1 70 BELL SYSTEM TECH NIC A L JOURNA L 



shown in heavy lines. The reversing and control circuits, indicated in 

 light lines, are permanently connected to the external audio-frequency 

 circuit and to the transmitting and receiving carrier line circuits regardless 

 of the condition of the switching relays. Figure 3 shows a carrier program 

 system including two terminals and a branching point as it is connected to 

 a type K system. The program equipment is identified by double-line 

 blocks. The carrier program terminals are connected into the networks in 

 the same way as the audio-frequency facilities, through equalizers, ampli- 

 fiers, bridges, and reversing circuits. Connected as one leg of a reversible 

 bridge, a carrier program circuit may feed or be fed by any of the other 

 legs, which may include cable, open-wire, studio loop, or other carrier circuits. 



Terminal Circuit 



As Fig. 2 indicates, a carrier program terminal consists of three elements: 

 a modulator-demodulator or modem, a demodulator amplifier, and a re- 

 versing and control circuit. The heart of the terminal is the modem, which 

 translates the program material from its original audio band to its desired 

 position in the carrier-frequency spectrum or vice versa. It consists essen- 

 tially of the non-linear varistor to which the carrier and program material 

 are applied, and the band filter which selects the desired sideband from 

 the modulation products. The varistor is connected in the double-balanced 

 bridge arrangement in which the signal, carrier, and sideband circuits are 

 each balanced against the other two. It is composed of copper-oxide ele- 

 ments and, in order to meet the conflicting requirements for high carrier-to- 

 signal ratio and low transmitted carrier leak, a high degree of balance be- 

 tween the varistor bridge arms must be maintained. This is accomplished 

 by building up each bridge arm of 16 copper-oxide elements connected in 

 series-parallel. This modulator as compared to one using single-element 

 bridge arms, has the same impedance, 12 decibels better carrier balance, 

 12 decibels greater carrier power capacity, and with the higher carrier power, 

 12 decibels lower non-linear distortion products. An amplifier provides 

 the required power and a narrow-band filter gives additional suppression 

 to carrier frequencies of other channels which are fed from the same carrier 

 supply. 



The band filter, which represents a major development in itself and is 

 described in another paper, introduces a considerable amount of delay 

 distortion. This is corrected by delay equalizers incorporated in the modem 

 circuit as shown in Fig, 2. Most of the delay correction is done in the audio- 

 frequency branch of the circuit by a 31-section network which also includes 

 equalization for the small residual attenuation distortion of the filter in its 

 pass band. At the lower end of the audio-frequency band, however, attain- 

 ment of the required phase characteristic with audio-frequency elements is 



