1178 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER, 1953 



per second brings a correspondingly different signal band into the range 

 of the delay line. The magnetic tape advances relatively slowly during 

 the scanning process and for any one scan of a one-inch section may be 

 regarded as stationary. The revolving pick up thus delivers segments of 

 signal 1/120 second (8.33 ms) long for the correlation analysis. Our delay 

 line consists of 1.5 ms total delay with 200 taps spaced 7.5 iis apart. 

 During the first 1.5 ms of each scan, the delay line contains parts of 

 the response from two successive scans and hence is not suitable for 

 correlation measurement for lag times corresponding to all taps. We make 

 provision therefore for excluding this interval from the analysis and use 

 only the last 6.83 ms of each scan. The rotating switch advances one 

 tap on the delay line for each one inch scan, so that the value of short 

 term correlation corresponding to one value of lag time is computed every 

 8.33 ms. 200 values are computed in 1.67 sec after which a time of 0.33 

 sec is allowed for the return of the marking stylus to its initial position. 

 The rotating smtch thus makes one revolution in two seconds but the 

 last sixty degrees of the revolution are not used for display of correlation. 

 The recording paper advances 0.01 inch after each stroke of the stylus. 

 The rate of advance of the signal tape is adjustable by means of a gear 

 train. In terms of the original signal wave, one second of recorded time 

 is represented by the distance the paper moves in one second (0.005 

 inch) multiplied by the ratio of recording speed to speed of tape advance 

 past the scanning head. 



The tape scanning mechanism with the synchronized motion of stylus 

 and paper is due entirely to I. E. Cole, who designed this part of the 

 system and supervised the necessary shop work. Mr. Cole also cooperated 

 in the choice of a design plan for the rotating switch, which was manu- 

 factured to meet our special requirements by Applied Science Corpora- 

 tion of Princeton, N. J. The switch output is followed by an electronic 

 gating circuit which removes the effect of time jitter in the beginnings 

 and ei?ds of the contact intervals and trims off the previously mentioned 

 1.5 ms interval during which the tail end of one scan of the tape loop 

 remains in the delay line. The gating wave is generated from the common 

 60-cycle power supply which drives all the mechanical apparatus. The 

 output of the electronic gating circuit consists of segments of signal 

 6.83 ms long with 1.5 ms separation and with the delay increasing in 

 steps of 7.5 microseconds between one segment and the next. This con- 

 stitutes one input to the multiplier; the other input is the undelayed 

 signal in the case of autocorrelation or an independent signal for cross- 

 correlation. 



The multiplier consists of a bridge of germanium varistors with the two 



