Radio Microphone intermodulation problems

[JMackenzie]

Hello again.

At our last show, when using our full quota of radio mics, I had to give up using one of them altogether because of sudden rushes of RF noise which were really loud.

It only seemed to happen with one hand held, Audio Technica ATW-T28, on Frequency 174.5mhz.

 

The radio mics I use are , 3 Senhheiser G2 SK100 on 839.1, 843.4 and 844.3 Mhz.

2 Senhheiser SK 2012 on 176.4 and 177.0 Mhz (licensed)

All the Senhheisers are body pack lapel types.

 

I also use two Audiotechnica ATW-T28 Hand helds on 174.5 and 175.0 Mhz

I also have Audiotechnica Lapel mics on 173.8 and 174.1 Mhz as well as having Lapels on 174.5 and 175.0 available , if not using the handhelds.

 

on investigation of this RF problem, it seems that the 174.5 system is being swamped by the 177.0 Senhheiser. The problem can be removed by full squelch applied to the 174.5 receiver, but that seems to lose my signal.

I dont quite understand what "Squelch" actually does ?

Could someone explain this to me ??

The actual signal from the 177.0 mic is appearing on the 174.5 receiver too, not just the RF noise.

I was led to believe , by my supplier of these mics, that they should not interfere with each other. Do any of you know better?

 

I know that the Audiotechnica systems are not in any way as good as the Senhhesiers, but beggars cant be choosers. I normally try to avoid using the radios, and hire in High Quality AKG or Senhheiser gun mics and Boundary mics to cover the stage.

Unfortunately our Orchestra / Band always play so loud that all that happens is they get picked up by the stage mics. Soft singers and speakers really need to use the radios.

 

I dont know if the Senhhheiser is the problem, or if it's the Audiotechnica receiver.

 

once again, I would welcome your opinions.

thanks, John.

[paulears]

You will find your answer here. The UHF systems won't be the cause of any of your VHF troubles. The basic snag is that the frequencies that can be licenced (or free to use) are NOT intermod free. 174.8 often caused issues. That said, the noise problem as you described it does not sound like intermod. Which often introduces odd warbles, or 'grainyness' or plain simple odd and unpredicatble noises. A loud rushing noise sounds a bit like white noise, where the RF drops out, and the squelch (designed to close when it detects weak signals) fails to shut, or is simply turned down too far.

 

A radio mic receiver with the squelch turned off, is noise free with a strong signal and as it gets weaker, the noise increases. usually they are set to simply stop the row at the point when no signal is better than a noisy one! If the burst of noise is really nasty, then the signal must have either totally gone, or, a mic on another close channel has caused de-sense. This is where a transmitter on an adjacent frequency simply desensitises the receiver. In this scenario, a strong signal suddenly appearing close by the receivers can render a weaker one totally absent, and maybe, if the squelch control is set poorly, this may be where the noise is coming from. Lots of experimentation is needed. Start with just one receiver and transmitter set at the working distance. then turn on others near the aerials and see if the noise happens. If it does, then it's probably desense, and a squelch control in need of a tweak.

[JMackenzie]

Thankyou Paulears.

 

Can I ask another opinion on this subject?

I work from a balcony which runs along the side of our theatre , being positioned at the far end of it, about 30m from the stage. The mixing desk and lighting desks sit on tables, and the radio receivers are mounted approx. 1m apart, sitting on the wooden rail which fronts the balcony, and sit about 1m off the floor. The two older , and much larger SK2012 receivers sit on another small table beside the desk.

I have used these receivers mounted on tables beside my stage box in front of the stage, but it looks a bit knaff, and too many wires and ariels for kids or staff to damage in the dark.

I would presume that 30m should be an ok distance for these things, and it is virually "line of sight".

Could the receivers be too close together?

When testing this over the last few days, all the mics were beside the desk an therefore all were about 1m or so from all receivers.

I already did as you suggested and went through the one by one setting up proceedure.

In the middle of all this I found that one of the handheld mics was cutting off. A simple resoldering of the wire from the capsule fixed that. ( it was the 174.5 ).

This compounds the problem a little.

Anyway, I continued with the procedure and eventually discoverd that with the 174.5 receiver on, and its own mic switched off, I was getting the RF light coming on, only when I switched on the 177.0 lapel mic, which , of course was sitting within 1m of the offending receiver. No other combinations caused a problem. Turning the squelch control full up stopped the RF signal appearing, but I am sure that I lost signal from the 174.5 mic by moving just 10m from the receiver.I could be wrong, and will recheck this on Monday.

Does the Squelch control actually affect the signal level? I am sorry, but I dont quite follow what it really does .

Should my receivers be positioned differently? Does it matter how close the mics are when in use on the stage?

I have tried to follow recommendations I have read elsewhere, which are sometimes contradictory.

Any other opinions gratefully received.

Thanks, John

[cedd]

Ok, Squelch; Imagine in audio terms for a second, a simple gate. When the signal level gets too small, the gate closes to cut off background noise. In the same way, squelch lifts at a certain level (somewhere around 10 microvolts on comms receivers, just as an example). Before that it prevents the audio from leaving the receiver - it's probably just background noise below that sort of level, or very unusable audio. It does not affect the sensitivity of the receiver, or the RF level.

