On 12/26/24 8:42 PM, Andrew Strachan wrote:
Hi,
I am please looking for some advice. live encoding from a decklink card to
MediaMTX (on windows) then using ffmpeg to save the live stream to file. I need
to be able to independently stop and start the saving of the file, that's why I
am using this setup. We are also going to send the live stream to a CDN, via
another instance of ffmpeg.
Here is my current command to encode;
ffmpeg.exe -rtbufsize 2G -re -format_code Hp30 -f decklink -i 35:00000000:00230600
-framerate 30 -video_size 1920x1080 -pix_fmt uyvy422 -vcodec libx264 -preset veryfast
-x264-params "keyint=120:min-keyint=120:scenecut=0" -tune stillimage -b:v 3000k
-minrate 3000k -maxrate 3000k -bufsize 3000k -acodec aac -ac 2 -ar 48000 -b:a 128k -g 120
-f rtsp rtsp://localhost:8554/mystream
then to save the file I am using this;
ffmpeg -rtsp_transport tcp -fflags +igndts -copyts -i
rtsp://localhost:8554/mystream -start_at_zero -c copy output.ts
I don't have the time to re-encode the .ts - just we will do a -copy and change
to .mp4 which will be the final output.
I keep getting Non-monotonic DTS errors, which I could just ignore, but I think
they are suggesting something is wrong. My test .ts files will play in VLC, but
I don't think they are 'good enough'. both seeking and audio sync does not look
right.
First of all, -video_size 1920x1080 and -pix_fmt uyvy422 appear to be
input options, which should go before the -i, so you would end up with
-f decklink -video_size 1920x1080 -pixel_format uyvy422 -i ....
Likewise, -copyts and -start_at_zero are both input options. However, to
answer your question, it looks like you have -fflags +igndts, which
causes the demuxer to ignore the DTS of the input files. DTS is "demux
time stamp." Sometimes, frames are decoded/demuxed in a different order
than they are presented - this is the case with B-frames
(bi-directional), which are frames that include both forward and
backward predictions.
By setting -fflags +igndts, you're telling FFmpeg to ignore the DTS, and
it likely is generating fake DTS values based on the PTS, which are
incorrect. The PTS are out of order, as you would expect when b-frames
are present, causing the DTS to also be out of order. DTS being out of
order is not legal (unlike PTS), so the MPEG-TS muxer is observing this,
fixing it, and throwing a warning.
In order to fix this, you may want to simply remove -fflags +igndts.
As for seeking the output file with VLC, mpegts is not an easily
seekable format and you should no generally expect that to work well.
- Leo Izen (Traneptora)
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