Picking up the radio on mics

[Glenn Chan]

I recorded some music recitals on Apex shotgun mics* (they have XLR connectors; presumably balanced) and during recording I noticed radio very faintly in the background (like you were almost tuned to a radio station). My questions are:

A- How come this happens?
B- How do you avoid this?
Does putting loops into the cable work?

*Shotgun mics are not for recording music. $100 Shure SM58s will likely do better, and those aren't for recording music.

[Ty Ford]

<<<-- Originally posted by Glenn Chan : I recorded some music recitals on Apex shotgun mics* (they have XLR connectors; presumably balanced) and during recording I noticed radio very faintly in the background (like you were almost tuned to a radio station). My questions are:

A- How come this happens?

*You may be too close to a transmitter.

B- How do you avoid this?

*RF can be demodulated into audio gear any number of ways. It may even come up an AC power line.

Does putting loops into the cable work?

*usually not, but sometimes you can null the rf by changing the position of the cable. I also removed a metal music stand from a studio and the rf stopped. Look around for a lot of extraneous metal.

Regards,

Ty Ford

[John Hartney]

There's always a bit of voodoo with RF.... I knew a guy who could pick up a certain fq in his fillings.

[Vic Owen]

This can be hard to track down. Are you sure it's coming from the mics? You can unplug each mic cable to see if it changes. If it is in the mic cables, in some cases, RF attentuation can be obtained by looping the cable several turns through a ferrite core (donut)-- not a cure-all, but sometimes effective. Checking power leads, grounds, lifting grounds in some cases, rerouting cables, AC isolating transformers, etc. Lots of fertile territory here.

[Jacques Mersereau]

Sometimes you are so close to a transmitter there is nothing you can
do. No one can afford a studio surrounded by Mu Metal. Most move location.

Make sure you use high quality XLR cables with heavy shielding. Belden 8412 has a thick woven shield and a rubber jacket.
I am unfamiliar with those mics and they may not even have interior shielding.
Just because they have an XLR connector, doesn't mean they are balanced.

As had been recommended, unhook everything and start by adding one
piece a time. Try other mics and see if they are affected in the same way.

[Greg Boston]

Gents,

Let me explain how this happens. I have a background in RF. I also play electric guitar. I will never forget how 'spooked' I was as a teenager one night around 2am. I had been playing my guitar and stopped. As there was no other ambient noise from inside or outside at this hour, I kept hearing faint voices and music. It turned out to be a radio station in Canada (680khz). And I was in Arkansas! I finally figured out that it was coming from my guitar amp.

Ok, now here is the technical explanation.

AM radio can be picked up with nothing more than an antenna, a diode rectifier, and amplification. Well, in the case if my guitar amp, I had at least 2 of the three components if not all 3.

Guitar strings = antenna
Guitar amp on overdrive channel =distortion, aka clipping, aka rectification.
Guitar amplifier = the ampification part.

As mentioned in an earlier post, it could get in through the power lines because power supplies use rectifiers to convert AC to DC. The cord would be the 'antenna' part in this case.

This phenomenon is more likely at night time due to the fact that the frequencies used in the AM broadcast band in the US and Cananda. These radio waves will bounce off the ionosphere and return to earth several hundred or thousand miles away at night. All AM stations except for 'clear channel' stations must do one of three things at local sunset to keep from interfering with other stations on the same assigned frequencies in different parts of the continent.

A. Go to reduced power
B. Switch to a directional antenna pattern
C. Go off the air completely.

Ok, for the cure I think several of the suggestions made are very good. Mainly, shielding! You should not lift any grounds because you can create an electrical shock hazard if you lift the AC ground. Lifting the audio ground using a direct box that can be purchased for this will help with removing AC hum (ground loops), but won't help with RF interference unless it contains components for that purpose.

Plugging in the audio equipment using a good RFI/EMI attenuating power strip could help.

Now for the kicker. If you were indeed picking up a distant signal and not a nearby strong RF field, you could probably set up the same configuration in the same place on a different occasion and hear nothing. Why? Because the signal you heard faintly arrived there under certain atmospheric conditions and the positioning of your cables and mics helped to form an antenna. Lacking an actual tuner, you would pick up whatever resonant frequency was formed by length of the cable.

Sorry for the long winded post.

Good luck,

=gb=