It's all about the Software

Do you have a simple signal generator, Simon? (As if I need to ask. A spare Lime can provide what is needed.)

Generate a very weak FM modulated signal with variable amplitude and variable modulation. To a large extent a second SDRC running its TX can do that. It's just for confirmation that the noise squelch works.

Split the demodulated FM audio into low pass filtered audio for output and a noise squelch channel. Optionally put some delay on the audio channel. The filter should have 40dB to 60 dB attenuation below about 3500 Hz with "0dB" attenuation about 4600 Hz. For the audio low pass filter "0dB" to about 3500 and 50 dB-ish at 4600 - or even tighter. It doesn't matter much.

for( int i = 0; i < InLength; i++ )
{
FTYPE mag = MFABS( sqbuf[ i ] ); // get magnitude of High pass filtered data
// mag is the squelch control exponential filter squelch magnitude.
m_SquelchAve = ( 1.0f - m_SquelchAlpha ) * m_SquelchAve
+ m_SquelchAlpha * mag;
}

Fish around for a good value range for the squelch setting. It is VERY good and insanely fast acting. Once you get the range it is not overly sensitive to set it but it picks up sub dB signal level changes down where there is noise on the demodulated signal. It is very sensitive to SNR so you can pretty reliably select the SNR you want and ignore EMI noise changes.

The only thing that spoofs it is intentional noise jamming. So a combination of signal amplitude squelch and noise squelch may be required where sophisticated jamming is expected.

{^_^}

Simon G4ELI wrote: Sat May 29, 2021 5:22 am I've just reminded myself of the current squelch - it's energy-based. I find it works fine with ham FM on simplex / repeaters. The SSB voice-based is good, so as I wrote before this could be used.

To test I need your IQ files.

ZIP file containing IQ WAV file with single carrier turned off & on attached. This carrier is from an HP8640B which has very low phase noise, so the phase noise that you see is from the LO of the SDR I used: an SDRPlay RSP1a. If you set the RX frequency ~50 kHz away from the carrier, then set the squelch to threshold while the signal is at a low amplitude (there was leakage through the sig. gen. even with the RF output set to off), the squelch will open when the carrier comes back up to full amplitude. This would never happen on any receiver with noise squelch even if it had poor LO phase noise characteristics.

Been 6 months & haven't seen any updates that address the amplitude squelch problem. Might this be looked into anytime soon?

Been another 7 months & still nothing. Also seems to me like SDRC development is slowing down?

I already got rid of my RSP1a because of lack of adequate software to use with it (SDRuno is a joke). I've been holding on to my RX-888 in the hopes I can use it someday with SDRC but not with amplitude squelch on NBFM.