176 



BELL SYSTEM TECUMCAL JOURNAL 



and channel 9 allocations marks the encl of the region in which the phase of 

 the blocking tilter can be successfully simulated. Outside of this region, 

 in parts of channels 5 and 9, the switching operation shifts both phase and 

 amplitude of the transmission and i)recludes the use of these channels for 

 voice-frequency telegraph or telephoto services. 



Equipment 



As previously stated a carrier program terminal consists of a modem 

 unit, a demodulator-amplifier panel, and a reversing panel. As shown in 



CHAN- 12 3^ 



NELs Cir-^t^crr^C:^-.! 



9 10 II 12 



80 



BOTH FILTER AND 

 NETWORK IN DURING 

 50-MILLISECOND 

 SWITCHING INTERVAL 



32 36 40 

 KILOCYCLES 



44 48 52 56 

 PER SECOND 



20 24 28 

 FREQUENCY II 



Fig. 6 — Transmission characteristics of blocking and by-passing circuit under normal 

 conditions and during switching. 



Fig. 7 two such terminals, together with fuse panels for 24 and 130-volt 

 battery supply for several bays, are mounted in one standard cable duct 

 type bay 19" wide and 11 '6" high. The equipment is mounted on the bay 

 in a group, from top down, in the order mentioned. The front or wiring 

 side of the equipment is provided with three separate covers which furnish 

 the necessary electrical shielding as well as physical protection. Connec- 

 tions to carrier systems are made through carrier program high-frequency 

 patching jacks on a 4-\vire basis and to the program circuits through audio 

 frequency testing jacks on a 2-wire basis from which point the carrier 

 program channel is lined up for program service at the proper transmission 

 levels for both directions of transmission. 



A through branching circuit consists of two sets of line bridging equip- 

 ment and two carrier program terminals mentioned above. As shown in 



