Phone Causes Interference

September 9, 2004 9:28 PM

When my I put my U.S. GSM phone near a speaker (computer, car, speakerphone) I sometimes here clunking sounds in the speaker. Why? I assume the speaker is picking up some radio signal my phone is sending, but what specifically is happening?
posted by Nelson to technology (13 comments total)

Yes, same here. Sometimes I can tell a call's coming in before my phone rings.
posted by lbergstr at 9:58 PM on September 9, 2004

The sounds you are hearing are a side effect of the hydrocoptic marzelvanes found in most recent GSM phones. If you disable them side fumbline will NOT be effectively prevented.
posted by Kwantsar at 10:05 PM on September 9, 2004

I like that my phone does this. It helps me hear my phone when I am rocking out.
posted by Quartermass at 10:08 PM on September 9, 2004

FWIW, it's not GSM-specific. My old CDMA phone did the same thing (as does my current GSM phone).
posted by jjg at 10:44 PM on September 9, 2004

I like it too. I set my phone to silent and leave it next to my monitor so that when I get a call, the whole screen jiggles and no one has to hear it rattling around on my desk.
posted by mcsweetie at 10:55 PM on September 9, 2004

disclaimer: I work for a mobile phone company but not in the mobile department, so the technical terms are different but I can never remember them.

What happens is the following: Your phone is 'connected' to an antenna somewhere in the area. Your phone listens to the antenna to see if there is an incoming connection. This listening state does not require the phone to send out any signals, when you switched on your phone or arrived in the reach of this antenna your phone and the network synchronised.

Now when a calls comes in to you the antenna will send out an alert that your phone picks up, and then builds up the call. (reply to antenna, authenticate, etc.)

When you move around your phone will switch antennas, which is fine. A number of antennas and base stations are combined in an area which will send out the 'call alert' to your phone. What happens when you are sitting in a car and move across a boundary of this combined area your phone will detect that you are now in a different area and will negotiate with this new area to register your phone there. Now the network nows where you are and can send out the call alert in this new area.

Now when a phone is stationary in an area it will still send 'alive' messages to the network to confirm it is still in the area and not switched off or out of range. The interval of these messages can be determined by the network operator. Thinking about it, it might actually be the network that starts these messages. I will ask and find out.

The reason you can hear it on your speakers is because it is a little antenna in your cellphone that sends out signals, which in this short distance can be picked up by speakers or amplifiers.
posted by sebas at 2:08 AM on September 10, 2004

OK, I asked and what happens is that the network gives the mobile a timeframe for the Periodical Location Update, on our network this is two hours.

After each two way communication between the mobile and the network a timer is set in the phone. When the time is up the phone initiates communication with the network.

If you are very brave you can have a look at this paper, which describes the GSM protocol in understandable terms.
posted by sebas at 2:54 AM on September 10, 2004

I know i read a detailed explanation of this somewhere, but i cant put my hands on it at all. In the meantime, i found a Google Answers question / response that has some detail. The article i remember reading dealt specifically with how un-shielded speaker wires are made to vibrate by transmissions from the phone, i'll keep looking and see if it turns up.
posted by kev23f at 5:53 AM on September 10, 2004

It's simply electromagnetic interference isn't it?

Hum, not sure - I'll have an educated guess: Mobile phones use a frequency below, but not far away from, microwave radiation (the stuff you cook with) - GSM is 900/1800/1900Mhz whereas as microwave is 2500Mhz. The power levels are vastly different, so don't worry about trying to cook your brain with a mobile phone. Radiation in this band has the peculiar property of being absorbed by water molecules, as well as fats, sugars, and interfere with some metal. What you see when you hold your phone next to the monitor (or what you hear when your phone is by a radio) is the result of radiation being absorbed by the various metal components in the monitor/radio causing slight changes in resistance of the circuit and then the minor interferences are amplified. I think. Don't quote me on this, but it sounds plausible.
posted by BigCalm at 6:21 AM on September 10, 2004

sebas:
disclaimer: I do work for a mobile phone company, in the technical department, but you really need an electrical engineer to answer the question as to interference.

I wrote a (rather hastily constructed) page on how mobile phones work a while ago which may be of interest.
posted by BigCalm at 6:26 AM on September 10, 2004

Thanks for the all the answers. I figured it was my phone sending out little messages to the towers. What I don't understand is exactly why speakers would pick up and amplify the EM coming from the phone. It's much less noticeable (although present) with a TDMA phone, and I've never heard it at all with a pager. Is there something special about GSM frequencies? Power?
posted by Nelson at 7:54 AM on September 10, 2004

CDMA vs TDMA (technical, but hopefully comprehensible)
posted by BigCalm at 8:29 AM on September 10, 2004

GSM uses a time-slotted transmission method (as does TDMA), so your phone will only transmit only on certain slots lasting about half a millisecond so it doesn't step on the toes of other GSM phones in your vacinity. The high frequency (850 or 1900MHz in the US, 900 or 1800MHz in Europe) pulses going out in these time slots can create lower-frequency oscillations in surrounding equipment that you can actually see and hear.
posted by zsazsa at 8:42 AM on September 10, 2004