[Vorbis-dev] Splitting Ogg Vorbis file

[xiphmont at xiph.org]

On Sun, Jun 28, 2009 at 6:06 AM, Conrad Parker<conrad at metadecks.org> wrote:
> 2009/6/28 ogg.k.ogg.k at googlemail.com <ogg.k.ogg.k at googlemail.com>:
>>> So my questions are: what does "overlap-add nature of vorbis" means?
>>
>> Decoding a Vorbis packet makes use of the two previous packets
>> (that's also known as 'preroll'). Thus, if you're missing those packets,
>> you'll get different data out (though I don't know *how* different
>> that output would be).
>
> My understanding is that for Vorbis, no data will be produced until
> the third packet is received, so if you're missing the 2 previous
> preroll packets then the output PCM stream will simply be shorter, ie.
> missing the first 2 packets worth of audio (a few hundredths of a
> second).

Two packets are needed to produce audio, not three.  The granule
position lists the first sample that will be produced from the
subsequent two packets.

Monty