Country Home Products Wireless Microphone Systems - Manual (Page 30)

BROADCAST RADIO
High-band VHF wireless FM systems are not generally
subject to interference from commercial AM or FM radio
stations. Both bands are well below the VHF band and in
particular, these systems are not typically sensitive to
moderate AM signals. UHF systems are even less likely to
respond to commercial radio sources since the UHF band
is >300 MHz above the top of the FM band. However,
occasional interference, in the form of distortion or short
range, can occur in cases of extreme proximity to a
high-power commercial radio transmitter.
OTHER RADIO SERVICES
Direct pickup of 2-way radio, pagers, business band,
ham-radio etc. is rare. However, since some of these
sources can be quite strong locally there is the possibility
of interference due to intermodulation or if the source
appears as an image frequency. For example, operating a
walkie-talkie near a wireless receiver can cause noise,
distortion or apparent loss of range.
In particular, the "travelling" wireless microphone
system frequencies (169-172 MHz) share space with a
number of unpredictable primary users including
government (Coast Guard, Secret Service), industry
(forestry, hydroelectric), land mobile and paging services.
Direct pickup or inter-modulation from these sources is
possible in this band. Symptoms may again include
noise, loss of range or actual audio pickup.
Of course nearby use of other (unexpected) wireless
microphone systems can create interference through inter-
modulation or direct frequency conflict. Given the finite
number of wireless frequencies available, especially in the
"travelling" channels, it is always possible to encounter other
wireless microphone users in locations such as hotels,
convention centers, sports facilities, and news events.
Remedies for such interference involve identifying the
interfering source if possible and relocating the source or the
wireless microphone system to reduce proximity. If this is
not sufficient it may be necessary to change the
operating frequency of the wireless microphone system
especially if the interfering source is a primary (licensed) user.
NON-BROADCAST SOURCES
Non-broadcast sources are those that produce radio
frequencies only as a by-product of their operation. There are
three main types which are of concern to wireless use: digital
audio/video devices, digital computers/controllers, and certain
AC power equipment. Digital audio devices include: digital
signal processors (DSP’s) such as delays, reverbs,
pitch shifters; digital recording and playback devices such as
DAT recorders, CD players, hard drive recorders and
samplers; digital electronic musical instruments such as
synthesizers, organs, and MIDI-controlled instruments. Digital
video devices include cameras, camcorders, video switchers,
video DSPs, and video editing systems.
Digital computing devices include: microprocessor
equipped units (PC’s, calculators), minicomputers
(workstations) and main-frame computers. In addition,
controllers for lighting, AV presentations, industrial
equipment and certain video equipment contain
microprocessors.
Digital devices can produce broad band RFI (radio
frequency interference) in close proximity to the source.
Any device which carries an FCC type approval label such
as "Class B computing device" can be assumed to be a
potential source of interference. The audible effect is
usually high frequency noise or distortion and it generally
only occurs when the receiver is close to the digital device
and the transmitter is at a distance. Unfortunately, this is
often the case when wireless receivers are located in or
near racks of digital gear while the transmitters are being
used on a stage.
The best remedy for this type of interference is to
locate the receivers and antennas at least several feet from
any digital device. In a rack of different equipment this
would suggest mounting the wireless receivers at the top,
analog equipment below that and digital equipment at the
bottom. In extreme cases, choosing higher wireless
frequencies may improve matters. However, as the speed
(clock frequency) of digital equipment increases this
technique will be less effective.
Finally, any equipment that uses or controls high
voltage or high current AC power can generate radio
frequency interference (RFI). Examples include lighting
dimmers and some types of gas discharge lamp supplies
such as neon or fluorescent ballasts.
Audible effects of this type of source include buzz or
hum in the signal. Again, the first remedy is to relocate the
offending source or the wireless equipment to minimize
pickup. In some cases special filtering may be applied to
the various power and connecting cables of both the
source and the wireless equipment to block RFI from
leaving the source or entering the wireless equipment.
A property of FM reception which can reduce the
audibility of many types of interference is the so-called
"capture effect. " When multiple signals (close to the
operating frequency) are present, the strongest signal
will capture or lock-in the receiver. If the desired signal is
sufficiently strong, the interfering signals may not be
heard. Since the strength of the desired signal (the
transmitter) is dependent on the operating distance, a
nearby transmitter can often overcome weak or distant
interference sources.
31
Selection
and Operation
of W
ireless Microphone Systems
C
H A P T E R
3
Wireless System Operation