CARRIER SYSTEM
A speech signal can, by amplitude modulation, be changed in frequency from its original audio frequency (of 300–3400 Hz) up to a higher carrier frequency. Most of the world's long distance telephony systems utilise 12-channel groups: twelve voice channels are all changed in frequency in this way so that a complete group of 12-channels occupies a 48 kHz bandwidth, basically from 60–108 kHz.
Figure 40.1 is a block schematic of the transmitting equipment required for channels 1 and 2 of a standard 12-channel group. The audio input signal to a channel is applied to a balanced modulator together with the carrier frequency appropriate to that channel. The input attenuator ensures that the carrier voltage is 14 dB higher, than the modulating ...
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