[FFmpeg-user] Frame rates and video accelerated

Werner Robitza werner.robitza at gmail.com
Mon Feb 9 19:24:20 CET 2015

On Sat, Feb 7, 2015 at 6:05 PM, Cristiano Chieppa
<cristiano.chieppa at gmail.com> wrote:
> I save the raw frames (NALs) in a file (some frames grabbed from a camera) and I invoke the ffmpeg from the sw - like a batch program - to encode the raw H264 in MP4.
> The conversion works well.

Out of curiosity, how exactly are you doing it? Is the file basically
an Annex B stream?

> Sometimes, under circumstance that I’m not able to understand,  I see that the generated mp4 video is like “accelerated”: the speed is very fast compare with the real stream.
> Is there a way to invoke ffmpeg and tell him, as a parameter, the timestamp of each grabbed frames to get the real synchronized video?

ffmpeg -r 25 -i input output

This is how you force the frame rate.

Actually, when you do "ffmpeg -i input -c:v libx264 output.h264", the
output file indicates the frame rate when you read it back in with
"ffmpeg -i output.h264". And when you force a different frame rate,
with for example "ffmpeg -i input -c:v libx264 -r 12 output.h264",
this frame rate change is visible when doing "ffmpeg -i output.h264".
This is due to the timing information being written into the Sequence
Parameter Set (SPS).

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