 

Conduct your trials in a "real world" setup - ok so the transmitter causes interference on the adjacent channel if you stand within 10m. If this isn't the situation in a show, I'd not worry about it. Sit your transmitters on the stage and make your checks.

I've found the aerial connections on the SK2012's a little iffy and a loose one can cause rushing noise like you describe. Route your audio through the speakers, stand on stage and give your aerial a good wiggle. If the sound occurs again, swap aerial.

I tend to put my VHF kit nearer the stage than 30m. It's true that VHF travels better line of sight than UHF, but it's also less likely to bounce off walls, seats and audience to get there. The argument over stage end Vs Front of house rages on, but for VHF kit, I'd really suggest that the stage is the best place to be. The signal won't scatter as much so you need all the help you can to get a decent and reliable RF level. A good pair of lintec dipole antennae will help out nicely and can be mounted remotely from the receivers (watch your cable losses - not as big a problem at VHF compared to UHF, but a decent quality cable will help).

[bruce]

Does the Squelch control actually affect the signal level? I am sorry, but I dont quite follow what it really does .

 

It shouldn't affect the signal level. It's basically a gate, keyed on the carrier signal strength.

 

[incredibly-simplified-picture]

Imagine a FM receiver, with no "squelch". When the transmitter is on, and in range, the receiver can lock on to that signal. The output is basically what goes into the TX unit.

 

Opposite extreme - the TX is turned off. The receiver will output whatever RF noise it receives.

 

Now imagine a middle ground where the TX is on, but signal is marginal. As the TX moves around, and the signal strength changes, the receiver output could vary between the two extremes above - from working perfectly to just outputting noise, and anywhere in between.

 

The receiver will have an automatic gate function, which mutes the audio output when the received signal strength drops below a certain level.

 

"Squelch" is the sensitivity control on that gate. If it's set too low, then the system is unlikely to emit unwanted noise, but it's more likely to cut out completely when the separation increases. If it's set too high, it's less likely to cause dropouts as the signal strength falls, but more likely to let RF noise through.

 

[/incredibly-simplified-picture]

[paulears]

30m is simply too far away. Signals will be low, but above the threshold where the squelch closes and the signal is muted. The receivers are designed to be as close as they can get - most live in racks anyway.

 

Turning on one mic and seeing a response on another is a good indication of intermod. Turning on a tx close to the receivers is also a good way of seeing the desense effect we mentioned. The signal strength of a local transmitter is amazingly high compared to what's left after 30m. (To be honest, it's never line of sight unless a handheld in front of the turns. The minute they turn away, or wrap their hands around the casing, line of signt goes out of the window).

 

Have a look at the RF signal strength meter as 30m away transmitters move around. It will be up and down like a yoyo, and the minute it gets really weak, the signal will be very noisy - or will cut off if the squelch is set high. With weak signals, you have to choose between no signal or noisy signal. Working at a distance, with frequencies prone to intermod, isn't a stable starting point. Many people work with the receivers at the mixing position, and report success - truth is, once you have transmitters on sweaty people, in pockets or tucked down inside their costumes on a busy stage with lots of people - you are on borrowed time. Hand held may work - but never guaranteed, but much better than lav packs!

[JMackenzie]

Well Well, I am impressed with the response to this question. Thanks to everyone for their inputs.

My thinking , in deciding where to put the receivers, was to try and minimise the proximity effect of having the problem "interference" of transmitters being too close to the receivers. Apart from the visual aspect of having them near the stage, (on a table infront of, but to the side of the apron), I have had problems with things getting knocked over, cables pulled out etc. I never wanted to enclose them in a box of any sort in case that compounde the reception problemss.

I have always had good signals with them positioned at the desk, and I can monitor audio levels received (on the Senhheisers only). What I might try next is to place all the seemingly compatible systems near the stage, and keep the 174.5 seperate from them, at a greater distance .

If I still get this problem, my other choice is to go for a 173.8 handheld Audiotechnica, since I do have the receiver.

I dont know if you can buy just the transmitter on its own.

As a extra question, by the way---sorry to be hoggin the space------when it comes to batteries, do you guys use rechargeables or disposable alkaline?

I would rather use good Alkaline batteries, because of the voltage, but I do tend to use NiMHydride rechargeables for rehearsals, presentations, and simple day to day occasions where it does not involve a paying audience . I get the feeling that the rechargeables quickly get to a point where cutouts etc occur. I just wish that PP3 cells were cheap . I can buy alkaline batteries in boxes of 40 for a reasonable price, but-do they have a decent shelf life?

I take all your advice gratefully, and I will follow your suggestions.

thanks for your time

John

[paulears]

Duracells have a sell by date on the cell, and it's quite a way in the future. There have been a number of topics about rechargeable batteries, but general concensus is that while ni-cad is rubbish, being less terminal voltage when charged (telling the transmitters they are half flat!) and the voltage dies with no warning, ni-mh is do-able. However after using 2000+ alkalines in panto and giving half empty ones away to anyone who wanted them, we still didn't think recharging was the way to go - yet.

[donald463]

Having used Audio Technica mics for a number of years I always found the quality good. However in a four mic system the 174.1MHz always picked up signals from the two adjacent mics causing what sounds like the noise you are experiencing.

Ditch the 174.1 and use 173.8, 174.5, 175.0 and 177.0MHz You should then have no problem.

As for distance this had no effect whether receivers were on stage or at mixing desk.