Wireless Static buildup causing interference

[rmccue]

We're coming up on the opening night of a school musical (Sweeney Todd) and we're noticing interference coming through the wireless microphones. We've noticed that though we start off with no interference, it gradually increases, and we've put it down to a static buildup, due to the microphone belt packs being situated inside lyrca/nylon holders underneath the actors' costumes.

At the moment, our solution to this is to get the actors to discharge before they go onto stage, however some of the leads stay on stage for quite a while and the interference is getting too great.

Aside from replacing the holders or anything like that (too late to do so), can you guys think of any way we could stop this from occurring?

 

[gpforet]

I'm not so sure it's the result of static buildup. I would suspect more that the transmitters are getting hot, and drifting slightly. This happens alot with low and mid-shelf wireless. An easy test would be to set a transmitter down, send some signal thru it by placing the mic in front of a radio or some other sound source (so the transmitter is actually transmitting a signal, otherwise, it'll be idling and won't be working hard enough), cover the transmitter with something to trap the internal heat build-up, and see if you get interference after an hour or so of operation. My guess is that as the transmitter gets warm, both from body temperature, battery heating as the result of current drain, and the heat from the transmitter circuitry, you're getting some frequency drift.

 

[epimetheus]

What brand/model of wireless might help as well.

 

[museav]

You say interference, but that seems to imply an external noise source or bodypacks interfering with one another. Is it actually increasing interference or instead decreasing signal or maybe noise getting in via the mic or mic cable? Are you experiencing hearing other RF sources, dropouts, some varying noise or a fairly consistent noise? Are there any other differences than time, such as characters, and thus possibly transmitters, being much closer physically or significantly different set pieces or larger number of transmitters on stage?

Sweat and moisture can affect bodypacks and antennas, in effect grounding them to the actor and thus reducing the effective radiation. That could be a factor but both that or thermal drift would tend to cause dropouts and not just interference or noise. The mic cable constantly rubbing against costumes, wigs, etc. could cause noise.

 

[mbenonis]

I tend to doubt static is causing an RF problem, but I suppose it could cause a problem on the audio end of things. That said, the sweat ought to keep static buildup down. I'm leaning more toward the thermal issue though. What brand and model wireless is this?

Like someone above said, have you tried leaving the pack out and on? Try covering it too to cause heat buildup and see if you can replicate the problem.

Also, have you ruled out other types of interference, such as intermodulation products, high-power RF from TV stations, etc?

 

[rmccue]

An easy test would be to set a transmitter down, send some signal thru it by placing the mic in front of a radio or some other sound source (so the transmitter is actually transmitting a signal, otherwise, it'll be idling and won't be working hard enough), cover the transmitter with something to trap the internal heat build-up, and see if you get interference after an hour or so of operation. My guess is that as the transmitter gets warm, both from body temperature, battery heating as the result of current drain, and the heat from the transmitter circuitry, you're getting some frequency drift.

OK, I'll try doing this and see if it does anything. If it does turn out to be heat, what could we do to work around this?

What brand/model of wireless might help as well.

It's a model made by a local company, IJS. I don't remember the actual model number off-hand.

You say interference, but that seems to imply an external noise source or bodypacks interfering with one another. Is it actually increasing interference or instead decreasing signal or maybe noise getting in via the mic or mic cable? Are you experiencing hearing other RF sources, dropouts, some varying noise or a fairly consistent noise? Are there any other differences than time, such as characters, and thus possibly transmitters, being much closer physically or significantly different set pieces or larger number of transmitters on stage?

Sorry, I meant to say noise. We're having several microphone changes between characters, however this does not seem to be affecting it. The noise is only occurring on those packs which are dedicated to a single person.

Sweat and moisture can affect bodypacks and antennas, in effect grounding them to the actor and thus reducing the effective radiation. That could be a factor but both that or thermal drift would tend to cause dropouts and not just interference or noise. The mic cable constantly rubbing against costumes, wigs, etc. could cause noise.

The noise occurs while the actors are standing still, unfortunately.

Thanks for the help guys, I'll try testing for heat buildup and get back to you.

 

[mbenonis]

It's a model made by a local company, IJS. I don't remember the actual model number off-hand.

