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Home  Telephone Toll Service and Systems Type A Carrier Telephone System |
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Type A Carrier Telephone SystemThe original carrier used by the Bell System was designated type A27. The carrier component and one sideband were suppressed, and one sideband from each channel was transmitted. In this system four talking channels, operating at different frequencies, were provided over one open-wire pair. The same frequencies were used for talking in each direction as indicated by the double-headed arrows of Fig. 18. Because of this it was necessary to use a carrier-frequency hybrid coil to separate the "talking" and "receiving" sidebands. This hybrid coil and the required balancing networks, which operated at carrier frequencies, were objectionable features. The type A system transmitted a master 5000-cycle frequency throughout the system. This was distorted at the various carrier terminals, and the harmonics were selected by filters and amplified to supply the carrier frequencies necessary for modulation (page 418) and demodulation (page 421). Several type A systems were installed about 1918, one of which remained in service until about 1940.29
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Last Update: 2011-05-30