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

[Anton Khirnov <anton@khirnov.net>]

Quoting Gyan Doshi (2022-07-11 08:46:48)
> 
> 
> On 2022-07-11 12:21 am, Anton Khirnov wrote:
> > 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.
> > You did not answer my question.
> > If both streams use the same clock, then how is offsetting them by
> > different amounts improve sync?
> 
> Because the clocks can be different at different stages of stream 
> conveyance  i.e. capture -> encode -> network relay -> ffmpeg reception.
> As long as both use the same clock at a given stage, they represent the 
> same sync relation but with some jitter in the mix added with each stage.

Why would you send the streams separately and not synchronized before
network transmission?

-- 
Anton Khirnov
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