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Old May 15th, 2004, 01:16 PM   #1
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Radio reception on XLR cable

On a recent indoor shoot I experienced radio reception on a setup I've used before indoors with no problems. The setup is a Sony lav (EMC44B) connected to a 10-ft XLR cable, in turn connected to a Studio box plugged into my trusty, classic TRV900.
The XLR cable is Comprehensive brand Premium EXF, shielded and so forth, and has produced good results except for this occasion.
If I were to use Star Quad cable instead, will that avoid radio reception 100% of the time, or is this a problem that can occur on occasion regardless of cables and equipment?
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Old May 15th, 2004, 01:56 PM   #2
 
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That's exceptionally rare, no doubt, and anytime you may have a broken shield or loose ground, you can get RF in there.
Nothing can guarantee ZERO interference, even the best cables too close to a powerful coil might see noise, but you could say that a star quad will nearly always prevent noise, yes.
Been hanging around Tesla's lately? :-)

Are you absolutely sure you don't have a leaky shield in either the mic cable or in the transformer?
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Old May 15th, 2004, 02:45 PM   #3
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Spot,
Appreciate the quick reply.
On a visual inspection, the Studio box and the thin cable attached to the mic both look intact. So does the XLR cable. I didn't think to change the ground switch on the Studio box, though, to see if that would help.
From what you're saying, it sounds like I should do some tests to see if there is still RF, and if so to determine in which component it occurs. If the problem is my XLR cable, I'll replace it with a star quad type.
Thanks again.
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Old May 15th, 2004, 11:53 PM   #4
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Can the Star Quad, double-balanced cables be fitted into standard XLR plugs and perform the extra shielding properly? If special plugs are needed for them, can you buy them pre-fitted with plugs in short lengths?

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Old May 16th, 2004, 03:31 PM   #5
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You can build your own or have Markertek build them. They do a nice job and they are not very expensive.

www.markertek.com
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Old May 16th, 2004, 05:30 PM   #6
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That's interesting, a couple weeks ago while recording one of our operas all of a sudden I picked up some sort of mobile radio, like a cab or police car outside on the street. It was rather startling to hear on the headphones! Now I haven't been back to listen to the tape yet, but I *think* I heard it on both channels. I was using the original XLR box on my PDX-10 with a line level feed from the house sound board on ch 1 and the on-camera Sony mike on ch 2. Have never experienced this before or since. I talked to people backstage and none of them heard this over the house system.

I have read a number of anecdotes of Sony cameras picking up RF, so I wonder if that's what happened? But then I really don't know much about this stuff... would the camera itself be able to pickup a radio signal without having cables plugged in as "antennas"?
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Old May 16th, 2004, 05:37 PM   #7
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Star Quad cable uses standard XLR's. Standard cable has two conductors loosely twisted within a shield.If each conductor receives noise at an equal level then the "balance input' will be able to reject the noise completely. If the interference attacks from the side of the pair one conductor will pick up more noise than the other and the pair will not balance (cancel)each other out.

Starquad has 4 conductors , two for each leg. This creates a round cross section making it more likely that each conductor will pick up the same noise and cancel the opposite polarity out.

Sorta kinda paraphrased from Jay Rose's book.

Just solder the two conductor of the same colour to one pin. It's a bit more work but worth it.

The cost isn't that much more or at least it shouldn't be
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Old May 16th, 2004, 05:49 PM   #8
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Although XLR cables (and circuits) are designed to minimize line noise I don't think this phenomenon is quite as rare as you might think. Last summer while shooting a scene outdoors near a museum I encountered two such oddities in the same day.

First, we were using commercial-grade Motorola handheld radios to cue and coordinate a car. While I did not pick up any voices I did notice (later in post) that I picked-up a rather prominent low "click" each time I pressed the transmit button on my unit while the camera was rolling.

Later in the day I began picking up a constant low hum from a different cable. After some experimentation we were able to position the cable to prevent this. My theory, given the location of the shoot, was that there was an underground power transformer immediately below the cable.
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Old May 16th, 2004, 06:49 PM   #9
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One of the big differences between cameras made for BQ and prosumer cameras is the amount and type of shielding. In BQ cameras the bodies are a heavier gauge metal with plenty of shielding around key audio and video components. Consumer cameras are mostly plastic and are not as easily shielded from RF.

In your case Boyd, the radio was probably not within FCC spec and was transmitting either off band or over power, or both. An example of this type of RF is the guy that hooks up a 50 watt transmitter to his CB. The FCC limits the band to 5 watts, but at 50 watts you would hear the CB over a phone line, house PA, camera etc.
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Old May 16th, 2004, 06:50 PM   #10
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There's anomalous RF interference kicking around all over and sometimes moving just a few feet or turning sideways will change what you pick up.

I bought a Sony wireless headphone set, as I thought it would let me shoot without fighting the wire from the camera. I use a shoulder-mount rig that has room to attach the tranmitter on the underside. But, in about half my locations, I was picking up more 9MHz analog cell-phone conversations than my own audio. Some of that stuff was scandalous, but I was there to make video.

This whole setup was a bust, but I now use the wireless headphones daily, when I crank away on my noisy exercise machine while watching TV.

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Old May 16th, 2004, 09:20 PM   #11
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<<<-- Originally posted by Boyd Ostroff : I have read a number of anecdotes of Sony cameras picking up RF, so I wonder if that's what happened? But then I really don't know much about this stuff... would the camera itself be able to pickup a radio signal without having cables plugged in as "antennas"? -->>>

Quite easily. Prosumer cameras have almost no shielding and run some very sensitive internal signals in unshielded conductors (lots of flat ribbon 'cables'). Pro cameras usually have real metal shielding over the sensitive circuit boards and are more carefully designed and constructed.

Sony Image Stabilization in the 2000/150/2100/170 series cameras is very susceptible to RFI. Get near a police car when they are transmitting and the camera will physically jump from the OIS hitting the stops. The PC-110 is also effected but that's not an optical stabilizer so the image just jumps, not the camera too.

If you get an audio connector with a little bit of corrosion between the pin and socket, sometimes that acts like a crystal detector (remember the crystal radio sets?) and you can hear a radio station.
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