View Full Version : Interesting interference

December 19th, 2006, 09:50 AM
I have a nasty yellow rotary telephone which I decided to convert to an internet telephone thingy via 2 headphone jacks. I didn't detatch any of the leads from the terminals on the telephone board, rather I just piggy backed the jacks onto the existing connections. It works fine, however if I connect the headphone function to my speakers, and turn up the volume past a certain point (not very far), I get a whine akin to a theremin or sythesiser which can be varied by the volume control knob, when left alone it leaves a steady tone.

I'm sure this is easily solved, I'm not bothered how to fix it. I just want to know why it happens.

Incedentally, I made a recording of this strange noise. I'll send it to someone if they want to host it (~500kb zipped wav, 20 secs)

nige the hippy
December 19th, 2006, 01:04 PM
It's probably because, in the telephone, the earpiece and microphone are connected via some components. You're probably sending a slightly (microseconds) delayed version of what's coming into the mic input, back out through the speaker output, which is then getting fed-back at reasonable volume, into the mic input, and so on in a self-oscillating loop (an oscillator is just a loop of amplification, and delay).

I'm actually quite surprised that the old phone microphone is working, as they are usually carbon-granule types, which require a dc current through them to work (they are just a resistance that varies with changes in (sound) pressure on the granules), maybe you've accidentally tapped into the 5v supply that some microphone inputs have on the "ring" of the jack!

"Nasty yellow" is a matter of taste, they're quite collectable you know!

December 19th, 2006, 01:59 PM
It working at all is probably a fluke :)

Should be useful for confusing people! ("Who are you talking to?" "No-one, just ACDC!")

I found that the problem stops when I unplug the microphone, so maybe it is some sort of feedback like you said (Is that the right term?) Betwixt mic and ear.

It's still a pretty cool noise, even if it isn't supposed to make it.

If it is collectable (It's the common BT rotary phone. Don't know what model but I think it's three digits and starts with 7) I haven't done anything destructive. If I want to revert to phone I did a sketch of where everything goes.

December 19th, 2006, 02:07 PM
I just realised, I didnt trace where the connections I made actually go. You say the microphone and earpiece are connected, then maybe the microphone doesn't work at all and they are connected in someway so that the earpiece itself is also a microphone, so instead of making an 'oscillating loop' (?context?) between earpiece and microphone, the speaker just goes between it's own mic input and speaker output.

...I think I might be right. I just swapped connector round from headphone to mic and vice versa on the PC motherboard and it made no difference at all.