Need circuit for mute the unwanted audio (Squelch control circuit) - Page 1
Forum section: electronics / rf-microwave

author: jtronix
author_rank: Regular Contributor
timestamp: November 10, 2017, 09:35:44 am
content: Hi,This is a basically audio without squelch control.I want to mute the speaker when there is no signal present. Is there any way to mute this audio??Please share the schematic

author: German_EE
author_rank: Super Contributor
timestamp: November 10, 2017, 02:15:23 pm
content: OK, think of this as an exercise in electronics.1) Most of a voice signal occupies the band of 300 Hz to 3000 Hz2) The white noise that you get between stations is very wideband, probably up to 20 KHz or higher.So, after detection (AM or FM) split your audio into two channels, one below 3 KHz and one above (say) 5 KHz, use a low pass and a high pass filter, there are plenty of designs out there.The audio below 3 KHz gets passed to your audio amplifier and the speaker via a switch, use either a relay or a CMOS switch or find an audio amplifier with a mute pinThe audio above 5 KHz is amplified and then used to drive your switch circuit. If there is only white noise then the switch is opened and the audio is muted.As an alternative, amplify the output of the signal strength meter and use that to switch your audio output.

author: Peabody
author_rank: Super Contributor
timestamp: November 10, 2017, 03:09:28 pm
content: The attached circuit is one I recently participated in at my local OSH group. It uses a walkie talkie set to remotely trigger a motion sensor that unlocks a door. Normally there is no audio, but when the WT receives a ring tone, that signal is amplified and rectified and charges C1 until the voltage there exceeds the divider of R6 and R7, and then the output of U1B goes high. There is no filtering or tuning - it's just looking for any audio energy. But it takes a while for C1 to charge, so the circuit doesn't generally respond to noise spikes.It takes a material fraction of a second for C1 to charge to the R6/R7 level, but it continues to charge to the LM358 output voltage minus the diode drop so long as audio is received. When the audio stops, C1 discharges through R5, and that takes several seconds. So the output actually lasts longer than the audio input.The gain of U1A and the relationship of R4 and R5 determine how the circuit responds to audio - how long it takes to register that there is legit audio, and how long it takes to "release" when the audio stops. But this might be teakable to work for you if there is enough difference between your background static and actual audio signal. If the noise is as loud as the audio, then this circuit wouldn't work, unless a little filtering out of the high frequencies would be enough to make it work.Then you would have to rig up something that lets the output of U1B switch the speaker on and off.I should add that the input assumes there is no DC offset, which is the case with the WT audio output. If there is offset, you would need to run it through a capacitor. Also, you have to be careful that the input signal doesn't exceed the absolute maximum rating for the voltage applied to any input, the meaningful one in this case being 0.3V below the negative rail.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: November 11, 2017, 01:15:29 am
content: Generally, squelch control is done probably in the IF stage of the receiver with something like a detector so that when the signal coming out of the RF amp is greater than the threshold set by the squelch control, it can pass through to the audio amp.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 11, 2017, 01:32:12 am
content: What does "signal" mean here? Did your sample include any of your desired "signal", or only the "noise"? Without some real-world comparison of "signal" vs. "noise" IN CONTEXT, you question cannot really be answered. What is the source of this signal? If it is from a radio receiver do you not have access to any RF signal strength information (RSSI)?https://en.wikipedia.org/wiki/Received_signal_strength_indicationThe techniques for doing this (or evaluation whether it can even be done) depends greatly on details not disclosed in your question. What is the source of the signal+noise? What is the SNR (Signal to Noise Ratio)? How can the "signal of interest" be distinguished from the channel "noise"?There are techniques for doing this in the audio domain where it is called "gating". But it relies on some dependable way of identifying the presence of the signal amidst the noise. Without even a brief sample of the signal+noise, we really don't know the scope of the problem.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 11, 2017, 02:12:42 pm
content: There are, or were, special ICs which were used in Radio comms equipment to do this.Google for "squelch IC" or "mute IC".

author: German_EE
author_rank: Super Contributor
timestamp: November 11, 2017, 08:17:20 pm
content: 'were' being the problem. I have not seen a squelch IC for over a decade, hence my proposed solution using the filters.

author: cncjerry
author_rank: Unknown
timestamp: November 11, 2017, 08:32:12 pm
content: If you are using SDR then you just calculate the amplitude of the signal as SQRT(i^2+q^2) and when the signal exceeds a specific value you turn on the audio. This only works for carrier based signals as SSB or audio alone won't work.

author: jtronix
author_rank: Regular Contributor
timestamp: November 13, 2017, 03:23:24 am
content: for this audio does any one has spectrum analyzer's waveform