[FFmpeg-user] Decklink with ffmpeg audio desync

Marton Balint cus at passwd.hu
Mon May 16 22:15:44 CEST 2016

On Mon, 16 May 2016, Dpto. Datos Television Costa Blanca wrote:

> El 14/05/2016 a las 2:44, Marton Balint escribió:
>>> I've removed -re from the command, but if that doesnt help, how can I
>>> fix the desync? I have no way of "not having" the NO INPUT SIGNAL ->
>>> Also, when the signal interrupts I see in the stream the colour bars. Do
>>> they come from ffmpeg or from the decklink card?
>> Bars are coming from ffmpeg, when no input is detected. Apparently
>> your inputs are not synchornized... Can you try to feed synchronized
>> signal to ffmpeg to rule out this, as a probable source of A-V desync?
> The quick answer is no, I cant feed with sync. One of the sources is a 
> complex PC VGA to S-VIDEO to SDI (Yes, 3 converters, dont ask) and they 
> dont have sync in/out.
> We have a blackmagic SDI to Analog A/V that works perfectly (Never get 
> desync), but with the quality loss of the Analog A/V so is a discarded 
> solution.
> While Im looking on how to sync the inputs, is there any other way to 
> fix this? Maybe telling ffmpeg to ignore any time code and just encode 
> whatever get in?

The problem may well be that when the decklink card loses input, it 
provides video frames only but no audio. I am afraid you have to dig into 
the code, to figure out what is really going on. You can try to modify it 
so it only adds a video frame to its packet queue if audioFrame is not 

You may also try using newer decklink drivers from blackmagic...


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