[FFmpeg-devel] [PATCH] avdevice/decklink: add option to drop frames till timecode is seen

Gyan ffmpeg at gyani.pro
Fri Nov 15 08:46:19 EET 2019

On 15-11-2019 03:02 am, Marton Balint wrote:
> On Wed, 13 Nov 2019, Ilinca Tudose wrote:
>> My previous email was incorrect about "-wait_for_tc 1". The 2 
>> commands used
>> were:
>> (1)
>> ffmpeg -loglevel trace  -timecode_format rp188any  -wait_for_tc 1 -f
>> decklink -probesize 15M -analyzeduration 5M -i "DeckLink Quad (1)" -t 30
>> -preset veryfast output.mov
>> (2)
>> ffmpeg -loglevel trace  -timecode_format rp188any  -wait_for_tc 1 -f
>> decklink -i "DeckLink Quad (1)" -t 30 -preset veryfast output.mov
>> The second command is the one that produced 1 frame offset in 3 of the
>> captures. The first command had 0 frames offset for all files tried.
> These two command lines are only different in probesize and
> analyzeduration. I don't understand how these two options can make any
> difference regarding the TC offset. Can you figure out which one 
> actually causes the issue?

It's not related. Initially,before my patch, they were trying to capture 
TC with format_code set. Because of the lag, the probe limits would 
usually be exhausted before the TC arrived.
With my initial patch in July, which compensated for the lag, extended 
probe was still required. With the current one, it's not.

Ilinca can supply details but the offset occurs infrequently and on 
repeating the capture, would not show an offset. BM Decklink support has 
been unable to reproduce the issue with their capture utility.


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