1176 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER, 1953 



speed forms a practical technique for securing the desired resolution in 

 both frequency and time. Likewise in the correlatograph we can dispense 

 with individual multipliers and integrators if we are willing to give up 

 some of the otherwise available integration time for each r value. One 

 multiplier and one integrator are then put in series with the switch arm 

 and the contacts go directly to the delay line taps as shown in Fig. 3. 

 A complete analogy with the simplified spectrograph would require a 

 line of variable delay with a single output tap. This could be done with 

 magnetic recording and moving heads, thereby eliminating the rotating 

 switch. We felt however, that the fixed delay line would furnish the more 

 stable and accurate component and chose the arrangement of Fig. 3. 

 An auxiliary recording and reproducing process made it possible to 

 accommodate a wide variety of signals with the one delay line. 



DETAILS OF APPARATUS 



Fig. 4 shows the arrangement of apparatus chosen. Our program was 

 aimed at signals such as might occur in a nominal speech band extending 

 from 200 to 4,000 cps recorded on magnetic tape at 15 inches per second. 

 The reproducing element was a spinning double-ended pickup coil which 

 successively scanned a one-inch loop of tape with one end beginning its 

 scan just as the other end left the tape. The coil made 60 revolutions per 

 second and hence the reproducing speed was 120 inches per second or 

 eight times the recording rate. Our speech band was thus made to 

 occupy the range from 1,600 to 32,000 cps, and this was therefore the 

 range chosen for the delay line. A recording speed other than 15 inches 



TAPPED 



DELAY 



LINE 



CO 



To-Ar 



'.. V 



Ar 



a 



^'MECHANICAL DRIVE-, 

 / M ULTIPLIE R IN TEGRAT OR \ 



- X 



T 



^ 



MARKING 

 STYLUS 



tt 



SIGNALS 



AUTO I CROSS 



CORRELATION CORRELATION 



CO 



Fig. 3 — Mechanism of correlatograph with common multiplier and integrator. 



