detecting packet lengths in Vorbis-streams

[Scott Wheeler]

Quoting Erik Montnemery <erik at babes-inc.com>: 
 
> How would one implement the following scheme with minimal use of 
> resources: 
>  
> Every Vorbis packet should be preprocessed to a certain extent, that is, 
> the beginning of every audio packet should be parsed and some decoding 
> steps executed. The result of this predecode as well as the rest of the 
> packet should then be sent as output for final processing. 
>  
> I have skimmed through the standard, and as far as i can tell there is no 
> field containing the packet length, however the 'end-of-packet'-condition 
> is mentioned at severeal places, and it is also checked for at various 
> places in the decoder source code. 
 
The packet lengths are read from the "lacing values" at the end of Ogg page 
headers.  (Also referred to as the "segment_table" in the RFC.) 
 
http://www.ietf.org/rfc/rfc3533.txt 
 
Each Ogg page contains a set of packets, which in the case of a "degenerate 
Vorbis stream" (i.e. a normal Ogg Vorbis file) will all be Vorbis packets. 
 
-Scott 
 
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