Interesting - can you please take some pictures of the transmitters and the receivers for me?

 

[rmccue]

Interesting - can you please take some pictures of the transmitters and the receivers for me?

I'll get some ASAP.

We've started sticking aluminium foil to the top of the antennas on the belt packs, and that seems to be working for now.

 

[Chris15]

IJS - IJS Logistics, Brisbane Concert Sound & Lighting and audio visual specialists, professional audio systems.
Apparently UHF and 193ch...

 

[rmccue]

IJS - IJS Logistics, Brisbane Concert Sound & Lighting and audio visual specialists, professional audio systems.
Apparently UHF and 193ch...

We're actually running the microphones on 180-200Mhz (VHF), not UHF. That does sound like the company though.

We had our technical consultant come in and he advised us to daisy-chain the antennas for the receivers, and everything seems to be working now. Previously, we had one or two antennas on each receiver.

Since I wasn't actually there to talk to him, does anyone here know how that would have fixed the problem?

 

[museav]

We're actually running the microphones on 180-200Mhz (VHF), not UHF. That does sound like the company though.

We had our technical consultant come in and he advised us to daisy-chain the antennas for the receivers, and everything seems to be working now. Previously, we had one or two antennas on each receiver.

Since I wasn't actually there to talk to him, does anyone here know how that would have fixed the problem?

Some type of antenna distribution would make sense as far as potentially improving reception and avoiding some RF related problems. Are you saying that the receivers have internal antenna distribution that allows connecting the antennas to one receiver then passing that through to the next receiver and so on?

 

[Chris15]

Ok, so let's consider the bigger problem here... You're sitting right on top of the main Brisbane VHF transmitters... The frequencies you are talking about are channels 7, 8, 9 and possibly 9A. 7 is 7 analog, 8 is 9 digital, 9 is 9 and 9 is now digital radio - a possible cause of issues if they have only started appearing in the last few months...

Basically VHF is useless in metro Aus and in this case the Gold Coast will probably be close enough to Brissy for it to be a problem...

 

[rmccue]

Some type of antenna distribution would make sense as far as potentially improving reception and avoiding some RF related problems. Are you saying that the receivers have internal antenna distribution that allows connecting the antennas to one receiver then passing that through to the next receiver and so on?

From memory, I think we have a sort of "master", which has 4 antenna outputs and 2 antenna inputs, so it passes the signal on. The outputs from these are plugged into the inputs for the receivers. I could be wrong on this however.

Ok, so let's consider the bigger problem here... You're sitting right on top of the main Brisbane VHF transmitters... The frequencies you are talking about are channels 7, 8, 9 and possibly 9A. 7 is 7 analog, 8 is 9 digital, 9 is 9 and 9 is now digital radio - a possible cause of issues if they have only started appearing in the last few months...

Basically VHF is useless in metro Aus and in this case the Gold Coast will probably be close enough to Brissy for it to be a problem...

I'm reasonably sure that the mics can't be tuned outside of 170-210Mhz, so we'll have to deal with that. Do you know where I could find out the exact frequencies of those channels?

 

[Chris15]

From memory, I th"ink we have a sort of "master", which has 4 antenna outputs and 2 antenna inputs, so it passes the signal on. The outputs from these are plugged into the inputs for the receivers. I could be wrong on this however.


I'm reasonably sure that the mics can't be tuned outside of 170-210Mhz, so we'll have to deal with that. Do you know where I could find out the exact frequencies of those channels?

You have an antenna splitter - this "master"

Ok... So you cannot operate below 174 MHz, it's allocated to licenced users. TV channel 6 is 174 - 181 M, 7 is 181 - 188, 8 is 188 - 195, 9 is 195 - 202. All of those channels are in use. 7 & 9 are currently analog, 6 & 8 are digital. You got no hope of squeezing into a digital channel. You MIGHT be able to slip into the gaps in the analog transmissions (between the video and audio carriers and in the guard bands).

But, consider this... you are allowed to run these radio mics by authority of the Low Interference Potential Devices Class Licence. Under that licence you cannot operate within the licence area of an active TV station. I don't know whereabouts on the Gold Coast you are but you may be within the Brisbane Licence Area (there is some overlap between the GC licence area and the Brisbane one according to ACMA.)

