[FFmpeg-devel] [PATCH] added parameter to dash encoder for start available time

[Jerome Berclaz]

Hi all,

Thanks for the comments on my patch. I should have given more explanations,
but this is my first time using git-send-email and I wasn't able to add
text to the patch.

The reason for the patch occurred while trying to live stream a Dash feed
with low latency. I did not use a timeline, hence having a precise start
time is crucial, otherwise the player will try to download chunks at the
wrong time. In the current implementation (branch 4.2) the start time is
set to the system time, which may not correspond exactly to the live feed
start time. This new parameter allows the user to pass the exact value for
the start time.

I've seen that the dash encoder has been refactored in branch 4.3, so I
don't know if this change is relevant anymore. The patch was intended for
branch 4.2.

Thanks,
Jerome

On Thu, Mar 18, 2021 at 9:20 PM Gyan Doshi <ffmpeg at gyani.pro> wrote:

>
>
> On 2021-03-19 03:22, Jan Ekström wrote:
> > On Thu, Mar 18, 2021 at 5:07 PM Jan Ekström <jeebjp at gmail.com> wrote:
> >> On Thu, Mar 18, 2021, 17:01 Gyan Doshi <ffmpeg at gyani.pro> wrote:
> >>>
> >>>
> >>> On 2021-03-18 20:12, Jan Ekström wrote:
> >>>> So right now we do have an option in ffmpeg.c called itsoffset. It
> >>>> lets you configure the input offset in time (seconds - such as
> >>>> "1337.123" or time format - such as "00:00:01.123"). The only problem
> >>>> with it right now is that unless you set -copyts it will not get
> >>>> applied for anything else than subtitles, as the offset gets applied
> >>>> before and is not taken into account in the "start from PTS 0"
> >>>> calculation for video/audio streams.
> >>> Not the case.
> >>>
> >>>       ffmpeg -f lavfi -itsoffset 3 -i nullsrc=r=1:d=3 -vf showinfo -f
> null -
> >>>
> >>> gives
> >>>
> >>> [Parsed_showinfo_0 @ 0000019cc1b88540] n:   0 pts:      3 pts_time:3
>  ...
> >>> [Parsed_showinfo_0 @ 0000019cc1b88540] n:   1 pts:      4 pts_time:4
>  ...
> >>> [Parsed_showinfo_0 @ 0000019cc1b88540] n:   2 pts:      5 pts_time:5
>  ...
> >>>
> >>> I've used it many times to adjust sync among streams.
> >>>
> >>> Regards,
> >>> Gyan
> >>
> >> Very interesting!
> >>
> >> I will have to retest, I just recall that the last time I tried it got
> applied when there were multiple streams in an input when the start_time
> based adjustment was done.
> > Right, so taking in a Matroska file it works fine, but when taking in
> > an MPEG-TS file with a start_time it not only transforms (start_time +
> > input_ts_offset) to (input_ts_offset), but just to 0 :) .
> >
> > Example:
> > ffmpeg -v verbose -debug_ts -itsoffset "1616090389" -i
> > 'https://megumin.fushizen.eu/samples/switchingaudio.ts' -c copy -bsf:a
> > aac_adtstoasc -movflags dash+frag_discont+delay_moov -vframes 5 -f mp4
> > - > test.mp4
> >
> > Probed start_time:
> >    Duration: 00:00:15.25, start: 19516.478644, bitrate: 23823 kb/s
> >
> > And how ffmpeg.c then adjusted it:
> > demuxer -> ist_index:2 type:audio next_dts:NOPTS next_dts_time:NOPTS
> > next_pts:NOPTS next_pts_time:NOPTS pkt_pts:1756483078
> > pkt_pts_time:19516.5 pkt_dts:1756483078 pkt_dts_time:19516.5
> > off:1616070872521356 off_time:1.61607e+09
> > demuxer+ffmpeg -> ist_index:2 type:audio pkt_pts:0 pkt_pts_time:0
> > pkt_dts:0 pkt_dts_time:0 off:-19516478644 off_time:-19516.5
> >
> > Thus, ffmpeg.c is not only adjusting the timestamp to start at zero
> > with the start_time, but also takes the itsoffset off as well.
>
> This affects formats with AVFMT_TS_DISCONT flag, ffmpeg checks for a
> discontinuity and readjusts the file offset to make output timestamps
> smooth.
>
> Add -dts_delta_threshold 1616090389 alongside itsoffset to prevent this
> readjustment. Perhaps, both the itsoffset and ts_scale adjustments
> should be moved to after the discontinuity checks.
>
> Regards,
> Gyan
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