[FFmpeg-devel] Experiences in using ffmpeg to transcode broadcast video

[Mika Raento]

Created a feature request to support columnboxing input that changes
aspect-ratio mid-stream.

https://trac.ffmpeg.org/ticket/4065

On 29 October 2014 13:49, Mika Raento <mikie at iki.fi> wrote:

> Tickets for sub2video and async not working with non-monotonic input,
> using -copyts:
>
> https://trac.ffmpeg.org/ticket/4062
> https://trac.ffmpeg.org/ticket/4064
>
> Hm. Lots of my issues seem to relate to using -copyts. I had trouble
> keeping all video and all audio if there were periods where one or the
> other were missing (esp. in the beginning). Maybe I could make this simpler
> if I'd relax that requirement (I don't know how much happier my users are
> if they get audio but no video, rather than nothing).
>
>     Mika
>
> On 29 October 2014 10:38, Mika Raento <mikie at iki.fi> wrote:
>
>> segment muxer PIDs were fixed in
>> https://github.com/FFmpeg/FFmpeg/commit/502fc3b3d4b36015562d19d74f27d0a4ff835c4e
>> by me
>>
>> On 29 October 2014 10:17, Mika Raento <mikie at iki.fi> wrote:
>>
>>> Hi
>>>
>>> Thanks for the feedback.
>>>
>>> I'll try to capture at least some of these in tickets, and reply on this
>>> thread as I go along.
>>>
>>> Async not working with damaged audio input is
>>> https://trac.ffmpeg.org/ticket/2693 (existing ticket)
>>>
>>> Sub2video not working with negative timestamps is
>>> https://trac.ffmpeg.org/ticket/4062 (new ticket)
>>>
>>>     Mika
>>>
>>> On 27 October 2014 01:00, Michael Niedermayer <michaelni at gmx.at> wrote:
>>>
>>>> On Wed, Sep 24, 2014 at 10:43:40AM +0300, Mika Raento wrote:
>>>> > Dear all
>>>> >
>>>> > This mail is meant mainly as a note to other potential users, but
>>>> > possibly as input to development - time allowing I might be able to
>>>> pick
>>>> > up some of the pieces myself. It's also a thank-you for all the hard
>>>> > work in ffmpeg.
>>>> >
>>>> > I've successfully implemented a transcoding pipeline for producing
>>>> > multi-bitrate fragmented mp4 files from broadcast DVB input. More
>>>> > concretely, I'm taking in broadcast TS captures with either mpeg2
>>>> video
>>>> > and mp2 audio (SD, varying aspect ratio) or h264 video and aac audio,
>>>> > both potentially with dvbsub. From that I'm producing ISMV output with
>>>> > multiple bitrate h264 video at fixed 16:9 aspect ratio and multiple
>>>> > bitrate aac audio, with burned subtitles. The ISMV outputs are
>>>> > post-processed with tools/ismindex and with the hls muxer.
>>>> >
>>>> > There are number of limitations in ffmpeg that I've had to work
>>>> around:
>>>> >
>>>> > - I haven't gotten sub2video or async working without reasonably
>>>> >   monotonous DTS. Broadcast TS streams can easily contain backward
>>>> jumps
>>>> >   in time (e.g., a cheapo source that plays mp4 files and starts each
>>>> file
>>>> >   at 0). The fix is to cut the TS into pieces at timestamp jump
>>>> locations
>>>> >   and using '-itsoffset' to rewrite the timestamps and then
>>>> concatenate.
>>>> >   I'm using the segment and concat muxers for that.
>>>> > - Sub2video doesn't work with negative timestamps, so I use
>>>> '-itsoffset'
>>>> >   to get positive timestamps
>>>> > - For HD streams, I need to scale up the sub2video results from SD to
>>>> >   HD. Sub2video doesn't handle the HD subtitle geometries. I'm not
>>>> >   enough of an expert to know whether that's the issue, or whether
>>>> that's
>>>> >   just the way it's supposed to work with SD subs (typical) with HD
>>>> video.
>>>> > - For columnboxing, I use the scale, pad and setdar video filters.
>>>> These
>>>> >   work fine, but their parameters are only evaluated at start, so I
>>>> need
>>>> >   to cut the video into pieces with a single aspect ratio first and
>>>> >   concatenate later.
>>>> > - Audio sync (using aresample) gets confused if the input contains
>>>> >   errors, so I need to first re-encode audio (with '-copyts') and only
>>>> >   after that synchronize.
>>>> > - The TS PIDs are not kept over the segment muxer, so I given them on
>>>> >   the command line with '-streamid'.
>>>>
>>>> a bit late reply but
>>>> bug reports or feature requests for all these are welcome unless there
>>>> are already tickets for them assuming these issues still exist
>>>>
>>>> thx
>>>>
>>>> [...]
>>>>
>>>> --
>>>> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>>>>
>>>> Why not whip the teacher when the pupil misbehaves? -- Diogenes of
>>>> Sinope
>>>>
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>>>>
>>>>
>>>
>>
>