Microphone Technology 663compared as to strength with the stronger signal at any instant being switched to the
remainder of the single receiver circuitry.
In a variation on this technique, the signals from both radio frequency amplifi ers are
summed and then fed to the rest of the circuitry of a single receiver with no switching
being involved. Finally, two receivers set to receive the same carrier frequency are
employed, one for each receiving antenna. The automatic gain control voltages that
are developed at each receiver’s detection stage are compared with the audio output
circuitry being switched to that of the receiver having the larger control voltage. This last
technique is the most expensive and, even though it involves switching, has perhaps the
best performance overall.
Wireless microphone transmitters employ a relatively small frequency deviation in the
frequency modulation process. The modulation index is thus small. This restricts the
dynamic range that is available for program material and weak signals may be lost in the
noise fl oor. A long-term average of the spectral density associated with both voice and
music programs exhibits a broad maximum in the vicinity of 500 Hz accompanied by
a roll off in density beyond about 2 kHz. The spectral density is the average power per
unit frequency interval. This being the case, it is necessary to pre emphasize the higher
frequencies in the audio material prior to further signal processing.
The normal range of the audio material to be transmitted may well be as large as 80 d B
while the available range in the small deviation FM transmitter may be only 40 dB. The
80-dB range of the audio material is squeezed into the 40-dB range available by 2 into
1 compression prior to the modulation process. After transmission and reception at the
receiver, the recovered audio material occupying a 40-dB range is fi rst subjected to a 1
into 2 expansion in order to restore the full dynamic range of 80 dB.
This is then followed by a de emphasis of the audio material above 2 kHz in order to
restore the natural spectral balance of the audio material. Figure 22.11 displays typical
preemphasis and complementary deemphasis curves with the upper curve being that of
preemphasis. The combination of the two yields a fl at response across the audio band.
The process of compressing the audio dynamic range prior to transmission and expanding
the range of the audio material following reception has been termed compansion. A
typical compression curve employed in the audio circuitry of the transmitter, followed
by the complementary expansion curve employed in the audio circuitry of the receiver, is
displayed in Figure 22.12.