[FFmpeg-devel] [PATCH 1/2] avformat/dv: allow returning damaged audio

[Dave Rice]

> On Aug 1, 2020, at 5:26 PM, Marton Balint <cus at passwd.hu> wrote:
> 
> 
> 
> On Sat, 1 Aug 2020, Michael Niedermayer wrote:
> 
>> On Sat, Aug 01, 2020 at 07:28:53PM +0200, Marton Balint wrote:
>>> 
>>> 
>>> On Sat, 1 Aug 2020, Michael Niedermayer wrote:
>>> 
>>>> Fixes: Ticket8762
>>>> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
>>>> ---
>>>> libavformat/dv.c | 49 +++++++++++++++++++++++++++++++++++++++++-------
>>>> 1 file changed, 42 insertions(+), 7 deletions(-)
>>> 
>>> If "dv remux loses sync", then the timestamps should be fixed, not
>>> additional packets should be generated based on previously read packet data
>>> (which is a fragile approach to begin with, e.g. what if the first frame is
>>> the corrupt one?).
>> 
>> Ticket8762 is about stream copy, so if no packets are returned for audio
>> but are for video and just timestamps are updated this would at least on
>> its own probably not work that well.
> 
> If the timestamps are good, a good player should be able to play it correctly, even if audio stream is sparse.
> 
> None of the demuxers generate packets because the timestamps are not continous, I just don't think it would be consistent if DV suddenly started to do this. E.g. what if the user wants to drop video with no audio?

In practice, when dv frames with video and no audio are interleaved within a dv stream that otherwise has both, it is because the playback videotape player of the dv tape is in pause mode or the tape is damaged. These frames most common are filled with only video dif blocks that note concealment (so the image is a copy of a prior image) and the audio source pack metadata is missing, but the paylock of the audio dif blocks are filled with error code so they would decode as silence.

I did a test of 114 dv tape transfers
	61 no difference in demuxing between pass and drop (using this patch)
	31 the difference is only in the final frames so the impact of the demuxer option would be hard for the user to see, no loss of sync, just video with no audio at the end
	22 the difference occurs mid-stream, with the drop option the demuxer outputs video and audio at different rate when hitting frames with no audio source pack, so the output of the demuxer loses sync

>> about the case of a damaged first frame. Do you have a testcase ?
> 
> No, but it can happen, can't it? If the stream starts with no audio for 1 second you will have 1 second A-V desync, as far as I see, that is why I believe fixing the timestamps is the proper fix.

Yes this happens (though it is more rare and didn’t occur in the test noted above). In that case, ffmpeg shows no audio at all and I’d have to read the stream at later frames using -skip_initial_bytes.

> Regards,
> Marton
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