Any hints on how to get started designing this circuit?

MisterBill2

Joined Jan 23, 2018
13,125
The start is to be able to describe the function of the circuit, which here it appears to be enabling a microphone when it receives a speech input. There already exist speech detecting squelch systems that are used to cut off inputs unless there is speech detected.

So that would be a good point to start your search. It can be done either in hardware or in software.
 

Ya’akov

Joined Jan 27, 2019
6,561
The paper is from 1992. Today an analog approach would be an odd choice.

With the high power and low cost of microcomputers, a software based solution is far more sensible. On the electronics side, you can use the same VCAs as in the paper, or something natively digital.

On the software signed, you can hand write an algorithm or used a deep learning strategy which actually seems well suited to the problem. If not out of fashion, fuzzy logic looks as if it was invented for this application but AI makes more sense because of wide support.

It doesn’t seem to be a heavy problem, so once the model was trained it could run on something designed for edge AI which would be cheap and low power consuming.
 

MisterBill2

Joined Jan 23, 2018
13,125
A speech detecting squelch system using analog parts can add far less distortion than a micro, and be built with parts available anywhere in the world, almost. Even better, it can be serviced easily and rapidly almost anywhere, now and in ten years. And every stage can be diagnosed with just a multimeter. It is much harder to fix anything that uses a micro.
 

ronsimpson

Joined Oct 7, 2019
2,388
I have used "THAT" ICs and find them wonderful. Their application notes are a good place to learn audio.

DSP Digital Signal Processing is a good place to look. They are something like a microprocessor but made to do "analog" type things with math and programs. There are some good application notes on how to do Limiters & Compressors in DSP, which is very much related to the Noise Gate.

If you are into solder and hardware, use THAT. If you like programming, get an audio, DSP development board.
 

MisterBill2

Joined Jan 23, 2018
13,125
A speech triggered squelch system was published in QST magazine a bunch of years ago. It had about four analog sections and used both halves of a CD4038 dual monostable. THree opamps, two comparatos, and two one-shots. Not horribly complex.
 

MisterBill2

Joined Jan 23, 2018
13,125
This package does not seem to provide the squelching function for non-speech signals, which is what was requested. It does provide remote gain control and also dynamic compression, which are not the same as squelching..
 
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ronsimpson

Joined Oct 7, 2019
2,388
The first one I built in about 1970.
At the output of the "volume control" I added a Cad Cell that would short out the audio when light was present. The Cell is in a piece of dark tubing along with a LED. When I wanted to reduce the audio, I sent power to the LED.

Many of these cells have above 1meg ohm in the dark and 1k ohms in the light. They are a little slow in responding but that is good in a noise gate. There are many ways to build a variable gain amplifier. This is one of the simplest.

RonSimpson
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