Country Home Products Wireless Microphone Systems - Manual (Page 27)

multipliers. However, both types of transmitters can
produce other spurious emissions due to power regulating
circuitry, parasitic oscillations, carrier harmonics, etc. These
emissions can all be controlled through careful design.
It can be seen that the calculation of both local
oscillator conflicts and image frequencies depends on the
intermediate frequency (IF) of the receiver while calculation
of crystal harmonics depends on the number of multipliers
in the transmitter. If receivers with different IFs or
transmitters with different multipliers are being used
together (i.e. units from different manufacturers) this must be
taken into account in compatibility analysis. Unfortunately,
only a few proprietary computer programs for frequency
selection have this capability. Input to most of these
programs assumes that all units are of the same design. For
this reason, accurate prediction of compatibility between
systems of different design is not always possible.
NON-SYSTEM
RADIO INTERFERENCE
Even though a group of wireless microphone
systems may be carefully chosen to avoid mutual
interference there always exists the possibility of
interference from non-system sources. These sources fall
into two categories: broadcast (including television and
other defined radio sources) and non-broadcast (narrow
band or broadband sources of radio noise). We will look
at each of these sources in terms of potential problems
and possible solutions.
BROADCAST TELEVISION
In the US, and some other countries, broadcast
television is undergoing a transition from analog to
digital. This transition affects wireless audio systems in
several ways: more "occupied" TV channels, no "open"
space in DTV channels, and future "re-allocation" of
existing TV channels.
Television stations are presently broadcasting both
traditional analog signals and digital signals (DTV).
Though both types of signal occupy similar channel
"blocks", the nature of the signal within the channel is
quite different. An analog TV transmission consists of
three separate signals, each at a specified carrier
frequency within a 6 MHz block (in the US). (See Figure
3-11a.) The picture or "video" information is an AM sig-
nal at 1.25 MHz above the bottom (low frequency end)
of the block. The sound or "audio" information is an FM
signal located at 0.25 MHz below the top (high
frequency end) of the block. The color or "chroma"
information is an AM signal at 3.58 MHz above the
video signal. The energy distribution and occupied
bandwidth of these three signals is not equal: the
video signal has the highest power and widest
bandwidth, followed by the audio signal and finally the
chroma signal with the lowest power and smallest
bandwidth.
A digital TV transmission consists of a continuous
signal that occupies the entire 6 MHz block. (See Figure 3-
11b.) All of the video, audio, and color information is
digitally encoded into this signal along with a variety of
other data, control, and secondary audio information. It is
possible for the DTV transmission to carry one
high-definition television signal (HDTV) or up to four stan-
dard-definition television signals. The energy distribution
within a DTV channel is essentially uniform. However, the
average signal level of a DTV transmission is somewhat
less than the levels of the video and audio signals in an
analog TV transmission.
As indicated previously, the primary users of both
high-band VHF and low-band UHF frequencies are
broadcast television stations. In the US these are VHF
TV channels 7 through 13 and UHF TV channels 14
through 69. Each TV channel is allotted a 6 MHz block
for its transmission. VHF channel 7 begins at 174.0
MHz and extends to 180.0 MHz, channel 8 occupies
Selection
and Operation
of W
ireless Microphone Systems
28
C
H A P T E R
3
Wireless System Operation
Figure 3-11a: analog television channel spectrum
Figure 3-11b: digital television channel spectrum (DTV)