[Libav-user] How to perform drift compensation in MPEG-2 recorder

"René J.V. Bertin" rjvbertin at gmail.com
Thu Dec 20 18:36:36 CET 2012

On Dec 20, 2012, at 11:12, Mike Versteeg wrote:

> If this does not introduce any other problems, like in editing
> software, then yes it should work. Unfortunately I'd still need a
> trick to speed up.

Speeding up (probably) means reducing the duration of the preceding frame - which seems easy enough to accomplish with a simple caching scheme.

> No. The MPEG-2 encoder seems very picky (possibly part of the
> protocol) when it comes to that. In fact a time base other than 1/25
> or 1/30 and it either returns an error or smacks me with an AV. Won't
> even do 29.97 fps or 50 or 60 fps.

I'm not talking about frame rate or time base. I'm talking about the unit in which presentation durations are encoded, a factor that makes it possible to encode a continuous variable (time) on an arbitrary fixed point scale that's most suited to the required frame rate. A 1/25 time base (25Hz, 0.04s) would be amount to a frame duration of 24 'ticks' with the standard (QuickTime) timescale of 600/s, or 100 with a timescale of 2500/s . You don't see any difference between the two when watching a video, but with the latter timescale one has more precise control over individual frame timing. I doubt things work differently in libav.


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