Understanding PTS and DTS in video frames

Question

I had fps issues when transcoding from avi to mp4(x264). Eventually the problem was in PTS and DTS values, so lines 12-15 where added before av_interleaved_write_frame funct

Answer 1

1. The time_base is just a unit of measurement. Different units may be used to represent the same times (approximately, if they are not exact multiples). In some cases a container format requires a certain time base and it will be set to that by the muxer. In other cases the container doesn't require a time base but it has a default that you might have to override. I'm not sure about 1/12800 specifically, I know 1/600 is a special value in mp4 spec.

2. The two time bases are the units of measurement of time for the codec and for the container. If using constant fps, the codec unit of measurement is commonly set to the interval between each frame and the next (the duration that each frame gets displayed), so that frame times are successive integers. It doesn't have to be set to 1/fps, however, as long as the pts times are correct in whatever units are used.

3. What you describe is simply what one would have to do to convert from one unit to another. (ie: multiply by old unit, divide by new). A time t in units of a/b can be converted to units c/d as t*(a*d)/(b*c).

4. The dts sequence can start from any value, there is no special significance to dts 0. At start of playback, the difference between wall clock time and the starting dts is computed, and all future dts are converted to wall clock using that. A video stream with dts=-10, -9, -8, ... is perfectly ok. The difference between successive dts is what is used, the absolute values don't matter.