[theora-dev] a new proposal
msmith at xiph.org
Wed Feb 25 21:24:59 PST 2004
On Thursday 26 February 2004 16:14, Matt Zimmerman wrote:
> On Wed, Feb 25, 2004 at 11:57:04PM -0500, Monty wrote:
> > ...sigh. And the confusion spreads like wildfire. We already have
> > perfect sync, Matt.
> Do you mind explaining briefly? As of November, it was my understanding
> that Ogg could only hold media streams with One True Rate which held
> perfectly forever---no dropped frames, no DSP drift, etc. Did things
> change since then, or does the state of the art still deal only with
> absolutely perfect streams?
Ogg stamps each page (not quite per-packet, but per-packet stamps can be
calculated) with a "granulepos". This has a unique mapping to a time value
(which is codec dependent, but that doesn't really matter).
So, given a known codec (e.g. "vorbis, at 44.1kHz"), and any page (pages are
typically no more than 4 kB - so these granulepos timestamps come along
pretty often)), you can trivially calculate an exact time offset for that
This has always been part of the ogg specification - it hasn't changed. Ogg
has never had any sync problems.
So, dropped frames just miss some data - but each granulepos value is an
absolute time offset (not relative to previous packet/page, or anything like
that) - so once you get good frames again, you have a correct timestamp. No
Of course, players can still be broken, and it may be that there are some that
have problems with this (I don't know, I haven't really looked into it) - but
those players are likely to be broken with any/all media formats, not just
ogg, because ogg has all the data neccesary to get this right.
> > Arc, I'll be in the channel tongiht to hash this out for as long as it
> > takes. Until then, I suggest you hold off reinventing the whole world
> > because you don't understand that one that already exists. You're
> > just going to confuse the Hell out of everyone who *hasn't* been
> > participating in the spec discussion.
> Which mailing list is said discussion taking place on?
Much of the discussion is happening on IRC (irc.freenode.net, #theora), and
bits of the logs are in cvs to help explain things. If you want full logs,
ask - someone should be able to send them to you.
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