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Old 24th Oct 2020, 7:45 pm   #1
G6Tanuki
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Default Audio-derived SSB Squelch?

"Listening" on 5MHz with my PRC320 is getting tiring: I feed the audio to a 10-Watt amplifier/speaker [so I can hear it around the house/nearby-orchard] and for 99% of the time it's reproducing nothing-but-"sharsh".

So I need an audio-derived 'squelch' that distinguishes between sharsh-and-speech.

Experiments so-far have split the audio through a low-pass filter turning-over at 1500Hz and a high-pass that starts at 3500Hz, my thinking being that "human speech through a SSB radio" has a significant syllabic component at 1500Hz but the sideband-filters will have stopped anything above 3KHz from being radiated so a significant audio-presence at such frequencies indicates noise.

I'm detecting the high- and low-frequency audio components using 'zero offset' rectifiers based around 741 op-amps, then feeding the DC levels to two comparators with different time-constants - the outcome then gets thresholded via TTL Schmitt-triggers and a pair of resettable TTL monostables to provide both a 'long' and 'short' signal - which is in turn combined to gate the audio.

It works - up to a point. it fails horribly on 'monkey-chatter" SSB signals a few KHz off-channel, the 500-milliseconds-of-25KV-spark-horror when the central-heating oil-furnace decides to come-online, and other random clicks.

OK, I know that the _proper_ way to do this is to digitize the audio, feed it into something like a Raspberry-Pi and there do phoneme-analysis to identify when there is human-speech present...

But can I do this with last-century analog/digital stuff too?

Suggestions please!!
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Old 24th Oct 2020, 9:23 pm   #2
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Default Re: Audio-derived SSB Squelch?

Noise-derived squelch splits the audio band and relies on speech having diminishing amplitude versus frequency while plain noise is flat.

Some AM receivers add an FM section to look for quieting. But that doesn't work with SSBSC. No carrier!

Noise squelch will work some of the time for you in a rigidly channelised service. In the palimpsests which are amateur bands, it'll drive you up the wall.

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Old 24th Oct 2020, 9:35 pm   #3
G6Tanuki
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Default Re: Audio-derived SSB Squelch?

Yes, I'm more falimiar with the reverse-thing where FM radios have an AM side-channel combined noise-and-logarithmic-carrier detector [RSSI] to drive both the squelch and a signal-level meter. Like you say, in the absence of a carrier that approach is nonsense.

Trying to identify and separate the signal- and noise-components and then 'recombine' them into some sort of DC/digital component that is consistent/stable over a few seconds and which I can use to trigger a squelch-gate - that's what's maddening me.

"Monkey-chatter" SSB a couple of KHz either side of the channel I'm monitoring really messes-up my attempts at analog time-domain 'noise' quantizing: if I could have something like four different time-constants to select I might just be able to make it work.

[OK, time to ingest a bit more Jim Beam in the hope that he may provide further insight]
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Old 24th Oct 2020, 10:53 pm   #4
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Default Re: Audio-derived SSB Squelch?

Jim Kirk or Jim Beam might help you.
I used the audio gate from a Pye PC1 controller as an external speaker box squelch for an IC202 - always an infernal racket other wise.
Lately I have put an op-amp comparator on the AGC line. That works tolerably well on IC202s and FT221s. Sorry no idea about PRC stuff.
Don't they use SL series ICs in the main?

The IC402 (70cm SSB) has those in the IF. I much prefer the others IC202, IC502 etc.
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Old 25th Oct 2020, 1:18 am   #5
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Default Re: Audio-derived SSB Squelch?

In the absence of anything very obvious or promising specific for SSB, I did some time ago stumble on an "audio levelling control" intended for use with an FM scanner. This brings all the signals, whatever their strength or level of modulation, to a common audio level. I couldn't comment on how that might or might not be of any use in dealing with SSB / sharsh, but it's a simple two transistor circuit bolted on to the receiver output, so fairly easy to try. It's demonstrated here https://www.youtube.com/watch?v=pUii_pDQxg8

There's an earlier Youtube on the same subject by the same guy https://www.youtube.com/watch?v=1h0FZJYXQ_w.

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Old 25th Oct 2020, 4:43 pm   #6
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Default Re: Audio-derived SSB Squelch?

Had a further think on this and I have come to reckon that actually mine was built using the RX audio gate from a Pye PC2 controller. This was a plug-in board about 3x3" with a couple of OpAmps and a few transistors on it. I suppose it was an active rectifier of some sort and a level comparator. The PC2 was a 600 Ohm Line device and this gate just cut off residual line hiss and hum from the operator in no signal conditions.

I preceded it by an 8 Ohm load so it could be driven from the headphone socket. I followed it by an LM380 Power stage and built the whole thing in a Pye Mobile Loudspeaker.

It seems impossible to find any schematic info on the PC2 today but I remember it worked quite well. The IC202 Volume pot became squelch level control, advance that until the gate was not quite open on noise. The Volume Pot for the LM380 set the desired listening level.

Here some pictures of my IC202 Bitsa for 4 metres. This is the one where the Smeter DC is used instead. A dual concentric pot does duty for Volume and Squelch.
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Old 25th Oct 2020, 5:11 pm   #7
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Default Re: Audio-derived SSB Squelch?

Back in the 80s we used Aussie Stingray HF SSB and also NZ Codan. Both had squelch as plug in option boards. They worked pretty well. I think, it was a long time ago, that they worked on the presence or not of noise in two audio pass bands. Certainly both boards only had a few op-amps on them.

It has always surprised me that squelch has never appeared on mainstream amateur HF SSB as it apparently can be done easily enough. I think I have a couple of the baords, I will try and find them.
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Old 27th Oct 2020, 3:47 pm   #8
G6Tanuki
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Default Re: Audio-derived SSB Squelch?

I've been playing a bit more: yes I've got the high-pass and low-pass audio stages separating the signal into noise- and noise-plus-signal streams; the issue seems to be what to do with them?

Simplistically, as I first thought, rectify them both [using a zero-offset rectifier built around a 741 and a pair of diodes] then use the 2 resultant DC levels to feed a comparator which feeds a schmitt and then into the audio-switch.

Which sort-of worked but was really susceptible to things like thermostat-clicks.

Second attempt - do away with the comparators, use the raw signal- and signal+noise streams into Schmitts [7413] whose squared-up putouts trigger a pair of retriggerable-monostables [74122] with different time-constants.

The idea I got from a 1970s US mag where it was used as part of the VOX circuit in a SSB transceiver.
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