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Author Topic: What is Squelch, how does it work, and can it work "for" you?  (Read 2208 times)

nick pell

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The Bay Area is tough for wireless mic users, as some of us know. All the clients expect them to work perfectly all the time (even if you're across the street from a military airbase).

Some people have suggested messing with the squelch, which I have tried and have not really achieved any perceptible results. What is squelch and does it have any impact on wireless mic operations that can be effective for users?
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Marcus Baeumler

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Re: What is Squelch, how does it work, and can it work "for" you?
« Reply #1 on: July 02, 2006, 05:54:24 pm »

Nick,

the easiest way to explain squelch is to compare it to a noise gate for RF.
With the trasnmitter turned off, set the squelch level so no audio is passed to the output (from potentially interferring RF sources).
When the transmitter is turned on you may want to turn the squelch level back a little bit to prevent early dropouts.
Hope this sounds understandable despite language barriers (I am from Germany).

Regards
Marcus
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Ron Hebbard

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Re: What is Squelch, how does it work, and can it work "for" you?
« Reply #2 on: July 02, 2006, 05:55:55 pm »

Hello Nick;

Think threshold setting for a gate.
Signals, usually unwanted, below threshold will not be heard.

I could be totally wrong on this but there'll probably be a pro along here any moment.

Toodleoo!
Ron Hebbard
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Henry Cohen

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Re: What is Squelch, how does it work, and can it work "for" you?
« Reply #3 on: July 03, 2006, 08:55:14 am »

Squelch is, as Ron indicated, a threshold at which the demodulated audio is either muted or umnuted (present or not present at the audio connector). The squelch circuit relies on one or more factors, depending on the quality and cost of the receiver; received signal strength (in microvolts or uV), pilot tone, also called tone grip; and signal to noise. The idea being that the threshold for audio quality at the output is dependent on the overall local RF environmental noise floor, the receiver's sensitivity and selectivity, other nearby carriers (both physically and in frequency), the anticipated RF signal level of the transmitter reaching the antenna, and the user's minimum acceptable audio quality. Thus, squlech is very situation dependent and the reason it's [usually] user adjustable.


You don't say specifically what kind of problems you're experiencing, but being in a highly RF congested area, you're  probably having other issues in addition to any squelch settings; are you sure you're using a clear frequency and necessary bandwidth, and if there are multiple wireless units, have you performed an intermodulation product calculation? Other possibilities are transmitter frequency error and/or the receiver being out of alignment.

Henry Cohen
Production Radio Rentals
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Henry Cohen
Production Radio Rentals
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"Every new radio emitter since Marconi’s 2nd transmitter has caused interference to other systems!" - Michael Marcus, Oct '07

Mike Slay

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Re: What is Squelch, how does it work, and can it work "for" you?
« Reply #4 on: July 05, 2006, 11:58:33 pm »

Good question.  Good answers and anologies. Something I've always wondered about.
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Chip Dixon

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Re: What is Squelch, how does it work, and can it work "for" you?
« Reply #5 on: July 06, 2006, 12:52:07 am »

_If_ a squelch adjustment can help your situation--you would increase the squelch if low-level signals (noise) are causing consistent interference, or decrease the squelch if your signal is dropping out too much because it is being disregarded as a weak signal.

Think of it as a threshold where disregarded noise meets live signal being passed on through.  Too low and you'll have noise to listen to during gaps in transmission, too high and your signal is disregarded at an inappropriate time.

To understand squelch get a CB radio with a squelch knob and play.  You'll understand in no time.
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Michael 'Bink' Knowles

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Re: What is Squelch, how does it work, and can it work "for" you?
« Reply #6 on: July 06, 2006, 01:55:33 pm »

Squelch can be useful in getting rid of outside interference but then so can things like aiming the antennas and turning the gain down on powered remote antennas so they don't pick up so much crap. Even simple whip antennas can be aimed: Point them at the trouble source while putting your stage at a 90-degree angle to the whip.

The trouble with RF mic dropout is that it can also be because you are too near another RF source or that you have spare mics powered up near the antennas or that your frequencies aren't grouped well such that they interfere with themselves. There are a bunch of things that can go wrong.   Confused

-Bink
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Jeff Pitt

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Re: What is Squelch, how does it work, and can it work "for" you?
« Reply #7 on: July 07, 2006, 08:34:54 am »

have you performed an intermodulation product calculation?

Can you elaborate on this?
Thanks.
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Henry Cohen

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Re: What is Squelch, how does it work, and can it work "for" you?
« Reply #8 on: July 07, 2006, 09:10:57 am »

Jeff Pitt wrote on Fri, 07 July 2006 08:34

have you performed an intermodulation product calculation?

Can you elaborate on this?
Thanks.


  Whenever there are multiple carriers (frequencies) present in a transmission line (the antenna itself; coax; multicouplers, aka antenna disribution; the front end of the receiver), the frequencies will mix - intermodulate - and create new frequencies - products. These new frequencies or intermodulation products can be exactly the frequency you have a receiver tuned to (a 'direct hit') or within the channel bandwidth and close enough to the receiver's tuned frequency as to cause a lesser degree of interference.

  The mixing is a fixed mathematical occurance: take any three frequencies and perform the following calculations:
A+B-C
A-B+C
2A-B
2B-A
2A-C
2C-A
2C-B
2B-C

  That's eight products (2 to the 3rd power) based on just three of your frequencies, and this can be carried out further (3A-2B . . .) but it's these first eight that produce the bulk of the destructive products. Repeat the eight calculations for each combination of three for all your frequencies **PLUS** any analog TV chroma, video and audio carriers of nearby stations **PLUS** any other nearby wireless mics/IFBs/IEMs/PL. As you see it all adds up to a lot of calculations and products, thus there are computer programs to do these calculations rather quickly, but you do have to understand  several other parameters to achieve valid results.

  I illustrated only a three frequency calculation as it is the most destructive. All other odd number (5, 7, 9 . . .) frequency mixing also takes place, but their products are generally (but not always) at a low enough level as not to be destructive and are thus ignored for most situations.

  If you want to learn more, there are some very good web based resources, both free and for purchase.

Henry Cohen
Production Radio Rentals
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Henry Cohen
Production Radio Rentals
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"Every new radio emitter since Marconi’s 2nd transmitter has caused interference to other systems!" - Michael Marcus, Oct '07
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