[Vorbis-dev] vorbisfile timestamp oddity

[Michael Smith]

On Monday 07 June 2004 14:39, Glenn Maynard wrote:
> I'm seeing some inconsistency in returned timestamps using vorbisfile.
>
> I open a file, read some data (grabbing timestamps with ov_pcm_tell),
> then seek back with ov_pcm_seek(vf, 0) and read it again.
>
> The seek lands back on 0, but on the second read through, there's a gap
> in the timestamps: they jumps up by 2112 frames.  This causes my vorbisfile
> code to insert silence to maintain sync (as if there was a hole), and one
> of my test cases fails: "reading data, rewinding and reading it again must
> always give the same results".  (This also fails if I disable
> hole-padding.)

Sorry for not responding to this before - I was going to, but forgot about it
until your recent followup.

Anyway, as ogginfo says on your file, it's invalid according to the vorbis
spec. Vorbisfile attempts to be liberal in accepting it, but due to the
nature of the corruption, it gets some details wrong when seeking around the
file.

This is very easily dealt with by simply using valid vorbis files - I don't
think this will ever occur with a valid vorbis file, and if it does, that's
clearly a (major) bug, and we'd look into it immediately.

> More specifically, ov_pcm_tell returns 0 after the seek, and 3136 after
> reading 1024 frames.  OV_HOLE is never returned.  (On the first read
> through, it returns 1024 after reading 1024 frames, as expected.)
>
> ogginfo says:
> Warning: Vorbis stream 1 does not have headers correctly framed.
> Terminal header page contains additional packets or has non-zero granulepos

Specifically, this is the problem. The header pages (illegally) containing
vorbis data confuses vorbisfile - when it seeks back to 0, it seeks to the
first possible place for vorbis data to exist (according to the spec), thus
missing the data in the header pages.

If you manage to find a reasonable fix for this specific problem, we'd be
happy to accept a patch, though.

> Vendor: Xiphophorus libVorbis I 20011231 (1.0 rc3)
> Warning: EOS not set on stream 1

This, whilst not ideal, doesn't cause any known problems with vorbisfile.


Mike