which is basically a voice coder. Essentially, a vocoder converts sounds into digital signals, which are then
multiplexed on a common line facility.
a. System use. Figure 1-2 shows how vocoders are applied to telephone channels of an-integrated satellite
communications system. The tone digital signals are placed on channels 1 and 2 of the telephone carrier set.
Figure 14. Vocoder applied to telephone channels of a satellite communications system.
b. Operation. The vocoder senses and separates the various sounds that make up a voice signal (buzz, hiss,
pitch, and so forth) and produces from these synthetic sounds a sequence of pulses. The pulses then key VF
oscillators that impart the intelligence to the tone carrier. When the tone signals are received at the distant
ground station, the corresponding channel vocoder demodulates the intelligence from the VF pulse sequence.
The intelligence so obtained keys a series of sound sources to produce an artificial telephone voice that
approaches the quality of a natural telephone voice.
c. Advantages. The advantage of the digital (pulse) method of voice transmission over the analog (wave)
method is that limiting circuits in the receiver are used to minimize the noise peaks without distorting the signal
as would happen with peak clipping of analog signal peaks. The received pulses then reconstruct the pulse
sequence.
1-17
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