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Author Topic: Wireless microphones in classrooms - interference  (Read 4316 times)

Martin Pacholuk

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Wireless microphones in classrooms - interference
« on: September 17, 2013, 05:56:24 pm »

I work at a school in downtown San Francisco. I currently run 10 wireless channels, one in each classroom, spread over three floors in one building (one floor has 5 channels). I'm using AKG WMS 450 receivers, PT 450 belt packs, and CU 400 charging units. The systems are operating in two different frequency bands: 500.100-530mhz and 570-600mhz.

The systems have worked well for the most part, except in the past year or so, I've started to experience more interference in the form of static getting picked up by the receivers, mostly when the transmitter is off.

The systems were setup by an AV contractor, who worked directly with AKG to find the best frequencies for our location (which granted, is a crowded spectrum).

I've played around with adjusting the frequencies and the squelch, but haven't had any luck getting an interference free system.

Things I'm considering:

- Purchase an external antenna - though I'm not sure this would help when the transmitter is off.
- Switching to a digital system - I'm looking at the higher-end digital AKG system (I need to have a drop-in recharger system, so that limits my options somewhat).
- Try to tune the systems by hiring someone who really knows wireless systems.

Thanks in advance for any advice.

Martin

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Tom Young

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Re: Wireless microphones in classrooms - interference
« Reply #1 on: September 17, 2013, 06:46:33 pm »

I work at a school in downtown San Francisco. I currently run 10 wireless channels, one in each classroom, spread over three floors in one building (one floor has 5 channels). I'm using AKG WMS 450 receivers, PT 450 belt packs, and CU 400 charging units. The systems are operating in two different frequency bands: 500.100-530mhz and 570-600mhz.

To start, we need to know which antennae you are using, where they are located and the distances involved.  Also any interface devices such as amplifiers, combiners and/or splitters.  Please provide model numbers and the type of antenna so I/we don't have to look this up in our attempt to help you.
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Tom Young
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Tom Burgess

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Re: Wireless microphones in classrooms - interference
« Reply #2 on: September 18, 2013, 11:07:55 am »

Interesting that they worked fine for some time and then started acting up intermittently.  That would lead me to believe that something has moved in on your space since the installation.  With the 450 series being frequency agile and having the ability to scan for available freq's, it may be as simple as reprogramming each set with respect to the new rf traffic.  It's not difficult but could be a little time consuming.  Can you contact the contractor and set up a service call?
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Jim Thorn

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Re: Wireless microphones in classrooms - interference
« Reply #3 on: September 18, 2013, 12:14:20 pm »

Martin,

   I was in the maintenance department of a large school district outside Houston, Texas, and we had many inexpensive wireless mics in use in classrooms and gyms around the district.  Many of them were fixed-frequency units, so I didn't have the luxury of retuning.  As the 500 and 600 MHz bands became more crowded, we began to get new interference, also.  In many cases, our transmitters were much stronger than the outside interference while they were transmitting, but we got bursts of noise when the transmitters were turned off, even at maximum squelch. 

   For one series, I found an internal coarse squelch adjustment on the receiver's circuit board.  When I set it so that the front panel squelch control would mute the outside interference when it was at the middle of its adjustment range, I found that our transmitters were still strong enough to remain unsquelched from any place in the desired area of operation, and they swamped out the interference.  If you still have a problem after following Tom Burgess's suggestion of scanning and retuning, AKG tech support may be able to provide a way for you to increase the squelch range.  It seems worthwhile to try it to avoid the cost of total replacement.  If you beat the problem, would you please post your success story?  I, for one, would be interested in hearing it!

Best wishes,
Jim Thorn

I work at a school in downtown San Francisco. I currently run 10 wireless channels, one in each classroom, spread over three floors in one building (one floor has 5 channels). I'm using AKG WMS 450 receivers, PT 450 belt packs, and CU 400 charging units. The systems are operating in two different frequency bands: 500.100-530mhz and 570-600mhz.

The systems have worked well for the most part, except in the past year or so, I've started to experience more interference in the form of static getting picked up by the receivers, mostly when the transmitter is off.

The systems were setup by an AV contractor, who worked directly with AKG to find the best frequencies for our location (which granted, is a crowded spectrum).

I've played around with adjusting the frequencies and the squelch, but haven't had any luck getting an interference free system.

Things I'm considering:

- Purchase an external antenna - though I'm not sure this would help when the transmitter is off.
- Switching to a digital system - I'm looking at the higher-end digital AKG system (I need to have a drop-in recharger system, so that limits my options somewhat).
- Try to tune the systems by hiring someone who really knows wireless systems.

Thanks in advance for any advice.

Martin
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Jason Glass

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Re: Wireless microphones in classrooms - interference
« Reply #4 on: September 18, 2013, 12:40:54 pm »

Martin,

   I was in the maintenance department of a large school district outside Houston, Texas, and we had many inexpensive wireless mics in use in classrooms and gyms around the district.  Many of them were fixed-frequency units, so I didn't have the luxury of retuning.  As the 500 and 600 MHz bands became more crowded, we began to get new interference, also.  In many cases, our transmitters were much stronger than the outside interference while they were transmitting, but we got bursts of noise when the transmitters were turned off, even at maximum squelch. 

