It's all about the Software

SDR Console's FM squelch appears to work off of signal strength. This means that any change in the broadband noise level will cause the squelch to blow open unless it's set so tight that weak but perfectly readable signals won't open it. Noise-based squelch has been the universal standard for analog NBFM radios for decades because it is largely immune to this problem: the noise floor can change but the noise output from the detector doesn't, so the noise squelch stays closed unless a carrier that quiets the receiver is present. This is especially problematic with SDRs because their LO phase noise is higher than typical analog NBFM receivers, so strong carriers within +/- 50 kHz or so of the receive frequency also cause the amplitude-based squelch to blow open.

Right now I'm using a slew of virtual audio cables & instances of KG-Tone as external squelch controls, but it's a kludgy long-term solution. Would it be possible to change the squelch method in FM modes to noise squelch?

Thanks

Bob NO6B

Hi, It's a 'sort of' noise squelch. I can add voice squelch as used on AM/SSB, this would be better I expect.

Thanks Simon. Not sure what you mean by "voice squelch", but simply detecting demodulated audio & using that to control the squelch probably wouldn't work on FM. What is needed is to high-pass filter the demoded audio above ~7 kHz, then detect & filter the amplitude of the result & use that to gate the audio. Some fancier analog noise squelches also employ a differentiator to detect extremely rapid changes in noise level, such as a nearly full quieting signal suddenly dropping out within a few milliseconds (much more rapid than the usual mobile flutter) & using that to snap the squelch shut so no squelch tail is heard. This is more commonly known as "Micor" or "bilevel" squelch. I can sort of hear this type of action in your FM squelch implementation, for strong signals have no squelch tail whilst signals closer to the squelch threshold setting do have a tail. If only it weren't sensitive to the total channel amplitude, it would be great.

Hello, that would be a good idea because at my home I have a lot of electrical noise (city limit) and I have to mount the squelch so much that it no longer opens on certain signals.

73's Jean.

Simon G4ELI wrote: ↑Sat Sep 05, 2020 1:17 pm Hi, It's a 'sort of' noise squelch. I can add voice squelch as used on AM/SSB, this would be better I expect.

There is a really really good argument for allowing either, or, neither, or both for carrier squelch and noise squelch. Back in 1967 it seems Israel had this spat with its neighbors. Near the beginning of the war the Israelis had some severe trouble communicating on the Egyptian front.

In about 1970ish I was working on a proof of concept fast frequency hopper for the US Army. Their rep shared a story with us that related to our technology being "relatively" jam proof, while providing an annoying noise to other users in the frequency range. He related that the Israelis were being jammed and didn't realize it. The Egyptians had adopted some Soviet jammers that use noise jamming. It blocked the Israeli noise squelch from opening. The Israelis figured this out fairly quickly and went back to signal level jamming. If it had to be cranked too high they moved to another channel. The rest is bitter history for the Muhammadans in the area whose troop buildups prompted the Israeli opening of hostilities on their own time line.

Having both available and able to be used independently is a good thing.

{^_^}

I suppose - if I wanted to tell if the NBFM signals I was listening to were being jammed by spread spectrum. One of my HTs has an RF squelch option. I never use it.

I also just discovered that in NFM mode SDRC applies AGC to the demoded, de-emphasized audio. Makes for pleasant listening to underdeviated LASD traffic. But unfortunately I can't use NFM mode because of the squelch issue: right now I use WFM & VACs to multiple instances of KG-Tone. WFM is necessary for the noise squelch in KG-Tone to work properly.

Not spread spectrum as much as simple wide band noise. At least with the toy I am building I have both carrier and noise squelch that can be set none, carrier, noise, both. It seems to work. And noise squelch seems to work even better than I remember old time noise squelch (from the 70s) working.

{^_^}

It's been almost 9 months since I posted this request. Just wondering if it's something that's in the queue or not.

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.

Well, all I can say is if amplitude-based squelch is so good why do all analog NBFM receivers use noise squelch? I personally find energy-based squelch unusable: even when setting it to a level that requires ~20 dB quieting it still pops open on occasion. While piping the output to KG-Tone is a solution it gets old after a while, especially every time I want to use several VRXs & have to start several instances of KG-Tone, configuring each one to listen to a different VAC.

I'll work on generating an IQ file that will demonstrate what I'm talking about.

Thanks.