[Bug 18571] Note about Vorbis appears incorrect

https://www.w3.org/Bugs/Public/show_bug.cgi?id=18571

--- Comment #5 from Aaron Colwell <acolwell@chromium.org> 2012-08-17 23:57:15 UTC ---
(In reply to comment #2)
> Oh right, we don't get to choose how the frames align, which has nothing to do
> with what I quoted...
> 
> http://xiph.org/vorbis/doc/Vorbis_I_spec.html says that the frame size is 64 to
> 8192, which ought to mean that if one cuts an arbitrary sample the worst case
> mismatch is 4096 samples? That's a rather big gap to smooth over, is that
> something you see in practice?

Most of the Vorbis content I've seen tends to keep frame durations around ~23ms
(ie 1024 @ 44100). My guess is that the higher numbers are for higher sample
rate content.

> 
> Why is this not a problem for other formats?
> http://en.wikipedia.org/wiki/Advanced_Audio_Coding says that AAC has variable
> frame length.
I believe this is referering to the number of bits in the coded frame not the
number of samples output per frame. MP3 had a fixed set of coded frame sizes
which AAC does not. I'm pretty sure both MP3 & AAC output a fixed number of
samples per coded frame though. There may be counter examples, but in my
experience the Xiph codecs (Vorbis, Opus?) tend to be the only ones that have
varying output sample counts.

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Received on Friday, 17 August 2012 23:57:19 UTC