# Wireless Problem



## mbenonis (Dec 21, 2007)

It's been a while since I posted a question, so I thought I'd put one up. The following hypothetical scenario takes place in/near Washington, DC (important).

A theater has four UHF wireless mic systems, operating on 543.250, 551.450, 562.000, and 562.075 MHz. The make/model of the systems is not relevant. When any one system is on, and right next to the receiver, it works fine. However, when on stage (~75' from the receivers), they don't work very well - lots of cutting in and out, and occasionally the sound from one mic appears on another channel. The receivers each have two antennas attached to them directly. Assume batteries are fresh, everything is connected, and that the fault lies in the wireless mic system (i.e., not in the console or anything else)

Identify the various problems here, and how to fix them. Bonus points for identifying what problems will be associated with what mic(s).

HINT: You may need to use reference sources, such as the FCC's website, Shure/Sennheiser/A-T's wireless tools, etc.
http://www.fcc.gov/mb/video/tvq.html


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## Hughesie (Dec 21, 2007)

well im the first person, so im going to offer the simplest answer crosstalk, and not knowing the US radio frequencies i would say you might be picking up a radio station tv station and the receiver doesn't know how to interpret the new signal, the reason it works close to the receiver is the mic's signal is stronger than all over sources until you move away when other signals become stronger


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## mbenonis (Dec 21, 2007)

Yes! That is definitely one problem with the scenario. Bonus points for you, Alex, if you go to the US FCC's website (above) and figure out which mic has this problem.

There also a number of other issues with this setup (not all related to US frequency allocations)...


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## mbenonis (Dec 28, 2007)

Anyone else want to take a stab at this problem?


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## n1ist (Dec 29, 2007)

Those last two seem too close in frequency to keep out of each other. I haven't checked what's the deviation on these mikes, but 75KHz separation is probably not enough.


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## Stoldal (Jan 5, 2008)

Hmm One problem that i have at the place i help out at, normal Cell phone some times interfere with our wireless mics. 

It could also be ask what kinda building is this production in. I also did network admin at the same place i do lights at. In one building we where trying to put wireless in, we could not get the signal to go across the room. the room was make be 60-70' wide. this was before we had started to do productions in the area. 

When we started to do productions in there we had the same problem with the signal. SO what we had to do what put the wireless mic receivers next to the same, and snake them back to the mixers.


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## Stoldal (Jan 5, 2008)

I found this FCC Chart It is a Large PDF


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## mbenonis (Jan 5, 2008)

n1ist said:


> Those last two seem too close in frequency to keep out of each other. I haven't checked what's the deviation on these mikes, but 75KHz separation is probably not enough.



Yeah, that's one of the problems. Mics are definitely wideband, and should probably get 200-400 kHz between channels (if anything to keep one channel from desensitizing the other receiver).

73's de KI4RIX


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## mbenonis (Jan 6, 2008)

Mstoldal said:


> Hmm One problem that i have at the place i help out at, normal Cell phone some times interfere with our wireless mics.



Cell phones operate at 800 MHz and 900 MHz, as well as 1.8-1.9 GHz so they won't interfere directly with the RF side of the wireless mic (one would hope the filters on the front end of the receivers is tight enough tocut them out anyway).

That said, they can inject undesired noise into the audio path before the transmitter and after the receiver. I suppose it would also e possible to get intermodulation if the cell signal were really strong (although the chances of IM products landing on frequencies in use is very unlikely).


Mstoldal said:


> It could also be ask what kinda building is this production in. I also did network admin at the same place i do lights at. In one building we where trying to put wireless in, we could not get the signal to go across the room. the room was make be 60-70' wide. this was before we had started to do productions in the area.
> When we started to do productions in there we had the same problem with the signal. SO what we had to do what put the wireless mic receivers next to the same, and snake them back to the mixers.



The type of building is definitely relevant, or more specifically the construction. If the building tends to let lots of RF through at UHF, then noise from the outside (such as from TV stations) may cause problems with the mics. If it is fairly shielded, though, these problems may be reduced or eliminated altogether.

I'd encourage everyone to take a close look at the TV stations in the Washington, DC area... See the link I posted above.


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## Stoldal (Jan 6, 2008)

> I'd encourage everyone to take a close look at the TV stations in the Washington, DC area... See the link I posted above.



Well i think that channel 20 is in that frequency range.


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## mbenonis (Jan 6, 2008)

Mstoldal said:


> Well i think that channel 20 is in that frequency range.



Yes, exactly. And the astute reader will notice that one of the mics is sitting on top of the luminescence carrier of Channel 20.


