[FFmpeg-trac] #4924(ffmpeg:new): PTS/DTS timestamps get broken when concatenating MPEG-TS files using -f concat

FFmpeg trac at avcodec.org
Mon Oct 12 17:55:51 CEST 2015

#4924: PTS/DTS timestamps get broken when concatenating MPEG-TS files using -f
             Reporter:  jsnajdr  |                     Type:  defect
               Status:  new      |                 Priority:  normal
            Component:  ffmpeg   |                  Version:  unspecified
             Keywords:           |               Blocked By:
             Blocking:           |  Reproduced by developer:  0
Analyzed by developer:  0        |
 == Steps to reproduce: ==
 1. Download two *.ts files and a list.txt from this Dropbox URL:
 2. Concat them with this command:
 {{{ffmpeg -f concat -i list.txt -c copy output.ts}}}
 3. Look at the PTS/DTS timestamps of the output.ts file:
 {{{ffprobe -show_entries packet=codec_type,stream_index,pts,dts

 == Expected result: ==
 The timestamps are in a continuous range and the transitions from one
 chunk to the next are seamless.

 == Actual result: ==
 The timestamps from the first chunk are OK, but at the start of the
 second, they start to be shifted by a big time interval backwards, which
 causes a timestamp wrap by 2^33. This causes a huge discontinuity and a
 resulting file duration of 26 hours instead of 6 seconds.

 Fixing the delta-computing algorithm in
 libavformat/concatdec.c:concat_read_packet fixed the issue for me (see the
 patch below). But I suspect it doesn't work in the general case. My case
 is special in that the timestamps are not-overlapping, following one
 sequence across files. Concatenating files where timestamps always start
 from zero probably won't work. I don't know how to fix it, the timestamp
 shifting/scaling logic is very complex and confusing.

 == Used software: ==
 ffmpeg bleeding-edge from Github master. The issue is new in 2.8 - it
 didn't happen in 2.7.

 == Proposed patch: ==
 diff --git a/libavformat/concatdec.c b/libavformat/concatdec.c
 index 832b7f4..1151b10 100644
 --- a/libavformat/concatdec.c
 +++ b/libavformat/concatdec.c
 @@ -580,7 +580,7 @@ static int concat_read_packet(AVFormatContext *avf,
 AVPacket *pkt)
             av_ts2str(pkt->pts), av_ts2timestr(pkt->pts, &st->time_base),
             av_ts2str(pkt->dts), av_ts2timestr(pkt->dts, &st->time_base));

 -    delta = av_rescale_q(cat->cur_file->start_time -
 +    delta = av_rescale_q(cat->cur_file->file_start_time -

Ticket URL: <https://trac.ffmpeg.org/ticket/4924>
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