560 Chapter 16
- B band of 722–734 MHz
- 728.125–740.500 MHz band.
The FCC has assigned most of the DTV channels
between channel 2 and 51, and only four channels
between 64 and 69, which is where most of the profes-
sional wireless microphones operate.
16.10.1.2 Adjustment of the System’s Operating
Frequency
Many of the professional wireless microphones are
capable of being tuned to many frequencies. In the past
the systems were fixed frequency, often because that
was the only way they could be made stable. With
PLL-synthesized channels (Phase Lock Loop), it is not
uncommon for systems to be switch tunable to 100
different frequencies in the UHF band and have a
frequency stability of 0.005%. This is especially impor-
tant with DTV coming into the scene.
16.10.1.3 Capture Ratio and Muting
Capture ratio and muting specifications of the receiver
are important. The capture ratio is the ability of the
receiver to discriminate between two transmitters trans-
mitting on the same frequency. When the signal is
frequency modulated (FM), the stronger signal controls
what the receiver receives. The capture ratio is the
difference in the signal strength between the capturing
transmitter and the captured transmitter that is blan-
keted. The lower the number, the better the receiver is at
capturing the signal. For instance, a receiver with a
capture ratio of 2 dB will capture a signal that is only
2 dB stronger than the second signal.
Most systems have a muting circuit that squelches
the system if no RF signal is present. To open the
circuit, the transmitter sends a special signal on its
carrier that breaks the squelch and passes the audio
signal.
16.10.1.4 RF Power Output and Receiver Sensitivity
The maximum legal RF power output of a VHF
high-band microphone or intercom transmitter is
50 mW; most deliver from 25–50 mW. Up to 120 mW
is permissible in the business band (for wireless inter-
coms) under FCC part 90.217, but even this represents
less than 4 dB more than 50 mW. The FCC does not
permit the use of high-gain transmitter antennas, and
even if they did, such antennas are large and directional
so they would not be practical for someone who is
moving around. Incidentally, high-gain receiving
antennas are also a bad idea because the transmitter is
constantly moving around with the performer so much
of the received radio signal is actually caught on the
bounce from walls, props, and so on (see Section
16.9.2).
Even if an offstage antenna is aimed at the
performer, it probably would be aiming at the wrong
target. Diversity receiving antenna systems, where two
or more antennas pick up and combine signals to feed
the receiver, will reduce dropouts or fades for fixed
receiver installations.
The received signal level can’t be boosted, given the
restrictions on antenna and transmitted power, so usable
range relies heavily on receiver sensitivity and selec-
tivity (i.e., capture ratio and SNR) as well as on the
audio dynamic range. In the pre-1980 time frame, most
wireless microphones and intercoms used a simple
compressor to avoid transmitter overmodulation. Today,
systems include compandor circuitry for 15–30 dB
better audio SNR without changing the RF SNR (see
Section 16.10.3). This is achieved by building a
full-range compressor into the microphone or intercom
transmitter, and then providing complementary expan-
sion of the audio signal at the receiver—much like the
encoder of a tape noise-reduction system. The compres-
sion keeps loud sounds from overmodulating the trans-
mitter and keeps quiet sounds above the hiss and static.
The expander restores the loud sounds after reception
and further reduces any low-level hiss or static.
Companding the audio signal can provide from
80–85 dB of dynamic range compared to the 50–60 dB
of a straight noncompanded transmit/receive system
using the same deviation.16.10.1.5 Frequency Response, Dynamic Range, and
DistortionNo wireless microphone will provide flat response from
20 Hz–20 kHz, nor is it really needed. Wireless or not,
by the time the audience hears the broadcast, film, or
concert, the frequency response has probably been
reduced to a bandwidth from 40 Hz–15 kHz. Probably
the best criteria for judging a handheld wireless micro-
phone system is to compare it to the microphone
capsule’s naked response. If the transmit/receive band-
width basically includes the capsule’s bandwidth, it is
enough. Generally speaking, a good wireless micro-
phone should sound the same as a hard-wired micro-
phone that uses the same capsule. Wireless intercom
systems, because they are primarily for speech commu-