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## Chris15 (Jan 8, 2008)

As others have noted, there is a potential for problem with mics 3 & 4 - you would have to have pretty darn good filters to pull them apart from each other...

One thing I did note Mike mentioned, each of these mics has their own set of antennae, now the done thing would normally be some form of RF distribution. This makes things less cluttered and allows you to use external antennae with directionality and hence passive if not active gain. Something in the back of my mind has a vague recollection of reading somewhere sometime that antennae in close proximity to each other interfere with each other and it makes sense even if my engineering mind can't quite remember the details of why at the moment...

Or am I confuzzled?


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## mbenonis (Jan 14, 2008)

Using antennas connected directly to the receivers can cause a few problems. However, surprisingly the biggest problem isn't interference but simply lack of diversity. Simply put, the farther apart the pair of antennas is from each other, the better the probability that the wireless system will always have a good signal.

That said, having multiple receiver antennas in close proximity can cause direct interference, if the receiver's output stage is not properly designed. These antennas can actually radiate as well as receive, and in some cases this radiated signal can cause interference on other units. Good multicouplers/distribution amplifiers isolate the receiver ports so that input signals do not make it from one receiver to another.


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## superdoo (Jan 25, 2008)

I strongly recommend antennae distros. I work for an outdoor theatre company in Bismarck, ND. Needless to say interference on the open prairie is a little more than a problem!
Last year we got 8 new mics complete with antenna distribution. We went from saying "maybe the mics will work okay at best?" To holy crap! All 8 aren't even cutting when the actors are in the building behind the stage!

My nights got so much easier...


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## Charc (Jan 25, 2008)

What does that do? / What is it?


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## avkid (Jan 25, 2008)

charcoaldabs said:


> What does that do? / What is it?


Look at the info about this Shure product:
http://www.shure.com/ProAudio/Products/Accessories/us_pro_UA844US_content


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## mbenonis (Jan 26, 2008)

A multicoupler, or active splitter as it is sometimes called, is a device that takes an RF signal from an antenna and splits to a number of separate outputs. It also amplifies the signal slightly to compensate for the loss that occurs when the signal is split.

Multicouplers often act as a power supply for receivers to minimize the number of power cables needed inside the rack. Additionally, they have filters that prevent spurious signals from being coupled from one receiver unit to another. Finally, they usually apply a band-pass filter to the input (the cheap ones usually have a broadband filter that passes 470-806 MHz, while the better ones have a tighter window on the order of 50 MHz).


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## Charc (Jan 26, 2008)

avkid said:


> Look at the info about this Shure product:
> http://www.shure.com/ProAudio/Products/Accessories/us_pro_UA844US_content




I had noticed that in my research, but your sound jargon runs circles around my dinosaur sized brain.


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## Chris15 (Jan 27, 2008)

See when you mentioned a multi coupler, my mind immediately went to an active combiner, quite a different beast. Combiners take RF signals from either multiple antenna or multiple transmitters - normally IEM, and combines them to a single RF output to go to either a transmit antenna or to the input of a splitter, active or passive.

What's the rule of thumb? Only one passive split and the rest need to be active? And on the flip side, only one active combiner and the rest must be passive...


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## mbenonis (Jan 27, 2008)

Chris15 said:


> What's the rule of thumb? Only one passive split and the rest need to be active? And on the flip side, only one active combiner and the rest must be passive...



There really isn't a rule of thumb here. You need to know how strong your signal is at the input of your receivers with various levels of splitting. If you have a very strong signal, you can certainly use two or three passive splits. However, if you have a weaker signal, you should probably use an active splitter with a narrow bandwidth. The second part is very important, as you absolutely do not want undesired signals, such as adjacent TV stations, amplified as well - they'll only serve to desensitize your receivers (especially if they're lower end models with poor filtering inside).


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## Chris15 (Jan 27, 2008)

mbenonis said:


> There really isn't a rule of thumb here. You need to know how strong your signal is at the input of your receivers with various levels of splitting. If you have a very strong signal, you can certainly use two or three passive splits. However, if you have a weaker signal, you should probably use an active splitter with a narrow bandwidth. The second part is very important, as you absolutely do not want undesired signals, such as adjacent TV stations, amplified as well - they'll only serve to desensitize your receivers (especially if they're lower end models with poor filtering inside).



Now I remember where I pulled those numbers from, the Shure stuff on antenna distribution and placement. They say that 5dB of loss is about the maximum acceptable level. A passive split drops 3dB, so a second passive split would be 6dB and opening the door to problems. If you then have say a 10m or more coax run between antenna and receiver, then you need also to consider the loss caused by that cable.

