[Vorbis-dev] Lacing Values

[Michael Smith]

On 6/2/05, Christopher Egner <disciplezero-nl at syrrax.com> wrote:
> I noticed that, when decoding an ogg vorbis file that was encoded with
> the xiph library, that the comment header and setup header are encoded
> on one page. Okay, the vorbis documentation says you can do this, no
> problem. My question is, the lacing values seem to indicate where the
> packet boundaries for the two of these are, is this required, or is this
> just a hint?
> 
> Further, I'm seeing that the lacing values for audio packets are rather
> wild. A hex string looks like: 9D 9893 FFFF 08FF FF15 5352 514E 5595
> 8F92 FFFF 1BFF FF11 FFF8 FF
> 
> Thats a set of lacing values for one page. Is this also giving me hints
> (or perhaps an absolute) as to where to find packet boundaries?

The lacing values are the only possible way to find packet boundaries
in ogg vorbis (in general, there are some common cases where you could
do it on the raw vorbis data, but it is wrong to do so). So, it's
telling you precisely where the packet boundaries are, and it's
neccesary to use that information.

The lacing values you see are just the result of VBR encoding (plus
the fact that vorbis uses two block sizes), seeing them vary widely is
normal.

Mike