Configuring channels for multichannel mode

Multichannel mode explained

If you have ever used any graphical SDR console (like SDRSharp or GQRX) you already have an idea of how multichannel mode works. You tune the receiver to a particular center frequency and then you can observe a specific chunk of frequency spectrum all at once. The width of this chunk is equal to the sampling rate configured on the device. Once you have the waterfall running, you may click anywhere on it to listen to any narrowband channel that fits in the spectrum chunk that you see. RTLSDR-Airband does the same thing - except that it can simultaneously demodulate multiple narrowband channels and produce a separate audio stream for each one.

Channel configuration

channels is a list of channels to be received. Each channel is a separate group of settings enclosed in braces { }. As we are working in multichannel mode, you may define any number of channels you want.

Syntax

Settings which are commented out with # are optional and their default values are shown.

devices: ({
  mode = "multichannel";    # this can be omitted as it's the default setting
  ...                       # other device settings
  channels: (
    {
      freq = 123.45;
#     modulation = "am";
#     label = "";
#     ampfactor = 1.0;
#     afc = 0;
#     ctcss = N/A;          # default is disabled (no value)
#     notch = N/A;          # default is disabled (no value)
#     notch_q = 10.0;
#     bandwidth = N/A;          # default is disabled (no value)
#     highpass = 100;
#     lowpass = 2500;
#     squelch_threshold = -35;
#     squelch_snr_threshold = 20;
#     tau = 200;
      outputs: ( ... );
    }
  );
});

Want more channels? Just repeat the braced section several times and separate individual sections with a comma, like this:

devices: ({
  mode = "multichannel";    # this can be omitted as it's the default setting
  ...                       # other device settings
  channels: (
    {
      freq = 123.45;
      outputs: ( ... );
    },
    {
      freq = 123.6;
      modulation = "nfm";
      ampfactor = 1.0;
      afc = 0;
      ctcss = 110.9;
      notch = 110.9;
      bandwidth = 8000;
      outputs: ( ... );
    }
#   , .... more channels here
  );
});

Remember, do not put a comma after the closing brace of the last channel. This is a syntax error.

Explanation of keywords

• freq (frequency, required) - this is the frequency of your channel of interest.

• modulation (string, optional) - channel modulation, either am (the default) or nfm. The latter is only available if NFM support has been enabled when building the program, otherwise you will get "unknown modulation" error on program start.

• label (string, optional) - name / description of the channel, used for logging, icecast metadata, and others.

• ampfactor (float, optional) - amplification factor, ie. the the volume knob for this stream. Any positive value is allowed, values between 0.0 and 1.0 cause the stream to be attenuated (quieter), while values above 1.0 cause it to be amplified (louder). You can use this feature to change the volume of all outputs from this channel. NOTE: ampfactor can be set on a mixer output as well and will be applied afterward.

• afc (integer, optional) - automatic frequency correction. The default is 0 (off). When turned on (by putting a positive number here) it will attempt to switch to adjacent FFT bin in case when the signal power there is higher than in the FFT bin chosen for the configured channel frequency. Most probably you won't need this in practice.

• ctcss (float, optional) - See CTCSS.

• notch (float, optional) - See Notch filter.

• notch_q (float, optional) - See Notch filter.

• bandwidth (frequency, optional) - See Limiting channel bandwidth.

• highpass (int, optional) - See Audio filters in MP3 outputs

• lowpass (int, optional) - See Audio filters in MP3 outputs

• squelch_threshold (integer, optional) - see [Manual squelch setting].

• squelch_snr_threshold (integer or float, optional) - see [Manual squelch setting].

• tau (int, optional) - Channel value, overrides global and per-device values if set, see NFM deemphasis

• outputs (list, required) - a list of outputs where the produced audio is to be sent. Each channel must have at least one output configured.

Now you may want to read about:

• Configuring channels for scan mode

Or if you don't need this, jump straight to:

• Configuring outputs
  • Icecast
  • PulseAudio
  • Audio file
  • Raw IQ file