Type C Carrier Telephone Systems

  
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The type C system was first installed about 1925 and has been very extensively used. It incorporates the most desirable features of the systems just considered. Like the type A system, the carrier is suppressed, and hence the effect of line attenuation is minimized. Like the type B, different frequencies are used for transmitting and receiving, an
Type C Carrier Telephone Systems - schematic

d filters, instead of carrier-frequency hybrid coils, are used to separate the talking and the receiving signals. The original type C carrier telephone system was extensively modified about 1938 to take advantage of developments in communication. Original Type C Carrier Telephone System. 30 The schematic diagram of the original type C carrier is shown in Fig. 20. To explain the operation of this system, assume that a person whose telephone is connected to channel 1 at the left is speaking to a person whose telephone is connected to channel 1 at the right. The incoming speech signals pass through the voice-frequency hybrid coil, and to the modulator at the top. This is a balanced modulator (page 421) that eliminates the carrier but produces both sidebands. The band filter selects one sideband and rejects the other. The selected sideband passes down to the common wires leading to the transmitting amplifier, where all the incoming signals to be transmitted are amplified to the proper level for impressing on the line. The amplified signals from the three channels pass through the high-pass filter and hence to the line. Each of the three modulators is fed with a different carrier frequency, and hence each of the three sidebands that are transmitted simultaneously is at a different frequency as Fig. 18 shows. Figure 20. Schematic diagram of original type C carrier telephone system, and of carrier and voice-frequency equipment at...



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