[FFmpeg-user] Decklink output latency, high

Marton Balint cus at passwd.hu
Sat May 9 22:21:06 EEST 2020

On Sat, 9 May 2020, Martin wrote:

> Hi!
> Anybody have an idea why I get a couple of seconds of latency when
> outputting to a decklink device? I
> ffmpeg -fflags nobuffer -i srt://x   -f decklink -pix_fmt uyvy422 -c:a
> pcm_s16le -ar 48000 -ac 2  "UltraStudio SDI"
> Input is a TS stream SRT or UDP, input type does not r matter. Im getting
> almost 4 seconds of delay on the SDI output compared to the stream. Seems a
> bit high? One would expect a couple of frames. I have seen this question on
> a few places but never seen an answer or solution.
> Decklink driver 10.11.4, Windows
> I only see this behaviour in ffmpeg not in other programs that uses
> decklink output, Premiere, OBS etc.

Latency comes from multiple sources:

1) The most significant is probably input probing. You can limit this with 
-probesize and -analyzeduration.

2) decklink preroll - you can set this with -preroll, but a very low value 
will cause choppy output.

3) other buffering - I am sure there are other places :)

By setting 1) and 2) you can probably can reduce the latency, but I'd 
think it will still be higher than special purpose software.


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