agreement of the curves seems to justify the extension of the analyzer 

 system in this direction. 



Computation of cross spectra is not yet available as commercial 

 analog electronic equipment, so this matter must be pursued at the 

 users' level. Some cross- spectrum analyzers have been built in the 

 laboratory, and at least one is known to be successful. 



There are two known methods of performing cross -spectrum analysis that 

 are amenable to incorporation in the SEADAC: 



1. Single-filter, and 



2. Matched filters. 



The first method requires the combination of two simultaneous records 

 by addition and by differentiation and addition. The record which 

 results from these operations is treated like an ordinary record and its 

 spectrum is analyzed accordingly. The result played out on the X-Y 

 recorder is operated upon, in certain ways, to extract the co^ar^ quadrature- 

 spectra (the desired components of the cross spectrum) . 



The second method involves the simultaneous modulation and filtering 

 of the two records. The outputs of the matched filters are multiplied 

 together, once directly, and once after one signal is phase- shifted 90 degrees 



The results, after integrating and averaging, are played out on the 

 X-Y recorders as the co- and quadrature=spectra. 



To accommodate both types of cross- spectrum analysis in the SEADAC 

 (which is our intent), the following operations must be incorporated 



27 



