Allow to only "consume" backchannel but not inbound audio from rtsp source

[felipecrs]

For example, I would like to discard the inbound audio of my Dahua VTO:

streams:
  vto_audio:
    # AAC, OPUS
    - echo:/config/scripts/fix_vto_codecs.sh 
      ffmpeg:rtsp://admin:[email protected]/cam/realmonitor?channel=1&subtype=1#audio=copy#audio=opus
  vto:
    # H264
    - echo:/config/scripts/fix_vto_codecs.sh 
      rtsp://admin:[email protected]/cam/realmonitor?channel=1&subtype=0&unicast=true&proto=Onvif#media=video#backchannel=1
    # AAC, OPUS
    - rtsp://127.0.0.1:8554/video_porteiro_audio?audio=all

Note I'm using #media=video#backchannel=1 in the RTSP URL. That is because I want to discard audio from that stream, but I still want backchannel and video from it.

However, what happens is that #media=video discards all audio tracks, including the backchannel one.

I believe the best would be to NOT discard the backchannel track if #backchannel=1.