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
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