PolyCom SoundStation2 Service Manual

Application Note

Voice Communications
Division

Jeff Rodman
CTO
Polycom, Inc

Version 1, March 2005

RADIO INTERFERENCE AND ELECTRONIC DEVICES

Since radio was first invented, radio and TV transmitters have demonstrated an ability to
unintentionally interfere with other devices. A neighborhood FM station might be picked up on an
intercom system, or a passing police car on a phonograph. There have even been rare but
documented instances where a strong local transmitter was picked up on a rusty radiator pipe, or a
loose filling! Historically, these situations did not show up very frequently and were addressed as
they occurred, but the increasing use of radio transmitters in personal communications is resulting
in a growing incidence of these problems. 802.11 network interfaces, cellular telephones, cordless
telephones, personal data assistants, cordless headsets, and other kinds of wireless devices
contain transmitters that are powerful enough to cause interference in many common systems such
as telephones, stereo systems, speakerphones, personal computers, kitchen radios, and others.
Polycom systems, with their sensitive microphones and electronics, are not immune to this effect.

Interference of this nature can be reduced or eliminated by taking some simple precautions. These
are techniques that have been used for decades, and can be very effective.

a. Keep any transmitters as far from sensitive components, such as microphones or
consoles, as possible. The “field strength” at one foot away is ten times that at three feet
away, and much more likely to cause trouble. Whenever possible, maintain at least a
three-foot (one meter) separation.

b. The strength of personal wireless transmitters can differ by over 2000 times from device
to device. Many Bluetooth devices operate a power levels of 1 milliwatt, while some
personal information managers and cellphones can run at 2 watts (2000 milliwatts). A
two-watt transmitter at one foot produces as much signal as a major FM radio station only
100 feet away! When using a high-powered device, provide extra distance from sensitive
components.

c. Some wireless systems, such as access points, are designed with directional antennas
that concentrate the signal in a particular direction. Try to point these antennas away from
conferencing equipment and microphones.

d. Most wireless devices are in at least occasional communication, even when nothing
apparent is happening. A cellphone, for example, will transmit a strong signal when it is
called, even if it is set to “silent,” and this kind of signal can produce a “bz-t-bz-bz” sound
from a nearby loudspeaker. Keep cellphones away even when they’re not being actively
used.

e. Cellphones and other devices are very intelligent these days, and know how to turn up
their transmit power when they are in a weak reception area. A cellphone in a well­shielded building, such as a metal or concrete building, is likely to transmit at a much
higher level than a cellphone in the open because it is aware that its signal will be
attenuated. Again, extra separation from those audio components is beneficial.