   For one series, I found an internal coarse squelch adjustment on the receiver's circuit board.  When I set it so that the front panel squelch control would mute the outside interference when it was at the middle of its adjustment range, I found that our transmitters were still strong enough to remain unsquelched from any place in the desired area of operation, and they swamped out the interference.  If you still have a problem after following Tom Burgess's suggestion of scanning and retuning, AKG tech support may be able to provide a way for you to increase the squelch range.  It seems worthwhile to try it to avoid the cost of total replacement.  If you beat the problem, would you please post your success story?  I, for one, would be interested in hearing it!

Best wishes,
Jim Thorn

Hi Jim,

I applaud your very clever solution to the problem, but it is important to know that opening the case and altering the squelch characteristics to allow use on occupied DTV channels defeats intentional design safeguards and violates these federal regulations:

47 C.F.R. 15.5
(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator.

47 C.F.R. 15.15
(b) An intentional or unintentional radiator must be constructed such that the adjustments of any control that is readily accessible by or intended to be accessible to the user will not cause operation of the device in violation of the regulations.

Joseph D. Macry

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Re: Wireless microphones in classrooms - interference
« Reply #5 on: September 19, 2013, 11:19:00 am »

I work at a school in downtown San Francisco. I currently run 10 wireless channels, one in each classroom, spread over three floors in one building (one floor has 5 channels). I'm using AKG WMS 450 receivers, PT 450 belt packs, and CU 400 charging units. The systems are operating in two different frequency bands: 500.100-530mhz and 570-600mhz.

The systems have worked well for the most part, except in the past year or so, I've started to experience more interference in the form of static getting picked up by the receivers, mostly when the transmitter is off.

The systems were setup by an AV contractor, who worked directly with AKG to find the best frequencies for our location (which granted, is a crowded spectrum).

I've played around with adjusting the frequencies and the squelch, but haven't had any luck getting an interference free system.

Things I'm considering:

- Purchase an external antenna - though I'm not sure this would help when the transmitter is off.
- Switching to a digital system - I'm looking at the higher-end digital AKG system (I need to have a drop-in recharger system, so that limits my options somewhat).
- Try to tune the systems by hiring someone who really knows wireless systems.

Thanks in advance for any advice.

Martin

Have you considered using IR systems, vs RF? Signal won't go outside the room.
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Joseph Macry,
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Jim Thorn

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Re: Wireless microphones in classrooms - interference
« Reply #6 on: September 23, 2013, 02:24:07 am »

Jason,

   With the great 700 MHz exodus, I became acutely aware of the existence of every TV transmitter in the Houston area, so I know that my classroom systems were not on any active TV channels.  When I heard the interference, I could not hear any modulation of the signal, just a loud white noise.  I assumed that my flea-power wireless mic would not have enough signal strength outside the building to interfere with the competing signal (whatever it was).  Safe assumption?  I see by your signature line that you specialize in wireless issues.  Do you know what service would have been present on an unused UHF TV channel?

Thanks!
Jim Thorn
Hi Jim,

I applaud your very clever solution to the problem, but it is important to know that opening the case and altering the squelch characteristics to allow use on occupied DTV channels defeats intentional design safeguards and violates these federal regulations:

47 C.F.R. 15.5
(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator.

47 C.F.R. 15.15
(b) An intentional or unintentional radiator must be constructed such that the adjustments of any control that is readily accessible by or intended to be accessible to the user will not cause operation of the device in violation of the regulations.
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Jason Glass

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Re: Wireless microphones in classrooms - interference
« Reply #7 on: September 23, 2013, 01:12:59 pm »

Jason,

   With the great 700 MHz exodus, I became acutely aware of the existence of every TV transmitter in the Houston area, so I know that my classroom systems were not on any active TV channels.  When I heard the interference, I could not hear any modulation of the signal, just a loud white noise.  I assumed that my flea-power wireless mic would not have enough signal strength outside the building to interfere with the competing signal (whatever it was).  Safe assumption?  I see by your signature line that you specialize in wireless issues.  Do you know what service would have been present on an unused UHF TV channel?

Thanks!
Jim Thorn

Hi Jim,

Interestingly enough, even very low power transmitters like mics can generate enough signal to be disruptive to other communications outside your building, although it all depends on how well the structure shields the signal.  Since there are so many variables, it's tough to make safe assumptions without setting up test equipment and taking measurements.  Without knowing more, a conservative 1KM radius of coverage is a reasonable safety margin.

Other services besides DTV in the UHF spectrum include broadcast translators and low-power television stations.  Sometimes it's tough to weed out which of these will cause trouble based on FCC database geographic searches.  Land Mobile 2-way radios, including public safety systems, can be assigned from 470MHz to 512MHz in some major cities, including channel 17 in Houston.  Radio licenses to coastal oil and gas companies are also possible.

The most difficult sources of interference to predict are intermodulations generated by poorly shielded or malfunctioning high power equipment operating nearby.  Anything from electric motors & controls, to sodium vapor lamp ballasts, to neon lighting chargers can be the culprit.  Intermod from your own wireless equipment is easy to predict with software like WWB6 or PWS IAS, but outside sources are nearly impossible to figure out without test and measurement gear.
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