374 BELL SYSTEM TECHNICAL JOURNAL 



also in the high quality monitoring circuit to afford additional dis- 

 crimination against unwanted high-frequency interference as, for 

 example, interference from the carrier channels. This arrangement 

 of splitting the filter requirements enables a less expensive type of 

 line filter set to be employed. 



Overall Performance 



The initial application of this new program system was made on 

 two transcontinental circuits between Chicago and San Francisco. 

 One circuit, referred to as circuit 1, was routed through Omaha and 

 Denver over the central transcontinental line. The other circuit, 

 referred to as circuit 2, was routed via St. Louis and Kansas City to 

 Denver and thence over the same pole lead as circuit 1. The layout 

 of these two circuits is shown in Fig. 20. Circuit 1 was approxi- 

 mately 2,395 miles long and was routed through 17 repeater stations 

 involving 23 amplifiers in tandem. Circuit 2 was approximately 

 2,689 miles long and was routed through 19 repeater stations involving 

 29 amplifiers in tandem. Both circuits were routed through B-22 

 cable facilities between Sacramento and Oakland, California, and 

 non-loaded cable facilities in the transbay submarine cable between 

 Oakland and San Francisco. 



At San Francisco a listening studio was set up in the Grant Avenue 

 office where the program circuits terminated. A two-unit loudspeaker 

 with suitable connecting networks was set in a 7' x T baffle, the 

 response of this loudspeaking system being practically uniform from 

 about 40 cycles to above 8,000 cycles. The room in which the loud- 

 speakers were located was acoustically treated so as to obtain the 

 proper reverberation time. A powerful amplifier having a flat gain- 

 frequency characteristic from 35 cycles to well above 8,000 cycles 

 supplied the loudspeaker system. A high quality phonograph system 

 for furnishing test programs was also installed at the Grant Avenue 

 office. The records used were of the vertical cut type and included 

 several recordings of a 75-piece orchestra as well as various solo and 

 instrumental recordings. Two outside pickup points were used, one 

 at the studios of one of the broadcasting companies at San Francisco 

 and the other at a hotel. At both of these places the moving coil 

 type of microphones was used and the latest type of high quality 

 pickup amplifiers. The pickup system used at both these places 

 had a response characteristic within about 2 db of being flat over the 

 range of 35 to 10,000 cycles. 



Figure 21 is a photograph showing the special equipment placed in 

 the Grant Avenue offfce for carrying out the various overall tests and 



