[FFmpeg-devel] [PATCH v3] ffmpeg: add option -isync

[Anton Khirnov]

Quoting Gyan Doshi (2022-07-10 20:02:38)
> 
> 
> On 2022-07-10 10:46 pm, Anton Khirnov wrote:
> > Quoting Gyan Doshi (2022-07-08 05:56:21)
> >>
> >> On 2022-07-07 03:11 pm, Anton Khirnov wrote:
> >>> Quoting Gyan Doshi (2022-07-04 18:29:12)
> >>>> This is a per-file input option that adjusts an input's timestamps
> >>>> with reference to another input, so that emitted packet timestamps
> >>>> account for the difference between the start times of the two inputs.
> >>>>
> >>>> Typical use case is to sync two or more live inputs such as from capture
> >>>> devices. Both the target and reference input source timestamps should be
> >>>> based on the same clock source.
> >>>>
> >>>> If not all inputs have timestamps, the wallclock times at the time of
> >>>> reception of inputs shall be used. FFmpeg must have been compiled with
> >>>> thread support for this last case.
> >>> I'm wondering if simply using the other input's InputFile.ts_offset
> >>> wouldn't achieve the same effect with much less complexity.
> >> That's what I initially did. But since the code can also use two other
> >> sources for start times (start_time_realtime, first_pkt_wallclock),
> >> those intervals may not exactly match the difference between
> >> fmctx->start_times so I use a generic calculation.
> > In what cases is it better to use either of those two other sources?
> >
> > As per the commit message, the timestamps of both inputs are supposed to
> > come from the same clock. Then it seems to me that offsetting each of
> > those streams by different amounts would break synchronization rather
> > than improve it.
> 
> The first preference, when available, stores the epoch time closest to 
> time of capture. That would eliminate some jitter.
> The 2nd preference is the fmctx->start_time. The 3rd is the reception 
> wallclock. It is a fallback. It will likely lead to the worst sync.

In which cases is this fallback useful? I would expect that all
transports you'd want to use this with will have working timestamps.

-- 
Anton Khirnov