Filter join
  Join multiple audio streams into multi-channel output.
    Inputs:
        dynamic (depending on the options)
    Outputs:
       #0: default (audio)
join AVOptions:
  inputs            <int>        ..F.A.... Number of input streams. (from 1 to INT_MAX) (default 2)
  channel_layout    <string>     ..F.A.... Channel layout of the output stream. (default "stereo")
  map               <string>     ..F.A.... A comma-separated list of channels maps in the format 'input_stream.input_channel-output_channel.


