How to Avoid Mobile Phone Interference w/ Speakers

EnzoTen asks: "Everyone has been sitting at their desk rockin, jamming, or groovin to their favorite tunes. You are in a trance, getting work done... then... BZZZPT... BZZTP..BTT.. BZZZZZZZZPTT... the blood curdling noise of your cell phone interferes with your desktop speakers playing 4 times the volume of your music and it takes everything in you not to flip your desk upside down, or throw your mobile phone across the room. Is there anyway to avoid mobile phones interfering with speakers? Are there speakers available that are shielded from this type of interference?"

Simple

[Anonymous Coward]

Turn off your cell phone. :-)

Re:Never happened...

[EvilMagnus]

it happens to me with my ATT/Cingular GSM phones and affects both of my car stereo systems and my desktop speakers, but not my hifi/amp setup. It's not as annoying as the submitter describes, but it's a very audible, very noticeable series of clicks and burps. Lasts about three seconds, every ten, fifteen minutes. I assume it's interference from some kind of GSM syncrohnisation signal, but I'm not an EE. :)

Comment removed

[account_deleted]

Comment removed based on user account deletion

Sheesh.

[Reeses]

Sounds like you're getting Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI).

Both are the result of either bad or cheap shielding. Most consumer electronic devices should have at least some shielding.

Look for frayed speaker wires. Look for cracks in your cell phone case (the one it's made out of, not the cool "leather" one you got when you bought it).

Either get new speakers, or replace your cell phone. Or both.

Re:Um...no.

[McGregorMortis]

This problem affects GSM phones much more than CDMA phones. GSM uses a time-division scheme, where the phones transmit in 570ns bursts, at a rate of around 217 bursts/sec.

I have the same experience. It really is very, very annoying.

One of the RF techs at my company apparently modified his computer speakers to filter out the noise. I'd like to find out exactly what it was that he did.

Re:Um...no.

[JabberWokky]

One of the RF techs at my company apparently modified his computer speakers to filter out the noise. I'd like to find out exactly what it was that he did.

I moved my phone about 17 inches to the right. Worked fine.

Seriously, just move the speakers and phone placement until they don't interfere with each other. A couple decades ago every bit of electronics you bought came with a piece of paper that told you to do this if you got radio or television interference. I always thought it was silly, but maybe that slip of paper needs to go back into boxes.

--
Evan

Re:Um...no.

[JacobO]

In my experience this is the telltale sign of an incoming GSM call. There's been a couple of occasions when colleagues' cellphones have gone off and caused this interference on a speakerphone. This is is a faux pas since our parent company's cell network is _not_ GSM. Of course, this is not something I take seriously, rather I find it amusing.

This definitely does not occur with the CDMA phones that we use (and our company prefers, for other reasons.)

Better cell or more shielding

[subreality]

There are two places you can solve this problem:

#1, get a better cell phone. With TDMA phones (GSM, D-AMPS, iDEN) you get a lot of noise as the transceiver switches on and off several times a second, transmitting at full power. iDEN phones (NexTel) have always been *by far* the worst about this, in my experience. If you get a CDMA phone (eg, Verizon), the phones on a cell share a common, continuous, low-level signal, which does not cause this kind of interference.

#2, shield your amplifier. (In cheap computer speakers, it's built into one of the speakers, or the subwoofer.) Surround it in tin foil, and ground the foil. Other possibilities are poor grounding on the signal wire - replace it with a shielded wire, and ground the shield to your computer's case and where it reaches the amp.

Re:Sheesh.

[onemorehour]

Heh, no, don't look for cracks on your cell phone. Your cell phone is functioning properly when it's emitting lots of electromagnetic waves--it doesn't have to be cracked in order to do so.

The issue is shielding--most likely in the wires to your speakers, but potentially in the power cord or the guts of the machine producing the signal out. If there's terrible internal shielding, dirty power or the EMI on the circuit board itself can cause this.

I like it

[RalphSleigh]

90% of the time it happens just before I get a call/text, so I quite like the early warning.

Re:happens to monitors too

[mcbridematt]

Just about everyone in Australia with a mobile phone (CDMA never took off here) has heard this sound.

Some phones seem to be worse than others. My Treo 600 is notorious for causing problems with CRT's, while lower end phones aren't.

Speakers

[Jeff DeMaagd]

There a few different paths you might take. You don't say anything about the model or brand of speakers, that seems to make a difference. The cheaper ones might have insubstantial cable shielding at best.

My home computer is connected to a reciever that powers separate speakers, I don't really get any problems.

If you were really obsessed, you might get a higher end sound card with balanced audio out, with powered speakers with balanced inputs. If you have cheaper speakers with cheap built-in amps, then the long, unbalanced cable can act as an antenna. I've found that switching to balanced audio connections is the best way to minimize picking up stray noise on the analog, much better than getting "high end" RCA cables. Switching to XLR (balanced) cables drops the amount of noise significantly in just about any analog audio path.

Or you could switch to speakers that have a digital inputs. That's probably a lot cheaper to get into.

