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Author Topic: Direct TV interfering with RF mics  (Read 2464 times)

Karl Maciag

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Direct TV interfering with RF mics
« on: January 03, 2012, 10:53:56 am »

Need some help,  I've not seen anything like this before in an install.  It's a AV system that has 4 channels of Sennheiser 300 G3 wireless mics,  and we are using Sennheiser antenna and power distribution for the receivers.  The customer furnished a Direct TV receiver, which is new and their "professional" model.  They also provided a HDMI Distribution amplifier to send the signal to two displays.  We are using a Extron HDMI audio de-embedder to get audio from the HDMI signal into the audio system (clearone, crown amp, jbl speakers).  Power connectors on the Direct TV devices are two prong, not three.  The issue we are having is RF interference when the DTV receiver is powered on.  Turn the device off, and the interference goes away.  Also, the tech in the field noted that changing the channel on the receiver can also make the interference go away.  The problem happens if the audio from the DTV box is disconnected from the rest of the system as well.

Anyone have experience with these boxes emitting RF?  The spectrum in this room is already very crowded,  so i'm loathe to switch frequencies of the mics.  Thanks in advance for any help.
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George Dougherty

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Re: Direct TV interfering with RF mics
« Reply #1 on: January 03, 2012, 12:04:07 pm »

Running g3 mic's, easy to enough to scan and reset channels. Turn on the direct tv, turn off all but one unit and see if a scan turns up enough channels that aren't affected to be usable.
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Karl Maciag

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Re: Direct TV interfering with RF mics
« Reply #2 on: January 03, 2012, 12:08:55 pm »

Already have scanned,  but the RF real estate in this space is limited,  so having a device creating interference makes using the mics more difficult in an already challenging scenario.  I'm hoping to eliminate this extra source of interference...
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Brad Weber

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Re: Direct TV interfering with RF mics
« Reply #3 on: January 03, 2012, 01:30:54 pm »

What is the physical relationship between the Sennheiser receivers, the Sennhesier antennas and the DirecTV receiver?

Is the conclusion that the problem is RF into the wireless receivers based on the fact that changing receiver channel affects the results or have you actually measured RF emissions related to the DirecTV box operation?

This reminds me of a number of years ago when the Hughes-JVC 300 Series ILA projectors were released and we ran into multiple incidences of their wreaking havoc with wireless mics when the projectors were first turned on.  Took a spectrum analyzer out to one such site and found that sure enough, when the lamp was struck the projector emitted very high level, broadband RF noise.  Since the projector was properly certified we asked the manufacturer about this and their response was that the certification was for normal operating conditions, which was once the projector was on and did not include the initial lamp strike.  So we ended up having to mute all the wireless whenever someone turned on the projector.
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Karl Maciag

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Re: Direct TV interfering with RF mics
« Reply #4 on: January 03, 2012, 08:41:41 pm »

Brad,  the antennae (antennas? i never know which to write) are mounted in the rack using a Sennheiser front mount kit, in the same rack space as the antenna distribution device.  The receivers are mounted below that, 3 rack spaces total for the RF mics etc.  6 spaces beneath the RF mic gear is the DirectTV receiver.  Rack drawers are in between the RF mics and the DTV receiver. 

We were able to measure that when the Direct TV receiver was powered on,  and tuned to a standard def channel, RF noise was created,  centered at 519Mhz, but a rather wide bandwidth.  The squelch on the receivers has been set at -20dB to deal with the hectic RF noise floor in the space, so only mics tuned near 519 were experiencing troubles.  There was a lot of other noise in the space at the initial install, so that why these mics were put where they were.  When the receiver is tuned to a HD channel, the noise goes away.  The saving grace is the owner will only be tuned to one station, which is a HD station, so for the time being we are in ok shape....I'm just waiting for when someone changes the channel not knowing what could happen...

I find it hard to believe that it's normal behavior for the DTV receiver to act like this....it makes no sense to me,  but i'm not familiar with how they work,  so maybe i'm missing something.

We are planning on creating more physical space between the RF antennas and the DTV receiver, to at least see what will happen.  I think we will mount the antennas at the top of the rack,  and start with the receiver outside of the rack,  and finally at the bottom of the rack if no interference is picked up.  If interested, I'll follow up with any more findings.
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Hal Bissinger/COMSYSTEC

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Re: Direct TV interfering with RF mics
« Reply #5 on: January 03, 2012, 10:51:03 pm »

Looks to me like the antennas are way too close to the receiver. I would get them off the rack and out into the space with a remote antenna kit and cables.
 
-Hal
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Re: Direct TV interfering with RF mics
« Reply #5 on: January 03, 2012, 10:51:03 pm »


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