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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?"
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. :)
-EvilMagnus
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.
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
"$30 for the One True Ring. $10 each additional ring!" -- JRR "Bob" Tolkien
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.
90% of the time it happens just before I get a call/text, so I quite like the early warning.
Come as you are, do what you must, be who you will.
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.
See TDMA.
Mea navis aericumbens anguillis abundat
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
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 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.