[FFmpeg-user] Determining how FFMPEG calculates start time for MPEG-2 files
htessmann at control-tec.com
Sat May 27 00:03:51 EEST 2017
I'm trying to understand how ffmpeg calculates the start time for an
MPEG-2 file containing one video track and one audio track. For
example, when I run ffprobe, I get this output:
Input #0, mpeg, from 'file.mpg':
Duration: 00:00:11.06, start: 579.376000, bitrate: 10704 kb/s
As a guess, I looked at the system clock reference in the first pack
header and the PTS/DTS values in the first PES header. When I
calculate those values by hand, or examine the same file in Wireshark,
I get a value that differs from the one calculated by ffmpeg. The
first pack header gives me a system clock reference of 578.4, while
the first video PES header gives PTS = DTS = 579.4. If I look at the
first audio PES header, I get 579.375655555556, which is so close that
it may be just a matter of doing math in the wrong mode (integer vs.
floating-point). From looking at those values, I would guess that
ffmpeg takes the minimum value from all the PES headers, but I'm not
confident about that.
I see in av_dump_format() the lines that calculate & output seconds
(ic->start_time / AV_TIME_BASE) and microseconds (% instead of /), and
am working backwards from there. Any information or pointers on where
to look would be appreciated.
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