Microphone Technology 661been added. Frequency space has been made available in both the VHF and UHF
frequency bands with UHF units currently being more popular. The UHF band offers
more fl exibility with regard to the number of different frequencies that may be employed
simultaneously as well as a higher probability of fi nding unused frequency space in a
given locale. Additionally, required receiving antenna lengths are much more manageable
in the UHF band. For example, with a carrier frequency of 900 MHz and a wave speed
of 3 1 0^8 m/s, the wavelength becomes one-third of a meter or about 13 inches. The
required receiving antennas range between ¼ and ½ wavelength and thus have lengths
falling between about 3 and 6 inches.
There are several signifi cant technical innovations incorporated in current wireless
microphone systems that are worthy of note. Each of these will be discussed in turn.
- Receiver assisted setup.
- Space diversity reception.
- Transmitter preemphasis—receiver deemphasis.
- Transmitter compression—receiver expansion.
A diffi cult problem associated with setting up wireless microphone systems in the past
has been that associated with determining interference-free operating frequencies. This
was particularly true when the application required the simultaneous operation of a large
number of separate audio channels, each of which required an individual radio frequency
assignment. Receivers having assisted setup facilities have built in protocols for scanning
the entire operating band and identifying those potential operating frequencies that are
free of any radio frequency carrier at the time of scan. Several such scans performed
over a period of time usually are quite successful in defi ning interference-free operating
frequencies.
Space diversity reception solves a problem depicted in Figure 22.10(a) by means of an
arrangement suggested by Figure 22.10(b).
In Figure 22.10(a) , a single receiving antenna is employed. This antenna receives a
signal via a direct path to the transmitter as well as a transmitter signal that has been
refl ected by a nearby object and thus follows a longer more indirect path along its way
to the receiving antenna. The phases of these two signals having the same frequency
are different and hence they can interfere with each other. The interference may be