Low res smtpe bars with message and rtmp stream - taken from https://gist.github.com/nickferrando/6d572b044a205a507201e2fc69423a3e

colorbars.sh

docker run -d --rm --network=host bluenviron/mediamtx:latest

ffmpeg -re -f lavfi -i "smptebars=rate=30:size=320x240" \
-f lavfi -i "sine=frequency=1000:sample_rate=48000" \
-vf drawtext="text='YOUR MESSAGE:timecode=01\:00\:00\:00':rate=30:x=(w-tw)/2:y=(h-lh)/2:fontsize=12:fontcolor=white:box=1:boxcolor=black" \
-f flv -c:v h264 -profile:v baseline -pix_fmt yuv420p -preset ultrafast -tune zerolatency -crf 28 -g 60 -c:a aac "rtmp://localhost:1935/mystream"

vlc --network-caching=50 rtmp://localhost:1935/mystream