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BELL SYSTEM TECHNICAL JOURNAL 



circuit formed by the office equipment and intermediate cable. As 

 this transformer must pass both the voice and carrier channels, it 

 has been designed so as to have a high efficiency for all frequencies 

 between 250 and 30,000 cycles. Separation of the voice-frequency 

 circuit from the carrier system is performed by means of the usual 

 high and low pass filters which are located at the central offices. 

 These filters both have a cut-off frequency of 3,000 cycles, the low 



FREQUCNCY- MLOCVCuEi 



Fig. 8 



pass transmitting all frequencies below this value and the high pass 

 transmitting all above it. In the carrier system the transmitting and 

 receiving currents are separated from each other by a hybrid coil and 

 balancing network. Between the output of the six modulators and 

 the common transmitting amplifier, individual band-pass filters are 

 located. Each one of these filters is designed to transmit one of 

 the side bands produced by the modulator associated with it and to 

 suppress all other frequencies. The six receiving currents are sep- 

 arated in a similar manner. Each filter allows current of the proper 

 frequency to pass to the corresponding demodulator and excludes 

 all others. Each demodulator is also provided with a low pass filter 

 which allows the passage of the resulting voice-frequency current 

 but excludes all incidental higher frequencies which might be present 

 and render the circuit noisy. The input of each modulator and the 

 output of the corresponding demodulator are finally joined by means 

 of a voice-frequency hybrid coil and extended to the circuit terminal 

 as a two-wire circuit. At the San Pedro end, each two-wire circuit is 

 extended to Los Angeles over a loaded cable circuit. Phantoms are 

 employed for this purpose as they have a higher cut-off frequency 

 than have the side circuits, with a correspondingly better quality. 



Concerning the carrier system itself, the two ends are practically 

 identical while the general equipment arrangement for an individual 



