284 BELL SYSTEM TECHNICAL JOURNAL 



The Western Electric circuit is a four-wire 19-gauge H-44 circuit ^ 

 817 miles long, with a 1000-cycle time of transmission of 0.043 second. 

 Normally there is an echo suppressor at Pittsburgh, which is approxi- 

 mately at the midpoint of the circuit. In connection with other tests 

 which were going on at the time, the normal echo suppressor was 

 removed, and the circuit was looped via Bell Telephone Laboratories 

 where artificial (acoustic) delay circuits and two echo suppressors were 

 inserted to simulate two tandem circuits, each with an echo suppressor 

 at its midpoint. The extra equipment introduced no additional at- 

 tenuation or frequency discrimination. 



Recording Methods and Mechanisms 



All of the recording was done by mechanical means controlled by 

 engineers who observed the progress of the conversations. The 

 preliminary data on local calls were obtained with the aid of an inked- 

 roUer paper-tape recorder of a type formerly used in telegraph studies. 

 This machine had only two recording traces and was not adapted to 

 run at high speed, thus limiting the amount and accuracy of the 

 information obtainable. In view of the limited amount of data and 

 its relative lack of precision compared to the main body of data it does 

 not appear profitable to enter into a further description of the early 

 recording means. 



The principal part of the improved recording mechanism was a six- 

 string rapid-record oscillograph of the type already described in the 

 Bell System Technical Journal.* The several strings of this machine 

 were energized by speech power from the two talkers and by energy 

 from an oscillator under control of the echo-suppressor relays. This 

 arrangement is indicated in Fig. 1. The machine was started at the 

 beginning of each call to be observed and ran continuously at a speed of 

 about 20 feet of recording paper per minute. This resulted in a com- 

 plete pictorial record of the conversational interchanges. These 

 records are well adapted to measurement of the essential time relations, 

 but do not lend themselves to reproduction of the original conversa- 

 tions. Operation of the echo-suppressor relays is also shown on the 

 oscillograms, but analysis of this information is outside the scope of 

 this paper. 



To facilitate inspection of the speech traces the voice energy from 

 the circuit was routed through quick-acting automatic volume controls 

 which permitted the weak beginnings and endings of words to be 



- The designation H-44 means 44 millihenry loading coils spaced 6000 feet apart. 

 3 "An Oscillograph for Ten Thousand Cycles," A. M. Curtis, Vol. XII, No. 1, pp. 

 76-90. 



