[FFmpeg-user] Recording and synchronizing from two video input streams and an audio input stream

Evert Vorster evorster at gmail.com
Wed Aug 23 09:25:55 EEST 2017

Hi there...

Since you are doing the recording on a computer, have you given any thought
to starting the three streams at exactly the same time with a script?
Something like:
ffmpeg command1 arguments < /dev/null &
ffmpeg command2 arguments < /dev/null &
ffmpeg command3 arguments < /dev/null

The "&" instructs bash to background the run command, and start executing
the next one.

and the < /dev/null is to stop ffmpeg grabbing keyboard inputs.

FYI, stopping the recording should be as simple as pressing Ctrl-C on the
last running ffmpeg, and all the streams should terminate at the same time,

I hope that helps?
KInd regards,

On 22 August 2017 at 14:20, Kim Thomassen <kim at sportcaster.dk> wrote:

> I have two IP cameras and a USB microphone, and basically, I need to
> record/sync up these three devices to an accuracy of about a millisecond,
> though perhaps I can get away with them being a couple of milliseconds out
> of sync, and write them to a single output file. I need them to be so close
> to each other because they will later be connected into a single panorama
> output.
> The cameras themselves are some cheap no-brand ones, recording in 4k, and
> they can't keep track of time, so I have to do all the managing on my
> machine. The timestamps I get from them are all wrong.
> I early on experimented with just getting my streams through one ffmpeg
> command;
>     >ffmpeg -i rtsp:// -map 0 -y -t 13 "file-202.ts" -i
> rtsp:// -map 1 -y -t 13 "file-203.ts"
> However I wasn't able to get them to synchronize anywhere near close
> enough, so I abandoned this setup pretty quickly.
> What I'm attempting at the moment, is to use the "at" utility, to schedule
> the two video streams to start recording at the same time, and I then
> filter them through vstack to test the synchonizity, and while I do get
> pretty close, it's sometimes in the wind wether the recording is actually
> in sync.
> The command I use to record is, located in a record.sh script;
>     >ffmpeg -thread_queue_size 512 -rtsp_transport tcp -i
> rtsp://192.168.0.$THISIP/av0_0
> -y -t $TIME "$THISIP.ts"
> And pertinent parts of the connector script is;
>     >for ((n=0;n<$NOCAMERAS;n++))
>     >  do
>     >    at now +1 minutes <<< "$PWD/record.sh $THISIP $TIME" &
>     >  done
> And the part of the connector script which stacks the videos atop each
> other;
>     >ffmpeg -i "202.ts" -i "203.ts" -y -filter_complex
> "[1][0]scale2ref[2nd][ref];[ref][2nd]vstack" -c:v libx264 -crf 23 -preset
> veryfast "mixer-$NOW.mp4"
> I'm not excactly completely satisfied with the result but as it's somewhat
> close, I've started on also doing the audio recording;
>     >ffmpeg -f alsa -i hw:2 -rtsp_transport tcp -i rtsp://
> -y -c:a aac -vcodec copy -t 12 "VidAud.mp4"
> But this gives horrible results, the audio/video streams are sometimes out
> of sync by up over a second.
> I wanted to incoorporate the audio recording into the connector script, but
> I'm not sure that I have the right direction on solving this problem
> anymore, and I was hoping maybe someone had some input.
> Please let me know if you need me to expand on anything.
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Evert Vorster
Isometrix Acquistion Superchief

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