NFM deemphasis

Noise occurs predominantly at the highest audio frequencies within the baseband. In NFM mode high frequencies are boosted in the transmitter and attenuated by a corresponding amount in the receiver. Attenuating the high audio frequencies in the receiver also reduces the high-frequency noise. These processes of boosting and then attenuating certain frequencies are known as pre-emphasis and de-emphasis, respectively.

The amount of pre-emphasis and de-emphasis used is defined by the time constant of a simple RC filter circuit. RTLSDR-Airband performs de-emphasis on NFM channels with a default time constant of 200µs. You can tune the time constant by modifying the tau configuration option.

tau is an integer value expressed in microseconds. It can be set:

• globally,
• per device,
• per channel.

Example:

# this is the global tau setting
tau = 100;
devices: ({
  index = 0;
  gain = 25;
  centerfreq = 150.0;
  channels: ({
      freq = 150.5;
      modulation = "nfm";
# no tau setting here - a global value of 100 takes effect
      outputs: ( ... );
  },
  {
      freq = 150.6;
      modulation = "nfm";
# no tau setting here - a global value of 100 takes effect
      outputs: ( ... );
  });
},
{
  index = 1;
  gain = 25;
  centerfreq = 170.0;
# this overrides the global value for all channels on this device
  tau = 200;
  channels: ({
      freq = 170.5;
      modulation = "nfm";
# no tau setting here - a per device value of 200 takes effect
      outputs: ( ... );
  },
  {
      freq = 170.6;
      modulation = "nfm";
# this overrides the global value and per-device value for this channel only
      tau = 0;
      outputs: ( ... );
  });
});

Increasing the value of tau causes the signal to be deemphasized more aggressively (high frequencies will be attenuated more). Setting tau to 0 disables de-emphasis altogether.