1174 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER, 1953 



the short term spectrum. Comparison of the two short term functions 

 is a much more involved matter than Avhen the time interval is large in 

 both cases. The subject has been extensively treated in the literature,^ 

 but the conclusions are still somewhat obscure. In an investigation which 

 was started by the present author and the late Liss C. Peterson in 1950, 

 we noted that a complete correlation analogue of the audio spectrograph 

 had apparently never been constructed even though suggestions had 

 appeared concerning the possibility.^ It seemed that some of the ques- 

 tions concerning the merits of the correlation method of analysis could 

 not properly be answered without actually building and testing such a 

 machine and it also appeared possible that we would thereby add another 

 useful tool for the problems associated with speech and other signals of 

 interest. We accordingly undertook the design and construction of a 

 "correlatograph" following lines closely parallel to spectrographic ex- 

 perience in order to economize in new shop designs. 



In a spectrograph, such as used in visible speech for example, a 

 three-dimensional display is obtained in which time and frequency are 

 two rectangular coordinates and the short term power spectrum is 

 represented by the intensity of light or density of marking. The func- 

 tional mechanism is illustrated in Fig. 1. The incoming signal is delivered 

 to a bank of band pass filters with midband frequencies uniformly spaced 

 throughout the frequency range of interest. The envelopes of the filter 

 output waves are picked up in turn by a rotating switch arm to control 

 the voltage impressed on a marking stylus. The stylus moves across the 

 paper in synchronism with the collector arm and the paper advances 

 after each stroke. 



The analogous diagram for a correlatograph is shown in Fig. 2. We 

 recall that the correlation of two functions is defined as the average of 



BAND FILTERS 



SIGNAL 





Fig. 1 — Mechanism of spectrograph. 



