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

[Gyan Doshi <ffmpeg@gyani.pro>]

On 2022-07-13 06:00 pm, Anton Khirnov wrote:
> 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?

Because they may arise from separate machines e.g. a video 
teleconference with multiple participants on the LAN, conveyed with NTP 
time of start of stream.

Regards,
Gyan
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