[FFmpeg-user] xfade + acrossfade usage - keeping A/V in sync

[Rob Hallam]

On Wed, 17 Jun 2020 at 11:34, Paul B Mahol <onemda at gmail.com> wrote:

>> ie neither should change the length of the inputs they operate on.
>>
>> Is that a mistaken assumption perhaps?

> Yes, very mistaken.

> Duration of output is first input duration + second input duration -
> duration of transition.
> For xfade you need to set offset, acrossfade does not need that.

D'oh, of course! Mistake on my part, I hadn't had my morning coffee.
Thanks for keeping me right.

What I meant was that they should both result in equal duration,
**if overlap is set in acrossfade**. Is that a safe assumption?

>From the docs:

> overlap, o
>    Should first stream end overlap with second stream start. Default is
enabled.
https://ffmpeg.org/ffmpeg-filters.html#acrossfade

Also, from your earlier suggestion:

> I believe this two should be - 2 seconds instead. Because both
> transition durations for two xfade and acrossfade filters is 2
> seconds.

An offset of -2 produced desynched audio in the second segment (input
1 in the original filtergraph). The closest I can get to synch is an offset
of -1 for the first input and -2 for the second; but it's still a little
off.

I'll try to create an example using generated 1Hz chirps and
'showspectrum'. If I can replicate there I will post a bug as you
suggested.

Cheers,
Rob