Mysterious radio reception

[Bill Spearman]

I was doing a small commercial job yesterday using 2 mics - wireless and wired AT-987. The wireless running direct to a Beachtek on my GL-2 , and the wired running through a MixPre to the Beachtek. I was using the 897 to get ambient sounds form the activity.

My headphones were hooked up to the camera, not the MixPre. As I put them on to check levels, I was entertained by a noticable level of music. It was very cold here in SW FL (don't laugh, but 40 degrees is cold for us) and I earlier in the day has some RF interference on another job with a wireless mic, so first thing was to disconnect the wireless and was surprised the noise did not go away. To the best of my knowledge after checking, it was not within the building I was in.

It had to be the wired mic or the cables attached to it, I believe. The wireless was fine, and so I did the job with it and disconnected the wired 897. It's the first time i ever experienced this situation. Any thoughts, suggestions or comments?

[Jay Massengill]

The RF signal could be leaking in at a number of spots, but my first guess would be the cable between the AT-897 and the MixPre (since it's coming in to a mic-level input and being amplified). Hopefully it's not the mic itself that's leaking in the signal.
Did you ever listen with headphones at the MixPre?
How long was the cable between the AT-897 and the MixPre?
Were you running the mic on internal battery or phantom power from the MixPre?
I'd suggest getting a cable tester that can check XLR cables and latch onto intermittent problems in the wiring. Or if you already have a multitester you can check the cable yourself although it's easier to do if you have 4 hands to make sure you're getting good contact with all the leads.
You should definitely track this down, either to the cable or the mic or some other component. It's likely to show up again and at the worst possible moment.

[Bill Spearman]

I did not listen at the MixPre, should have thought of that.

It was a 15' cable between AT-897 & MIxPre
Mic was on internal battery,

Agree completely, I want to track it down before Mr. Murphy drops by again at the worst time, as you say. I have mostly new cabnles, but perhaps that was one of the older ones, I will experiment with that. Good thought, thanks.

[Mark Boyer]

What you have is another form of RF interference. You are too close to a high power transmitter and the RF is bleeding into your system through a non shielded cable. Running any unshielded cable (the connecton between the Beachtek unit and camera is not shielded) you take a chance on interference. Running a long un shielded cable is almost as bad as hooking your system up to a big antenna.

[Allan Black]

Bill you say you were entertained by the music, it wasn't the buildings wireless Muzak by any chance? If so it must be a new supplier, it sounds pretty ghastly down here.

When you plug the leak take all cables and everything back to exactly the same spot and give it another shot.

Cheers.

[Rick Reineke]

Radio signals can most definitively leak in via mic cables. However a cable tester will not normally indicate this specific issue.
Use star-quad cables or at least high quality 100% shield 2 conductor cables.
This is business as usual in midtown Manhattan, aka, RF HELL.

[Andy Tejral]

Back in my Live Van days, there were occasions, in the shadow of transmitter towers, where a wired mic would receive radio--even with star-quad. And yes, the only work-around was to use wireless. No cable between mic and transmitter; very short cable between receiver and Van. Plus, I had the receiver set to line level output.

FWIW, I've found Mackie preamps (1402) very susceptible to RF interference at mic level gains. In fact, there is almost a 'switch' on the pot. At one point, no RF; a small twist, you get RF.

[David W. Jones]

When I was doing A1 work on college football games for network TV, we always had a listing of available channels for each city we were going to. But I always used an RF monitor on site to double check frequencies because there was always some radio station doing remotes from the parking lot, or some super church near by using unauthorized audio channels that could interfere with your signal.

All the Best!

[Paul R Johnson]

Old fashioned AM radio is the worst culprit, leaks everywhere! Also I've had occasional problems with building that have loop systems for hearing impaired people - these often induce phantom audio into devices normally quiet!

[Bernie Beaudry]

I recently had an issue with an am radio station that had its tower within about five miles from our shoot. I was originally going to use a Lectro camera hop to send audio to the camera (HVX200). After hooking up and setting the input level, I could hear this very familiar station underneath the tone when I was listening to return from the camera and when I was listening straight off the mixer. The camera was actually sounding slightly distorted from it. Did a test record and it was for sure going down on the camera media. I then plugged directly into the camera via an umbilical and same issue. I was using a Microtech Gefell hypercardiod mic on a stand with an internally cabled boom pole. I then listened to my back up recorder (Marantz 661) which was being fed via y cables from the mixer main xlrs. Heard it there too. Took out the y cables, still there. I was powering the mic with a Denecke out board 48 phantom supply. The cable from the mixer to that was not the best. Changed it out with a better cable. Radio was still there but attenuated somewhat. The last thing to try was a different mic. I was saving my Schoeps Cmit5u for the run and gun part of the shoot so I wouldn't have to take apart my interview setup, but when I changed out the mic, problem solved. The moral of this long story. Have star quad or similar cable in your entire bag, and have a back up mic in case the rfi is getting in there too. All of the cables that weren't star quad in my kit have always worked fine in the past, but being that close to the source of AM ground wave propagation was just too much for them and even my very good sounding Gefell hyper.
Lesson learned.
Bernie

[Bill Spearman]

Thanks guys, those are all great suggestions. With 1 exception (which did happen to be in the connection that had the problem), the cables are all new AT cables which say "Dual inner copper shields and twin conductive PVC inner shields protect cable signal quality with 100% coverage". However based on your advice, I will invest in a some Star Quad cables also. I will be back to the same location in 2 weeks, so will try to set things up and see if it repeats.

[John Willett]

If you use XLRs - try using the Neutrik EMC-XLR series - I am using these on all new cables I make.

The EMC-XLR Series is a specifically designed version of the XX series to give enhanced RF screening for critical applications in live performance and recording where there are particular problems with radio transmission or mobile phones. The design guarantees a continuous RF shield connection from the cable to the chassis connector housing via a circular capacitor around the cable shield. An EMI suppression ferrite bead between pin 1 and the cable screen provides a low-pass filter for improved RF rejection.

[Bernie Beaudry]

Hi John,
Would it be best to replace the female xlr on my internally cabled Ktek boom with this connector to keep rfi from getting picked up by the mic? Or can it be used further down the line ie: between the mixer and the outboard phantom power, or between the coiled cable and the phantom power box? I would imagine it would be best to have them on all of the cables in the bag.
Regards,
Bernie