CTCSS

Continuous Tone-Coded Squelch System (CTCSS) support will only open squelch (pass audio) when the configured tone is present - see Wikipedia.

NOTE: Enabling ctcss does not filter the tone from the output audio, for that enable a notch at the same frequency - see Notch filter.

To enable CTCSS, set the ctcss channel option to the tone frequency:

devices: ({
  (...)
  channels: (
    {
      freq = 145.35;
      modulation = "nfm";
      ctcss = 103.5;
    }
  )
});

In scan mode ctcss can be provided as either a single value that will apply to all frequencies or a list of values. When providing a list of values, use 0.0 for any frequency where ctcss should not be enabled:

devices: ({
  mode = "scan";
  (...)
  channels: (
    {
      freqs = ( 145.35, 145.65, 145.85 );
      ctcss = ( 0.0, 103.5, 0.0);
#     ctcss = 103.5;
    }
  )
});