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

[Gyan Doshi <ffmpeg@gyani.pro>]

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.

Regards,
Gyan




>

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