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Author Topic: A radio station plays through a projectors audio output to sound system HELP!  (Read 807 times)

Eric Stageman

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I have an audio system installed in a school auditorium that has multiple mic's (wired and wireless) as well as DVD,CD recorder/player, and a Multi-media projector mounted at the rear of the stage (rear-projection). 

The problem (Mexican Radio) occurs when the projector is shut down/turned off. The projector then goes into "cool down" mode...this is when a local radio station 1440AM/102.5FM  comes through the projector channel. Once the projector finishes cooling down and totally shuts off the radio station audio fades out. We have changed out projector makes and manufacturer and it still happens.  We have tried both balanced and unbalanced. We have changed the input channel on the Ashly NE 24.24M w/FR-16 network remote...and still have the issue.

We have narrowed it down to what we believe is a Power issue.  The auditorium is 61 years old (in its current form) The auditorium was actually built in 1933 but suffered a fire in 1951.  The power to the school is very susceptible to spikes and surges and is also very "Noisy" according to the maintenance staff the power was destroying computers and the like. They do have fairly recently installed  "Isolated Ground Receptacles" of which the projector is plugged into.  We did also plug the projector into other non isolated ground receptacles and the radio audio also came through...in addition the non isolated ground receptacles added enormous amounts of hum/noise. 

We are contemplating putting power conditioners on the amp rack, mixer, and the projector and wonder if that would alleviate the problem.  Looking for help!
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Mac Kerr

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I have an audio system installed in a school auditorium that has multiple mic's (wired and wireless) as well as DVD,CD recorder/player, and a Multi-media projector mounted at the rear of the stage (rear-projection). 

The problem (Mexican Radio) occurs when the projector is shut down/turned off. The projector then goes into "cool down" mode...this is when a local radio station 1440AM/102.5FM  comes through the projector channel. Once the projector finishes cooling down and totally shuts off the radio station audio fades out. We have changed out projector makes and manufacturer and it still happens.  We have tried both balanced and unbalanced. We have changed the input channel on the Ashly NE 24.24M w/FR-16 network remote...and still have the issue.

We have narrowed it down to what we believe is a Power issue.  The auditorium is 61 years old (in its current form) The auditorium was actually built in 1933 but suffered a fire in 1951.  The power to the school is very susceptible to spikes and surges and is also very "Noisy" according to the maintenance staff the power was destroying computers and the like. They do have fairly recently installed  "Isolated Ground Receptacles" of which the projector is plugged into.  We did also plug the projector into other non isolated ground receptacles and the radio audio also came through...in addition the non isolated ground receptacles added enormous amounts of hum/noise. 

We are contemplating putting power conditioners on the amp rack, mixer, and the projector and wonder if that would alleviate the problem.  Looking for help!

The most common fault for letting in AM radio is a cold solder joint on a cable, but a cold solder joint in equipment may also demodulate AM radio. You also need well shielded cables with the shield correctly connected at both ends.

It will be easier to help you through troubleshooting with a complete description of the sound system, including makes and models, as well as how everything is connected.

Mac
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John Roberts {JR}

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Rectification (decoding radio audio from RF)  occurs in active circuitry when the speed/amplitude of the RF is too fast for the active circuitry to follow.

Your symptom of this occurring in an input when it is turned off, sounds like the normally low impedance output becomes high impedance when the power is removed, so the amplitude of the RF interference increases and causes rectification to occur.

Consider adding some caps across the the outputs that are small enough to not affect the audio when on, but large enough to attenuate the RF when off.

JR
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Tune it or don't play it... please

Mike Sokol

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The problem (Mexican Radio) occurs when the projector is shut down/turned off. The projector then goes into "cool down" mode...this is when a local radio station 1440AM/102.5FM  comes through the projector channel. Once the projector finishes cooling down and totally shuts off the radio station audio fades out. We have changed out projector makes and manufacturer and it still happens.  We have tried both balanced and unbalanced. We have changed the input channel on the Ashly NE 24.24M w/FR-16 network remote...and still have the issue.

Maybe I'm missing something obvious, but why is there an audio feed from the projector to the mixing console?

