Enhancement request: Noise squelch for FM

This probably only applies to the CB and 10m bands, but it would be nice if there was a noise squelch option since it gives much better separation between an empty channel and a readable signal for NBFM and is what walkie-talkies and most CB radios use. Often CB radios provide noise and RSSI squelch as options, on the CRT SS 9900 the squelch is turned left for noise squelch and right to switch to RSSI squelch.

A noise squelch works by triggering on the audio level via a high-pass filter that lets through frequencies from slightly above the highest modulated audio frequency, which will probably be about 3KHz and up. The audio is obviously sampled before any de-emphasis filter.

There may well be a cleverer way to discriminate between a quieting signal and noise in DSP but I am only familiar with the analogue way of doing this.

I find that the RSSI squelch is very fiddly to adjust for NBFM reception and FM is where the squelch is needed the most because of the harsh channel noise.


  • Our current NBFM squelch code comes from the CuteSDR project: https://github.com/jks-prv/Beagle_SDR_GPS/blob/master/rx/CuteSDR/squelch.cpp

    I believe it does so by measuring noise power above the filtered voice frequencies. But I am not an expert. If you know of other open source implementations worth looking at please let me know.

  • edited May 9

    Hi JKS,

    Yes, that is the type of squelch I am referring to. It's fantastic if it is already implemented. Does the squelch slider change mode on FM? Is there anything else I need to do to activate it? The squelch slider appears to behave as an RSSI squelch even on FM. Am I missing something?

    Update: I did some tests on blank carriers and it does indeed seem to behave as a noise squelch on FM. However listening to actual CB conversations it does seem to chatter a bit. It may be a bit slow to open/close. I will have to investigate further.


  • The noise squelch is enabled automatically for NBFM modes, but as you have noticed the hysteresis is not brilliant on some types of signals.

    CB is also bad because you get so many co-channel signals, which sometimes cause heterodynes that 'fool' the Squelch into believing that noise is present, when it is not.



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