All SEADAC components are commercially available but the analyzer 

 system* comprising 1) local oscillator, 2) filter networks, and 3) analog 

 computers is relatively new and may not be well known. 



2. DATA PREPARATION. 



The raw data are received from the transducer as an analog voltage, 

 preamplified and frequency modulated (FM) onto 1-inch, 14-channel , 1.5-mil 

 Mylar magnetic tape. Since the raw data will be subjected to a frequency 

 decomposition in the filter network of the analyzer system (to be 

 discussed) , consideration must be given to the relationship between the 

 range of frequencies in seakeeping events and electronically feasible 

 filter networks. Seakeeping events seldom contain energy at frequencies 

 outside the range 0.03 to 3.0 cps . Unfortunately, commercially available 

 filters that are fairly sharply tuned are too wide to accommodate this 

 frequency range. It is only since the recent advent of low- speed magnetic 

 tape recording that the conventional "wave analyzer" has lent itself to 

 use as a low-frequency analyzer. The principle is simply to bring the 

 signal frequencies to the filter rather than the filter to the signal 

 frequencies. This is accomplished by causing the filter to look at the 

 time-history of the seakeeping event, not in true time, but in a highly 

 condensed time scale. If a 20-minute record is speeded-up ten times, the 

 record will pass through the filter system in one-tenth the recording 

 time and the frequencies will appear to be ten times greater than they 



really are. Under such conditions, the "apparent" frequency range becomes 



*Instruction Booklet for TP-625 Wave Analyzer System, Technical 

 Products Corp., Los Angeles, Calif. 



