[FFmpeg-user] ffprobe -show_frames and coded_picture_number

[thljcl]

What you are saying is that some sources do not have constant frame rate of
24000/1001 FPS despite having the “average frame rate” close to 24000/1001
FPS, thus speeding up the audio would cause the video and audio to be out of
sync.
The truth is, to me, regardless of the original frame rate of the source, I
do not need to speed up or slow down the audio in order to keep the audio
and video in sync. I do understand that speeding up the audio is the
standard practice being used by television stations in PAL regions to keep
the audio and video in sync from the source of 24 FPS. Such practice makes
sense before digital age; without re-encoding the video/audio, speeding up
the audio would keep video and audio in sync. With ffmpeg, however, there is
no need to change the playback speed to keep the audio and video in sync.
Let me briefly explains how the video filter “fps” works.
Basically, with -vf "fps=25" as an output option, you can change the frame
rate to constant frame rate of 25 FPS; ffmpeg would duplicate frames if the
source is of lower frame rate; ffmpeg would drop frames if the source is of
higher frame rates. In fact, that’s how I create a slideshow which consists
a lot of duplicated frames (slides) as I wrote at
http://ffmpeg-users.933282.n4.nabble.com/Create-slideshow-with-ffmpeg-from-still-images-td4660277.html.
You can read those posts as reference. By duplicating and dropping frames,
there would be minimal change of playback speed, subject to the round-off
error. In practice, there would be no need for the change of audio speed or
to actually know how the frame rate of the source varies over time.




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