[FFmpeg-user] PTS resolution

[Jim DeLaHunt]

On 2021-02-22 18:53, Mark Filipak (ffmpeg) wrote:
> Are these correct?
>
> ffmpeg PTS resolution is 1ms.…

This at least is not correct AFAIK.

The Presentation Time Stamp (PTS) value which FFmpeg associates with 
video frames and audio data is a 64-bit integer. There is an associated 
time base attribute for each video or audio stream, which gives the 
number of seconds between successive values of PTS. This time base might 
be thought of as the resolution of PTS. Thus if you have two PTS values 
pts1 and pts2, then the difference in seconds between them is 
(pts2-pts1)*time_base.

The time base can be represented as a rational number, e.g. 1001/30000. 
In this case, 31 frames might have PTS values of 0, 1, 2, 3, …, 30. The 
difference in seconds between frame 31 and frame 1 is (30 - 
0)*(1001/30000) = 1.001 seconds exactly. The PTS resolution is slightly 
under 1/30 second.

A different media stream might have a time base of of 1001/30,000,000. 
31 frames might have PTS values of 0, 1,000, 2,000, 3,000, …, 30,000. 
The difference in seconds between frame 31 and frame 1 is (30,000 - 
0)*(1001/30,000,000) = 1.001 seconds exactly. The PTS resolution is 
slightly under 1/30,000 second.

FFmpeg does not promise a relationship between the value of 
(pts*time_base) and wall clock time. Some media streams could be 
authored with such a relationship, but FFmpeg does not enforce it.  If 
two media streams have the same time base, and one stream has PTS values 
of 0, 1, 2, 3, and the other has 422, 423, 424, 425, then FFmpeg should 
make the same presentation time calculations for both streams.

This is all based on my limited reading of the source. There are many 
developers on this list who know the source better than I do. Perhaps 
some of them might step in to correct whatever I got wrong.

Best regards,
      —Jim DeLaHunt