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dhodges82 · Posted - Wed Nov 28, 2001 11:44 am

Question for anyone familiar with interference coming from emf between audio cables and power cords. It includes a fun little story. I was recording one of my artists the other day when I started to hear what sounded like soft muffled talking in the background. I dismissed it and she finished the song. When she did, there was a radio signal that was recorded for about 5sec after she got down playing. We both found it odd (and hilarious). I unplugged everything I could try and the only way to stop the NPR station from coming in through the microphone was to turn the microphone off. I know that I have audio cables and power cords running paralell to each other (a no-no, I know). Is it possible for the microphone to pick up the radio waves since the NPR station was a low frequency on the dial? Or is this interference from other cables? Just a humerous and complexing problem. -Dave

Edited by - dhodges82 on 11/28/2001 11:44:50 AM

SteveG · Location: United Kingdom

In order for this to happen, you have to have something dodgy in the way of connections somewhere in order to 'demodulate' (AKA rectify) the RF signal that your mic lead is picking up. And it doesn't have to be power cables - the mic lead can do this on its own! This can sometimes happen with balanced mics if you have screen problems - also, it's not unknown for dry soldered joints on mic sockets to cause the same problem. You need to check out, or get someone to check out, your mic cables and anything else between the mic and soundcard.

Interestingly, speaker cables can exhibit the same behaviour, with an amp's output stage doing the demodulating. Fortunately, this doesn't happen too often!

Steve

Graeme · Member Location: Spain

... and, just to add to SteveG's very comprehensive reply, a transistor (or valve/tube) somewhere in the mic circuit, which is not neing operated at the correct point can also work as a partial rectifier and cause the problem.

jonrose · Location: USA

I've also had this problem at a thru-the-wall connection box. One of the XLR's was damaged enough to become a bit of a diode in and of itself. It probably wouldn't have been a problem, except the these recording rooms were built in a house situated halfway up one of the most transmitter-populated hills in the area... There was so much RF flying around there you'd think you'd be glowing afterward.
;-)

SteveG · Location: United Kingdom

We used to have a couple of real gems around here. There used to be an old Band 1 TV transmitter that used to put out about half a megawatt into the surrounding countryside. If you had a TV anywhere near it, the best way to receive its signal was to remove your tuner completely and just use breakthrough into the IF stages. If you used the tuner, the overload used to shut the IFs down completely!

But the best one was a Taxi firm transmitter on the roof where I used to live. If I was careful, I could position an electric toaster near it and almost hear the words on the elements... Actually, it was quite dangerous in some weather conditions - I could quite easily draw significant levels of RF energy and sparks from any metal objects that were even close to 1/4 of the tuned wavelength. Like cutlery...

Steve

SteveG · Location: United Kingdom

resistor man · Posted - Wed Nov 28, 2001 7:37 pm

SteveG · Location: United Kingdom

There were a lot of things which you might have needed, but didn't get. Like much sleep, for instance.

I should explain that there was a height restriction placed on the transmitting mast, and since this was in the centre of a town, the taxi firm just bumped up the power until they had the same field strength as they would have got from a higher mast. I discovered (actually it was pretty obvious!) that they were chucking a significant amount of power into their not-too-well tuned antenna, and I wasn't best part pleased, as you can imagine. It used to break through on everything. What they were doing was basically illegal. But this all pales into complete insignificance in comparison to what one of their drivers did with a lion...

Steve

Edited by - SteveG on 11/29/2001 02:07:40 AM

resistor man · Posted - Thu Nov 29, 2001 7:14 pm

SteveG · Location: United Kingdom

Oh, allright. It's more fun than interference, but it's slightly longer than a normal reply...

In 1976, one of the taxi drivers kept a lioncub locked up all day in a Ford Transit van parked just off the high street, just around the corner from where I used to live. Inevitably, it got bigger. I paid a visit back there, and without knowing anything about the lion, walked around the corner, and saw the van with the door open with what I initially took to be a large dog on a thin chain sitting inside it. I looked more closely, and gradually realised what it was.

The lion was actually quite friendly, but heck, it's a lion... Now, you can't ever find a policeman when you want one. So I went around to the police station and said to the duty sergeant "Do you know that there's a lion sitting in a transit van in Chersey Road?" He said "Boring! You're about the 200th person who's told us about the lion. It's harmed nobody, and there's nothing we can do about it. Would you like to make a formal complaint?"

So I asked who owned it, and when I found out that it was a driver in the bloody taxi firm I'd had so much trouble with, I said that I would complain about it. So I lodged a formal complaint about the size of the chain, which wasn't strong enough to hold a dog, never mind a lion.

A few weeks later, on the national TV news comes up a headline "Lion Attacks Woman in Park". Apparently, the owner of the lion had been taking it for its evening constitutional, and it had seen a lady wearing a leopardskin coat in the distance. With a few bounds, after snapping the chain, it had come up behind her, jumped up, put its paws on her shoulders and started licking her - he thought it was a playmate, or whatever. She collapsed under the weight of it. You don't do that with lions - they think you're dead and proceed to eat you.

Fortunately, the lion's owner got there at that point and pulled it off her - God knows how - it was fully grown lion by now. The woman was somewhat shocked, but actually physically unharmed, and extremely lucky.

The lion was rehoused in a private zoo, and a few months later, we had a Dangerous Animals Act, which, believe it or not, we didn't have on the statute books before...

There are some strange people in that part of the country...

Steve

Mike Ocean · Posted - Mon Dec 03, 2001 3:53 pm

DHodges82,
You may want to check the scmatics (I think I spelled that wrong) for your mixer. Some of them have a place in the circuit board to add a capicator to eliminate the rf you are picing up. This is usually for each channel.
Hope this helps.
Mike

oldman · Posted - Mon Dec 03, 2001 8:57 pm

Steve, I'm ROTFLMAO! Did you make up that story? If you did your in the wrong business man. I just can't get over the great things that you can find on this forum.

SteveG · Location: United Kingdom