Mike Caldwell

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Maybe I'm missing something obvious, but why is there an audio feed from the projector to the mixing console?

I've seen lots of installs were the various video source inputs had both audio and video ran to the projector and the projector is used as the audio source selector feeding back to the sound system.
AV guys and the designers of system installs have not figured out you can not run hundreds of feet of unbalanced audio cables through a building and not have problems, that applies to both the inputs and outputs of a projector. Even better when the projector output is a stereo mini jack and at the sound system input it is wired to a balanced input, depending on the source material the phase cancellation can be impressive!
« Last Edit: April 12, 2014, 09:00:11 pm by Mike Caldwell »
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Steve M Smith

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Even better when the projector output is a stereo mini jack and at the sound system input it is wired to a balanced input, depending on the source material the phase cancellation can be impressive!

I had that at a live show once. A company put on a laser show with their own music tracks on mini disc.  They had a lead with two RCA plugs into an XLR plug which they assured me had both inputs summedd to pin 2 with pin 1 and 3 grounded.  It seemed to work o.k. when testing it so it was used.  One track must have been almost mono as it almost cancelled out completely.  Luckily everything else was sufficiently different left to right that it sounded o.k.

After a bit of investigation, it turned out that they had a similar lead with an XLR socket and they would regularly use them at each end of a mic lead to make a long stereo RCA to RCA lead. The adaptors had left and right going to pins 2 and 3 and should definitely not have been used into a balanced input!

I now check everything which is offered as an input!


Steve.


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Mike Sokol

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Consider adding some caps across the the outputs that are small enough to not affect the audio when on, but large enough to attenuate the RF when off.

I've used Neutrik ECM XLR connectors with success on a few installations with "Mexican Radio". They cost less than $10 each and have built-in RFI filters: http://www.neutrik.com/en/xlr/xlr-cable-connectors/emc-series/

Frank DeWitt

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I would start with a audio isolation transformer at the projector with a ground lift and a balanced output. You may not be able to ground both ends of the balanced signal.

If you want to try something before buying a transformer just for this, come out of the projector with a very short cord into a DI box.  Then from the DI box to the board with a balanced mic level signal.  If that works, the isolation transformer will work and give you a line input.

Either way I would use the Neutrik ECM XLR that Mike suggested.
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Tim Perry

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I have an audio system installed in a school auditorium that has multiple mic's (wired and wireless) as well as DVD,CD recorder/player, and a Multi-media projector mounted at the rear of the stage (rear-projection). 

The problem (Mexican Radio) occurs when the projector is shut down/turned off. The projector then goes into "cool down" mode...this is when a local radio station 1440AM/102.5FM  comes through the projector channel. Once the projector finishes cooling down and totally shuts off the radio station audio fades out. We have changed out projector makes and manufacturer and it still happens.  We have tried both balanced and unbalanced. We have changed the input channel on the Ashly NE 24.24M w/FR-16 network remote...and still have the issue.

We have narrowed it down to what we believe is a Power issue.  The auditorium is 61 years old (in its current form) The auditorium was actually built in 1933 but suffered a fire in 1951.  The power to the school is very susceptible to spikes and surges and is also very "Noisy" according to the maintenance staff the power was destroying computers and the like. They do have fairly recently installed  "Isolated Ground Receptacles" of which the projector is plugged into.  We did also plug the projector into other non isolated ground receptacles and the radio audio also came through...in addition the non isolated ground receptacles added enormous amounts of hum/noise. 

We are contemplating putting power conditioners on the amp rack, mixer, and the projector and wonder if that would alleviate the problem.  Looking for help!

Assuming it is this station http://radio-locator.com/info/KTNO-AM  it is a 50,000 watt "flamethrower" in the daytime. When it goes 350 watts directional at night I'm guessing you wont have as much problem.

Place an audio isolation transformer at one or both ends of the audio path. 

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Stephen Swaffer

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Perhaps there is a good reason the most obvious and simple solution is not invoked.  If the projector is in cool down or off, the projector input is no longer needed-IMO good practice is not having any unused inputs on to add noise to the system.
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Steve Swaffer
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