572 BELL SYSTEM TECHNICAL JOURNAL 



the opposite directions of transmission as required by the allocation 

 of frequencies. The amplified currents pass through the high-pass 

 filter of the line filter set and thence to the line circuit. 



In receiving, the sideband frequencies, after separation from the 

 voice currents by the line filter set, pass through the directional filter 

 and an amplifier similar to that used at the transmitting terminal. 

 While the power output required at the receiving amplifier is usually 

 small as compared to that required at the transmitting amplifier, the 

 same unit is used for the two positions to provide flexibility in the 

 adjustments of the receiving gains of the separate channels and for 

 the purpose of economy in production. The different channel currents 

 in the output of the amplifier are selected by the respective receiving 

 band filters and thence pass into the demodulator circuits. In the 

 demodulators the voice frequencies are derived by the modulation of 

 the sideband currents with a carrier frequency supplied by a local 

 oscillator whose frequency is adjusted accurately to agree with that 

 of the corresponding transmitting modulator at the farther terminal. 

 This important problem of synchronization of oscillators is further 

 discussed later in the paper. It is, of course, obvious that if the 

 carrier frequencies of the modulator and the corresponding demodulator 

 of the same channel are not in sufficiently close agreement there will 

 be a serious distortion of the speech currents received over the channel. 



The output of the demodulator circuit includes a low-pass filter 

 for suppressing the unwanted components of demodulation, and the 

 circuit thence leads to the channel terminal through the hybrid coil. 

 The function of the latter is to provide a two-wire termination of 

 the channel and it prevents the output currents of the demodulator 

 from reaching in any substantial magnitude the input of the modulator 

 circuit, thus setting up a regenerative action which might result in 

 "singing." 



It may be noted that the circuit normally provides for a transmission 

 "gain" or amplification of energy from the switchboard termination 

 to the high-frequency line circuit of approximately 20 T\} ■' corre- 

 sponding to a current or voltage amplification of 10 to 1. In the 

 receiving direction a gain of the same order of magnitude is also 

 available. Of course, the exact amount utilized in a particular case 

 depends on the line attenuation and the desired overall equivalent 

 of the circuit. It is usually desirable at the transmitting terminal 

 to maintain the level at the maximum possible for the system. The 



2 R. V. L. Hartley, "The Transmission Unit," Electrical Communication, \. 3, 

 No. 1, July 1924, pp. 34-42. W. H. Mattin, "Transmission Unit and Teleiihone 

 Transmission Reference Systems," A. 1. E. E. Jl., V. 43, No. 6, June 1924, pp. 

 504-507, Bell System Tech. JL, V. 3, July 1924, pp. 400-408. 



