"DougSlug" wrote in message
...
In my office we often experience instances of a "galloping" sort of buzz
periodically coming from the PC speakers near co-workers who have Cingular
GSM cell phones. A preliminary Web search turns up many instances of this
problem. My understanding is that these phones periodically communicate
with
the network, and during these brief bursts the carrier is modulated at
around 200 Hz resulting is a clear, repeatable buzz pattern. How is this
buzz getting into the PC speaker amplifier, and, more importantly, how can
it be prevented?

The phones do communicate regularly with the network. The network must be
constantly updated with whose phones are switched on and which cell tower is
providing service. (It would be impractical to ring you for an incoming
call if the network had to query every tower in the world, "Hey, I have a
call for DougSlug. Who has him???")

This is a surprisingly common form of Electromagnetic Interference (EMI).
The RF is being coupled into the wiring and changing the conduction of the
small transistor amplifier inside one of the speakers, the one with the
light and switches.

At work, I usually know when my co-workers cell phone will ring, because MY
speakers emit a bup-bup-bup-buzz right before the ring. See, he usually
keeps his phone next to the cubicle panel on his side, but by chance just
inches away from my speakers. I have heard the same thing on TV, when a
performer is wearing both a wireless mike and a cell phone.

Rerouting the wires or relocating the phone is the obvious solution, as
distance is your friend in these cases. Otherwise, adding a few ferrites to
the speaker lines may work. They present a high impedance to RF traveling
along the wires, but have negligible effect on the audio signal. (You
probably already own ferrites pre-installed in some of your wiring -- you
just didn't know it. The various connecting cords for your camcorder have
those fat lumps in them. Those are ferrites.

Ferrites are available for purchase in many radio and electronic stores in a
form which closes, clamshell-style around the wire. You put two or three
along the wire and *presto* no more buzz. Wrapping the wire multiple times
around a ferrite before snapping it closed can help immensely. (This
assumes you have some available slack in the speaker wires.)

Some theory he http://en.wikipedia.org/wiki/Choke_(electronics)

http://www.radioshack.com/sm-snap-to...-pi-2103222.ht
ml has them, but overpriced compared to the $1.00 apiece I pay for them at
the San Diego-area ham radio swap meet.

Also around a dollar he
http://www.mouser.com/search/Refine....rPartNumber%7C
0%7C%7CP_SField

I cannot advise you what to buy, but If you spring for a few of each at the
$1.00 level, I really think you'll find something that works. Shipping is
extra but Mouser will sell in small quantities. Thanks, Mouser.

Good luck. If you go this route, please let us know what worked and how
many. TKS.