While the fundamental ideas as outlined above are old, the physical 

 means by which successful carrier current telephony could be made 

 practicable did not become available until the period 1913 to 1918. 

 In that period, the successful development of the vacuum tube for use 

 in telephone repeaters produced a device which, with different circuit 

 arrangements, could be used satisfactorily for generating carrier 

 currents, modulating them with telephone currents, and for reproducing 

 the telephone currents from the modulated carrier currents. At 

 about the same time, marked advances were made in the development 

 of means for separating into any desired groups a mixture of currents 

 of different frequencies transmitted over the same conductors. These 

 means may be considered in principle an elaboration of the elementary 

 apparatus of this type, called "composite sets," long in use for sepa- 

 rating telephone and telegraph currents transmitted over the same 

 circuit by reason of their difference in frequency. The more complete 

 solution of this general problem was made by the development by the 

 Bell System of the "electrical filter." 



With these new tools it became possible to develop carrier telephone 

 systems suitable for commercial service. Such systems were first 

 introduced into the plant of the Bell System in 1918. Since that time 

 their use has spread widely, particularly over non-loaded open-wire 

 circuits of the System. 



The most important type of carrier telephone system in general use 

 is the Type C. One terminal of such a system is indicated schemati- 

 cally in Fig. 17. With this system three carrier channels, marked A, 

 B, and C, and a voice-frequency channel, marked V, are transmitted 

 simultaneously over one pair of wires. The four circuits as they appear 

 at the toll switchboard are alike and are treated indiscriminately by 

 the operator. Coming from the switchboard as indicated at the left 

 of Fig. 17, the three carrier channels first pass through three individual 

 sets of carrier equipment. In each of these sets of carrier equipment, 

 the circuit is separated into transmitting and receiving paths. The 

 transmitting path is passed through a modulator in which the voice 

 currents received from the switchboard act upon a carrier and produce 

 modulated carrier currents, and through an electric filter to the general 

 transmitting circuit indicated on the drawing. The receiving channel 

 is connected to the general receiving circuit through an electric filter 

 and through a demodulator by means of which the received currents 

 are caused to reproduce voice-frequency currents similar to those 

 delivered to the circuit at the other end. 



As the next step the three transmitting channels are brought to- 

 gether through a common amplifier and transmitted through a "band" 



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