Notch filter

Notch filter is used to eliminate narrowband interference. For example, there might be some constant carrier present just to the side of your channel of interest, causing constant and annoying beep in the audio. Some communication systems use CTCSS tones to isolate multiple talkgroups from each other. CTCSS shows up as a constant tone during transmission and its pitch encodes a particular talkgroup. Notch filter can be used to eliminate that tone from the audio.

NOTE: If ctcss is enabled the notch filtering is done after squelch processing so the output audio is filtered withough impacting squelch.

To enable notch filter, set the notch channel option to the frequency of the interfering tone that you want to cancel out:

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

This should attenuate CTCSS tone at 103.5 Hz.

The same works in scan mode:

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

This sets the notch filter to 103.5 Hz for all frequencies listed in freqs option.

In case you need a different notch frequency for each individual scanned frequency, you may set notch option to a list of frequencies, like this:

devices: ({
  mode = "scan";
  (...)
  channels: (
    {
      freqs = ( 145.35, 145.65, 145.85 );
      notch = ( 103.5, 67.0, 0.0 );
    }
  )
});

This applies a notch filter of 103.5 Hz to the scanned frequency of 145.35 MHz, 67.0 Hz to 145.65 MHz and disables the notch filter for 145.85 MHz. The number of frequencies in the notch list must be equal to the number of scanned frequencies.

notch_q sets the selectivity of the notch filter. The default is 10.0. The higher the value, the more selective (i.e. narrower) the filter is. notch_q has the same syntax as notch, that is, either:

notch_q = 15.0;

or (in scan mode):

notch_q = ( 10.0, 10.0, 15.5 );

A value of 0.0 keeps the selectivity for a particular frequency at the default value.