APPENDIX 

 AMPLITUDE MODULATION APPLIED TO THE SEADAC 



The analyzer system of the SEADAC is essentially a beat frequency 

 analyzer commonly encountered in the field of acoustics; that is, a pure 

 carrier frequency is mixed in a certain way with the random signal (sea- 

 keeping event) being studied. The process of mixing the carrier with 

 the random signal, to assign amplitudes to the frequency components in the 

 random signal, is called amplitude modulation . The carrier frequency is 

 called the modulated frequency, and the frequencies in the random signal 

 are called the modulating frequencies. 



In the SEADAC, the oscillator produces a range of frequencies between 

 97,000 cps and 122,000 cps. This generation of frequencies by the oscillator 

 occurs in a continuous fashion so that the resulting modulated signal is 

 a Iway s c hang ing . 



For the sake of simplicity, we shall deal with the modulation process 

 which occurs at any particular instant of time and a modulating signal 

 which is a single frequency; we will then generalize for the random signsfl 

 containing many frequencies. 



Consider the unmodulated carrier signal to be 



a = A cos 00 t [A-1] 



c c c 



where c means carrier and cD is the frequency. The modulating signal 

 combining with the carrier is 



a = A cos OD t [A-2] 



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