Re:Cheap speakers

[Anonymous Coward]

Well, I have a magnetically shielded speaker and a GSM phone, and I get this static noise, which is annoying, of course.

Re:Never happened...

[Anonymous Coward]

hey mods-
do you know the difference between insightful and informative?

(apparently not)

Re:GSM

[Detritus]

Yes, it does. You have to distinguish between TDMA (the technique) and TDMA (the standard, D-AMPS or IS-136). Both GSM and IS-136 use TDMA (the technique).

See TDMA [wikipedia.org].

Technically....

[Anonymous Coward]

GSM is a TDMA (Time Division Multiple Access) system. This means a single physical channel is divided up in time for multiple people to share. A single physical channel is just a frequency range, such as 890-890.2 MHz. Each physical channel is divided up in time into what are called timeslots. Timeslots are grouped by 8's into what are called frames. Each timeslot lasts 0.577ms, and a frame is 4.615ms. When your phone is communicating with the base it is assigned a single timeslot in each frame. This means it is transmitting ON off off off off off off off ON off off... and the ON's are spaced 4.615ms apart (the frame duration). 1/4.615ms gives you 216.7Hz, which means that if (when?) it is picked up by other electronics, it is most definitely in the audio band and you will hear it. The RF transmissions die off very rapidly with distance from the antenna, which is why moving the phone a small distance away is sufficient. What else... when you hear the interference before receiving a call/SMS, this is what is happening 1) one or more base stations broadcast "hey you" to the geographic region where your phone is (your phone is always listening for this) 2) your phone contacts a specific base station requesting a channel (AUDIBLE) 3) the base station responds, assigning a channel 4) your phone goes to that channel, authenticates, etc. (AUDIBLE) 5) phone finally rings / SMS is sent

Re:Technically....

[Anonymous Coward]

Left off the answer to the question! Since the problem is RF emmissions from the phone, you have only a few choices: 1) move the phone a sufficient distance away 2) shield the electronics well enough that they don't pick up the RF, keeping in mind that any cables may be acting like antennas. There may be audio equipment that is shielded well enough by itself - but on the phone side, if you use GSM, you will have this problem. As others have pointed out CDMA, which transmits continuously, does not have this issue. If you live anywhere but the USA, almost everything is GSM.

see with most GSM phones and 802.11b transmitters

[daniel422]

As an audio applications engineer for a large semiconductor company -- this is the single biggest compaint we have from customers using CMOS IC amplifiers. You will get it from almost any audio system that's near a cell phone (or 802.11b RF transmitter -- they have 50Hz packet rates that are audible from the 2.4GHz baseband signal).
What ususlaly catches it is the speaker wires -- they act like giant antennas. 900MHz and up takes only a few centimeters to make a decent antenna, so long speaker cables are a prime target. The high frequncy (which you can't hear) couples on the output lines (typically) and goes back through the gain setting resistors on the amp to rech the amplifier input, where the packet rate (not the 2.4GHz to 900MHz baseband) is rectified and amplified. What you hear is the resulting buzz.
Your cell phone itself doesn't shield this -- then it wouldn't work! The best thing to do is to shield your speakers, wire, and audio system it may be near. A faraday cage (as described by others here) will work great. Plastic-metallic shielding (like that used in anti-static IC or computer part bags) also works wonders.

Use balanced cables&connectors

[Anonymous Coward]

Buy professional soundcard and speakers and use balanced wires & connectors.

Systems with XLR or TLR connectors/output can remove all the inference. When cables have 3 wires inferences can be eliminated 2 wires are used for normal circuit (L/R) and one for zero level (X=0) signal.

If inputs are L and X=0, inference is I then at the end of the output we have X' = I, L' = L+I.

And it is possible to remove all inference with simple substraction
X = L' - X'

An answer from an audio engineer:

[AEther141]

Use pro-level gear that has balanced connections on XLR or TRS jacks. Balanced cables have three conductors carrying ground, the signal, and a copy of the signal 180 degrees out of phase. Any interference affects both signal lines identically, so when they are put back in phase at the other end, any interference will be phase cancelled as it will now be 180 degrees out of phase.

Harmonic interference

[billcopc]

The source of this noise is the actual digital communication between your phone and the cell tower. There's nothing wrong with the phone, it's probably your speaker wiring that's picking up this signal as interference, as cheap wires have absolutely no shielding. If you don't know what kind of wiring you have, then you have cheap wiring :)

The cellular signal is rather strong and because of the way it is modulated, it has tons of noise outside its nominal band, almost all the way down to DC. In plain english (well sort of) this means it's feeding square waves into your speakers. Square waves are the "loudest" waves due to their harmonic constituents, which is why the noise can scare you shitless compared to your regular music content. Square waves are that loud annoying "bzzt" family of sounds in techno music, like Benny Benassi :D

Using better wiring will decrease the amount of noise picked up, but I find that even with extremely good wiring you can still hear a tiny bit of phone interference. I kind of like it because I have a rather potent car stereo and I couldn't hear the phone ringing if it weren't for the interference, but it is sufficiently dampened that non-audiophiles won't notice.