Amplifiers mounted to antenna can help to overcome cable losses etc. But you can have too much and sometimes less is more.

Wideband v Narrowband - It's all about the right tool for the job. It may be that you are running a substantial number of systems and hence need to have systems on more than one band, then you will need a wideband splitter. This given Log Periodic antennae or Helical or other such wideband antennae. But if you have that many systems, the better option would probably be to use a wideband as the primary split and then run all the secondary splits with narrowband. Best of both worlds then. Of course if you are needing this sort of number of systems, you really need to be using high end systems which will have decent filtering built in...


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## BNBSound (Feb 10, 2008)

Hughesie89 said:


> well im the first person, so im going to offer the simplest answer crosstalk, and not knowing the US radio frequencies i would say you might be picking up a radio station tv station and the receiver doesn't know how to interpret the new signal, the reason it works close to the receiver is the mic's signal is stronger than all over sources until you move away when other signals become stronger



There's a couple issues with interference from broadcast sources. The first is the FM capture effect, wherein an FM receiver locks on to the strongest signal and decodes that to the exclusion of all others. The other and probably more likely culprit (but from the same source) is front end overload. Strong RF from any source can overwhelm the receiver's front end. If it's strong enough it doesn't even need to be in the same frequency range. Example: TV channel 2 (neighborhood of 50 MHz) totally overwhelming a guitar wireless (way up in UHF) at a nearby club.


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## mbenonis (Feb 11, 2008)

BNBSound said:


> There's a couple issues with interference from broadcast sources. The first is the FM capture effect, wherein an FM receiver locks on to the strongest signal and decodes that to the exclusion of all others. The other and probably more likely culprit (but from the same source) is front end overload. Strong RF from any source can overwhelm the receiver's front end. If it's strong enough it doesn't even need to be in the same frequency range. Example: TV channel 2 (neighborhood of 50 MHz) totally overwhelming a guitar wireless (way up in UHF) at a nearby club.



Yes indeed - both of these are potential problems. One of the frequencies posted above sits on top of a TV station's video carrier, just killing the bodypack's signal.

With regard to the UHF system being overwhelmed by the channel 2 TV station, that must be a very poorly designed wireless system! One would hope that even low-end RF systems would have SOME kind of band-pass filter on its front end! Any half-decent system should not see any problems from a 50 MHz noise source.


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## Chris15 (Feb 11, 2008)

mbenonis said:


> With regard to the UHF system being overwhelmed by the channel 2 TV station, that must be a very poorly designed wireless system! One would hope that even low-end RF systems would have SOME kind of band-pass filter on its front end! Any half-decent system should not see any problems from a 50 MHz noise source.



One would hope so, but given some of the stuff coming out of Asia these days... But honestly, a band pass filter is what a few cents?


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## mbenonis (Feb 11, 2008)

Chris15 said:


> One would hope so, but given some of the stuff coming out of Asia these days... But honestly, a band pass filter is what a few cents?



Yep. It really makes you wonder why they're not included...


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## BNBSound (Feb 13, 2008)

mbenonis said:


> With regard to the UHF system being overwhelmed by the channel 2 TV station, that must be a very poorly designed wireless system!



I wasn't remembering the situation with total accuracy. The club is actually located within 1/4 mile of an FM broadcast station around 107 MHz. The RF in the room is so strong that some of the outboard gear is rendered totally useless, and wireless anything is right out. The outboard thing may be because of the patch cables being close to a resonant wavelength. Full wavelength at that freq would be a little over 4.5 feet, Hosa 3 meter patches would be about 2 wavelengths, while 2 meter versions would be less resonant.


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## Chris15 (Feb 19, 2008)

Reminds me of something I read. There was a regular gig in what constitutes the alps down here where a keyboard was picking up FM. Turn it 90 degrees and it all went away. I think something was muttered about needing balanced lines and shielding in instruments...


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## Hughesie (Feb 19, 2008)

mbenonis said:


> Yep. It really makes you wonder why they're not included...



well when things are done "on the cheap" things get left out,

seriously would you buy this system
ebay link

highlights 
*< OVERALL TRANSMITTER>. 
Frequency responce 100Hz-10000Hz 
RF Power output 20mW(max) 
Antenna Type Built-in type 
mic Type Dynamic mic 
Current consumption Less than 25 mA 
Battery Life 8 Hr 

Each microphone is marked with red and green stripes which represent two separate frequencies*


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