Jump to content
smadge1

freaky RF interference

Recommended Posts

I had just setup some new PCs here at work, and one of the teachers wanted me to test the audio recording.

 

When I tried recording, the mic didn't record my voice, instead, there was a faint voice speaking amongst the static. It didn't sound like anyone in the room with me.

 

I then tried recoding in Audacity, and did some cleaning up of the audio, and in fact, it was a radio station, that I was hearing. We do not have a radio in the room, or any other devices connected to the PC, or any other open applications. (Folk were talking about CIA and bin Laden, so I got a little paranoid for a sec!)

 

There is a very large radio antenna up the road, so I can only assume that it was the source of the interference, but heck, why buy a radio when all you need is a piece of wire and a speaker?

Share this post


Link to post
Share on other sites

There is a very large radio antenna up the road, so I can only assume that it was the source of the interference, but heck, why buy a radio when all you need is a piece of wire and a speaker?

Well, given the length of radio waves, if you wanted to change the station, you might be walking a fair distance.

Share this post


Link to post
Share on other sites

of course, I was speaking in hyperbole.

 

I'm just amazed at the clarity you can get with just "a piece of wire and a speaker"

 

I'm tempted to try recording more.

Share this post


Link to post
Share on other sites

Where about's are you? (Or rather, where abouts is the computer?)

 

Probably what is happening... is that the microphone cabling is acting like an antenna. The amplifier connected to the microphone cable will usually be a fairly high impedance type J-FET amplifier. For audio applications normally it is desired that the amplifier be perfectly linear, however in practice this is almost never the case. It'll be close, but not quite.

 

This nonlinearity, when in the presence of a strong AM station, can be enough to detect the AM station and demodulate it, causing the phenomina you hear.

 

The cause will almost certainly be poor grounding and poor design of the amplifier. A capacitor across the amplifier inputs should be enough to shunt the high-frequency station to 0V (a low-pass filter).

 

Double check the cabling to the microphone ... my bet is the shield is disconnected, leaving you with a long piece of wire (a shortened monopole) which is receiving the radio station -- if you lengthen or shorten this cable, you should be able to change frequencies. That or you've got a balanced microphone who's feedline is unbalanced. You might also notice a 50Hz hum in those cases.

Share this post


Link to post
Share on other sites

You want something even wierder?

 

We had a room in my school, it was THICK brick walls, even mobile phones dropped a few bars.

It was always unsupervised caus it was for us to do distance educatoin on subjects the school didnt normally offer.

 

Anyway, we had a set of powered speakers, which, as any powered speakers would, buzzed if you put your finger on the end.

As such I was touching the tip to anything I could to see if anything else buzzed (nothing but the PC case did). I then went to leave the room, so absent mindedly sat the audio jack into the keyhole of a nearby 3 draw filing cabinate.

 

Radio so clear you'd swear it was only 5mhz off.

It sounded like it was probably an AM station, but holy shit, basic powered speakers + filing cabinate = radio.

 

It was amazing, and to this day ive never bothered to find out WHY this is possible. Was the filing cab really that good an antenna?

 

*bracing for redhatter attack* lol

 

EDIT: I think the redhatter attack i was expecting is actually covered above. Same thing Red'?

Edited by Master_Scythe

Share this post


Link to post
Share on other sites

yeah, it's just a cheap PC headset connected to a cheap HP computer via the front panel connectors.

 

The school is in Bracken Ridge, probably within a stone's throw of that massive antenna by the highway.

Share this post


Link to post
Share on other sites

That "massive antenna" is the ABC transmitter site for "ABC Local Radio" (4QR; 612kHz) and "ABC Radio National" (4QG; 792kHz). If you listen on the hour, you should hear the news. Hence the talk of Bin Ladin.

 

My guess is that the headset microphone/cable is stuffed. I wouldn't be surprised if HP had been cheap on the filtering either ... if it's a real problem, a 100nF capacitor between tip and sleeve should fix it.

 

We had a room in my school, it was THICK brick walls, even mobile phones dropped a few bars.

It was always unsupervised caus it was for us to do distance educatoin on subjects the school didnt normally offer.

 

Anyway, we had a set of powered speakers, which, as any powered speakers would, buzzed if you put your finger on the end.

Yep... there's mains wiring in the room, generating an electric field. Your body being mostly water, is capacitively coupled to that mains power. Not enough to cause any health issues. Basically the wiring is one plate of a capacitor, and you standing on the ground is the other. When you stick your finger on the end, you provide a circuit from that other "plate" of the capacitor, into the amplifier to ground. Hence why you hear the buzzing.

 

It's a characteristic of high-impedance amplifier inputs.

 

As such I was touching the tip to anything I could to see if anything else buzzed (nothing but the PC case did). I then went to leave the room, so absent mindedly sat the audio jack into the keyhole of a nearby 3 draw filing cabinate.

 

Radio so clear you'd swear it was only 5mhz off.

It sounded like it was probably an AM station, but holy shit, basic powered speakers + filing cabinate = radio.

5mHz would be perceivable to the human ear. In fact, you can be 10Hz off and not notice. I find I begin to notice when I get more than 20~30 Hz... and only when using SSB. AM of course, the radio uses a small 455kHz band-pass filter to synchronise a phase-locked loop, the output of which is used in the product detector to bring the radio bang on frequency.

 

5MHz would put you out-of-band on the MW broadcast band; 540kHz ~ 1600 kHz is the range... 5MHz would put you somewhere on one of the shortwave bands. A bit further and you're on the 40m amateur band.

 

The filing cabinet probably is acting like an antenna or counterpoise to an antenna. The metal-to-metal contact can also have some P-N junction characteristics to behave like a diode, thus becoming an envelope detector -- the key element in a crystal radio receiver.

 

http://www.qsl.net/k3pd/chap04.pdf is worth a read. It covers a lot of what's going on here.

http://www.qsl.net/k3pd/book.html is the entire book, "Crystal Sets to Sideband", which covers a lot of the above.

Edited by Redhatter

Share this post


Link to post
Share on other sites

according to google earth, we were almost exactly 1km from the transmitter.

Share this post


Link to post
Share on other sites

Frequency ~=	 600 000 Hz
Velocity  ~= 300 000 000 m/s

			   300 000 000	  1 000
Wavelength ~= -------------- = ------- = 500m
				 600 000		  2

So you're two wavelengths away. That's bugger all in the scheme of things, of course your cheap consumer electronic equipment is going to hear the ABC. ;-)

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×