Basically the best way to reliable operation is to move to UHF mics.

 

[museav]

From memory, I think we have a sort of "master", which has 4 antenna outputs and 2 antenna inputs, so it passes the signal on. The outputs from these are plugged into the inputs for the receivers. I could be wrong on this however.

We had our technical consultant come in and he advised us to daisy-chain the antennas for the receivers, and everything seems to be working now. Previously, we had one or two antennas on each receiver.

Maybe I'm missing something but since it apparently helped I'm trying to understand how the daisy-chaining of the antennas and your having one or two antennas per receiver relates to your having antenna distribution. I am also not sure what you mean by "daisy-chain the antennas", do you actually have one antenna looped through another and then feeding an input of the antenna distribution unit?

 

[Chris15]

What I think might be happening is that the local oscillators of the receivers are coupling through the antennas if there was an antenna per receiver. Are these diversity receivers?

 

[mbenonis]

What I think might be happening is that the local oscillators of the receivers are coupling through the antennas if there was an antenna per receiver. Are these diversity receivers?

That was my thought, and would explain why an antenna distro would quiet the problem down (because of a decent amount of attenuation from output to output port).

 

[rmccue]

But, consider this... you are allowed to run these radio mics by authority of the Low Interference Potential Devices Class Licence. Under that licence you cannot operate within the licence area of an active TV station. I don't know whereabouts on the Gold Coast you are but you may be within the Brisbane Licence Area (there is some overlap between the GC licence area and the Brisbane one according to ACMA.)

Basically the best way to reliable operation is to move to UHF mics.

Correct me if I'm wrong, but isn't VHF TV being phased out? And unfortunately, we don't currently have the funds for 12 new packs.

Do you have a link to where I could find out about the areas? I've searched ACMA's site several times, but couldn't find anything useful.

Maybe I'm missing something but since it apparently helped I'm trying to understand how the daisy-chaining of the antennas and your having one or two antennas per receiver relates to your having antenna distribution. I am also not sure what you mean by "daisy-chain the antennas", do you actually have one antenna looped through another and then feeding an input of the antenna distribution unit?

I was told they were daisy-chained by one of the staff, not by our consultant, so I believe he meant we changed from having antennas on each receiver to using the ones on the antenna splitter.

What I think might be happening is that the local oscillators of the receivers are coupling through the antennas if there was an antenna per receiver. Are these diversity receivers?

I'm lead to believe so, as each has 2 antenna inputs (as do the antenna splitter boxes).

 

[Chris15]

Correct me if I'm wrong, but isn't VHF TV being phased out? And unfortunately, we don't currently have the funds for 12 new packs.

Do you have a link to where I could find out about the areas? I've searched ACMA's site several times, but couldn't find anything useful.

Alright, so yes I am correcting you if you are wrong... VHF Band III television is here to stay. RF channels 0, 1, 2, 3, 4, 5 & 5A are not being used for new assignments (3, 4 & 5 are actually the FM radio band). Channels 6 through 12 are expected to stay for the foreseeable future.

Analog TV will be discontinued over the next few years progressively, but the main metro areas are not scheduled to be switched off until some time in 2013.

Any free space created by the eventual digital transition is expected to be compacted down and the freed spectrum auctioned.

ACMA are rather useless for most things, this is one of them... As an idea, check the SBS digital coverage map here. It's not totally accurate being a UHF transmission but they are designed to offer reasonably similar coverage patterns in UHF and VHF...

 

[rmccue]

Alright, so yes I am correcting you if you are wrong... VHF Band III television is here to stay. RF channels 0, 1, 2, 3, 4, 5 & 5A are not being used for new assignments (3, 4 & 5 are actually the FM radio band). Channels 6 through 12 are expected to stay for the foreseeable future.

[...]

ACMA are rather useless for most things, this is one of them... As an idea, check the SBS digital coverage map here[/here]. It's not totally accurate being a UHF transmission but they are designed to offer reasonably similar coverage patterns in UHF and VHF...

Alright, so we're outside the Brisbane region, but by the looks of it we're inside the Gold Coast region anyway.

Basically, the best solution to this would be to get new microphones which operate on UHF instead, correct?