We're going to air, please turn down your radio

February 8, 2006 2:52 PM

Phoning a live radio show while you have your radio on causes some problems.

I was listening to a radio station this morning and three people had phoned in to be part of a competition. One of the callers had their radio on and it caused the most unholy squealing feedback giving the presenters conniptions.
Can anyone explain to me in layman's terms what's happening here? What configuration has to be in place for this to occur? Other answers I'd like - How close to the radio does your phone have to be (or is it the volume)? Do you or the station have to be making a sound before the feedback happens? Do you have to be tuned to same radio station or is it just the fact the you have the radio on?

posted by tellurian to science & nature (11 comments total)

I think part of the problem is that any (US) radio show that takes callers is, by FCC regulation, broadcasting on a delay (7 seconds, iirc). So if the radio is on you'll actually be hearing over the phone what was broadcast a few seconds ago.

I don't think that can explain the squealing, though.
posted by Kellydamnit at 2:57 PM on February 8, 2006

I think this is a feedback loop, caused by the delay that the radio station has in place.
posted by Who_Am_I at 2:58 PM on February 8, 2006

It's basically a feedback loop - the sound output is accumulating because the radio is amplifying the phone call, and the phone's microphone is sending the same signal back to the radio station, which in turn is causing the same cycle to repeat itself.

With every "loop", amplification is accumulated and as such the residual audio signal is combined and amplified resulting in the feedback noise.

As a brief aside, it's quite an interesting thing to observe by putting your microphone next to a speaker on a Skype call, as the latency makes the accumulation slower, but you can hear it happening for real - but make sure your speakers are turned down quite low, feedback always drives speakers very hard.
posted by rc55 at 2:59 PM on February 8, 2006

If there was a tape delay, there wouldn't be feedback, just a distracting lag that would confuse the caller. It's the fact that the radio is on to the same station, broadcasting live. The mouthpiece of the phone is picking up noise from the radio, the radio rebroadcasts the noise that the phone is picking up, the phone picks it up, etc., getting louder with each iteration -- hence the term "feedback." It's the same effect as putting a live microphone in front of a speaker in a cheap sound system.
posted by commander_cool at 3:02 PM on February 8, 2006

And in answer to your other questions, this happens when you are tuned to the same station that your voice is being broadcast over and your radio is loud enough to be audible through your telephone; distance doesn't matter, decibels matter. I used to screen calls for public radio in Seattle and one of the things we would always tell people is "turn your radio down NOW, if you wind up going on the air, it will be bad if your radio is on loudly"
posted by jessamyn at 3:02 PM on February 8, 2006

Besides any possible censorship delay, I'm sure there's some unavoidable delay just in the system of phone to radio station to antenna back to your radio.
posted by smackfu at 3:08 PM on February 8, 2006

There is now, especially with cell phones and VOIP, but there used to be a lot less, especially when callers are in the same city, which would be the case for a radio station. It was all analog in those days and it's hard to slow down electricity.
posted by kindall at 5:19 PM on February 8, 2006

Acoustic Engineers... Is there any chance of software detecting these loop patterns and reducing them?
posted by holloway at 6:03 PM on February 8, 2006

There are feedback exterminators that will target narrow bands of frequencies that are causing feedback, and bring the gain down for that small area of the spectrum. I don't think that they would be useful in this situation, though, as the telephone really cuts down on the bandwidth that you are using, so there is only a small selection of frequencies feeding back to begin with.
posted by Roger Dodger at 6:38 PM on February 8, 2006

The original 1963 Doctor Who credits were made using visual feedback, basically pointing a camera into a TV monitor, randomly flashing pulses of light, then recording the results. So you're basically watching a visual analogue of the feedback screech. Apparently they got the idea from a 1951 TV show.
posted by meehawl at 7:59 PM on February 8, 2006

Just to be crystal clear, tape delay has nothing to do with the howling feedback, in fact it tends to suppress it. Even a tiny bit of delay (e.g. in a cell call or VoIP call) might be enough to mitigate it, although you can hear it trying in the background.

By extension, if the feedback takes the form of the unholy howl, then the radio station is probably NOT using any tape delay, thus practicing unsafe talk radio, and you should call into and swear your fucking head off. Shut up, Beavis.
posted by intermod at 8:12 PM on February 8, 2006