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Author Topic: Understanding Squelch and Pilot Tones  (Read 3533 times)

A Man

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Understanding Squelch and Pilot Tones
« on: September 03, 2005, 04:34:49 pm »

Can someone point me in the right direction to understanding the how/whys of "Squelch" and "Pilot Tones".

I'm using the Sennheiser EM 500-G2 series with combiners and paddles for our IEM's and want a better grasp on the above subjects.
Anyone with some "tips & tricks" for the G2 systems, please feel free to post them here as well.

TIA
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Milt Hathaway

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Re: Understanding Squelch and Pilot Tones
« Reply #1 on: September 03, 2005, 06:24:01 pm »

I'm not sure I can describe it any better than this:

 http://www.absoluteastronomy.com/encyclopedia/s/sq/squelch.h tm
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Milt
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caseyw

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Re: Understanding Squelch and Pilot Tones
« Reply #2 on: September 07, 2005, 11:12:45 am »

So the squelch knob on most wireless units is actually controlling the threshold for a gate?
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David Buckley

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Re: Understanding Squelch and Pilot Tones
« Reply #3 on: September 07, 2005, 03:16:50 pm »

But not as we know it Jim.

The standard squelsh mechanism monitors the RF level received from the microphone, and if that level drops below some preset level it mutes the audio.

It does this because radio mics are FM, and FM radio links have a number of interesting and odd characteristics, the important one here of which is that that over quite a wide range of signal strengths there is no real variability in the background noise level.  The technical term for this is the "signal strength required for 'full quieting'".  If the FM signal drops below this level, then you get a world of hiss, just like what you hear on the walkie talkie when the other bloke releases the PTT - a nasty burst of white noise.  This is the last thing you need replacing the vocal in the middle of a quiet number.

Tone squelch is an additional layer of link integrity verification.

Simple RF squelsh cant tell whether you're getting a great signal from a radio mic on your stage, or from a TV station, after all, radio mics operate in the TV spectrum.

If you have tone squelch, then the receiver can have good confidence that it is actually receiving ignal from the radio mic, not channel 9 news.  This is because the radio mic is transmitting an ultrasonic signal superimposed on the audio path, and the receiver can detect the presence of this signal, and mute the audio in its absence.

This is important because of another interesting characteristic of FM radio links, the "capture effect".  With FM, the receiver will only receive one signal as a time, basically the stronger of any competing signals.  Unlike AM radio, the signals dont mix, one just wins.  So if you're all set up with good wireless and then the truck in the loading bay moves and channel 9 becomes the stringer signal, you lose wireless altogether.  With ordinary squelch the receiver will stay open (its still got signal), and heck only knows what you'll hear.  With tone squelch the receiver will mute.  Ok, so you've lost the mic, but it was lost anyway, you just hope silence is less embarresing than Jay Leno or whatever...

Thus, to make a wireless mic system work well, in my opinion the single most important thing is decent reception antennae and splitting, and get the antennae where they can see the mics.  If your signal level gets marginal you will then have hassles, if the signal level is high then its a whole category of problem you can just forget about.

Hope that helps a little Smile


(I've just noticed you're talking IEMs not mics, but its all the same)
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ProSoundWeb Community

Re: Understanding Squelch and Pilot Tones
« Reply #3 on: September 07, 2005, 03:16:50 pm »


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