[MPlayer-users] A-V-sync in mencoder

[Ville Saari]

On Sun, Mar 21, 2004 at 11:48:12AM -0500, D Richard Felker III wrote:

> > How come? If the input is constant framerate, the inverse telecine filter
> > uses constant input-output-ratio and the output is constant framerate, then
> > the only source of unpredictability is the phase of the telecine pattern,
> > which can cause a difference of one frame at most.
> 
> ROTFL! The phase is not set once, but hundreds or thousands of times
> during the movie.

That's the magnitude of the number of frames in the movie. If the telecine
pattern were to reset that many times, then we wouldnt have any pattern at
all. In my experience it only happens between a few and a few dozen times
during a movie (of course it shouldn't happen at all if the DVD was authored
right, but such disks seem to be extremely rare).

But the amount of discontinuities in the telecine pattern should not matter.
The filter should still output frames at a constant rate. If we are creating
a 23.976 fps avi from an NTSC DVD, then the filter should output exactly
4 frames for each ten fields read from the DVD (counting the repeated fields
too) no matter what happens with the telecine pattern. If it doesn't do that,
then it distorts the time axis and introduces a need to do something to keep
the audio sync.

If the filter does have constant input-output-ratio, then we can predict
perfectly reliably the mapping between source fields and ripped frames
in those positions where the ratio gives integral results. Between those
positions there's unpredictability of plus or minus one frame at most.

> > And if the inverse telecine filter doesn't use strictly constant
> > input-output-ratio, then it is plain wrong unless the video pipeline
> 
> No, your assumptions about how telecine is done are plain wrong.

My assumption is based on two facts:

1. NTSC signal has constant field rate.
2. Contents of an NTSC DVD are intended to produce that signal.

If you are not going to argue with those, then it should be quite difficult
to disprove that encoding an NTSC DVD to a constant frame rate avi would
require a constant input-output-ratio in the filter chain.

-- 
 Ville