THE CORRELATOGRAPH 



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their product with fixed time lag; i.e., when T is large, the expression 



^i2(r) = I jy,{t)f2{l - t) dt 

 gives the cross-correlation of /i(0 and/2(0, and the expression 



Mr) -\[ m)m-T) 



dt 



gives the autocorrelation of fi{t). In short term correlation T is finite. 

 In Fig. 2, the different lag times are obtained by a tapped delay line and 

 each tap is followed by its own multiplier and integrator. The integrated 

 values are picked up by a rotating switch arm to control the marking 

 stylus voltage as in Fig. 1. The rectangular coordinates are now t and r 

 instead of t and /. The marking intensity represents the correlation 

 function. 



In actual spectrographs it is usually found expedient to replace the 

 bank of filters by a single filter and use a swept frequency oscillator and 

 modulator to heterodyne the signal across the filter band. A rotating 

 condenser plate tuning the oscillator thus takes the place of the rotating 

 switch. The sweeping frequency must not change too rapidly for the 

 analyzing filter to respond adequately, and also must not change so 

 slowly that short signal bursts are imperfectly registered. A preliminary 

 recording of the signal wave with a subsequent reproduction at a different 



MULTIPLIERS INTEGRATORS 



MARKING 

 STYLUS 



Fig. 2 — Mechanism of correlatograph. 



