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Home  Telephone Toll Service and Systems Type B Carrier Telephone System |
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Type B Carrier Telephone SystemThe second carrier, developed and installed about 1920, was designed type B;27 one set remained in operation until about 1940.29 The carrier component and one sideband were transmitted. Because of this and the fact that stable oscillators had been developed, it was no longer necessary to transmit a master 5000-cycle control frequency throughout the system.27 Also, as Fig. 18 indicates, different frequencies were used for "talking" and "receiving," or for "east-west" and "west-east" transmission. Thus, the "talking" and "receiving" sidebands could be separated by filters, and carrier-frequency hybrid coils and balancing networks were not necessary. With the type B system, both the transmitted carrier and the sideband were attenuated by the line. Because the magnitude of the demodulated voice-frequency wave depends on the magnitude of both the carrier and the sideband, the voice-frequency wave was decreased by the square of the attenuation. Variations in transmission, caused by weather changes, were pronounced.27
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Last Update: 2011-05-30