3. Pulse width modulated signals 



These signals can be acoustical or electromagnetic transmissions 

 (including light). Most oceanographic measurements are telemetered 

 electrically or acoustically at the present time. 



An amplitude modulated signal is a time analog, in that it is continuous 

 rather than incremental. Inasmuch as the electric current is dependent 

 upon the resistance of the transmitting cable, variations in the current 

 must be measured relative to some standard level at the signal source. 

 Strictly speaking, a frequency modulated signal is incremental in that 

 it is a frequency count covering a very short period of time. For practi- 

 cal purposes such signals generally can be regarded as analogs when 

 appropriately recorded. Thus, they have the advantage of easy conversion 

 to analog or digital form. Pulsed signals are by definition incremental 

 but may be made to simulate an analog. Frequency and duration of 

 pulses may be significant. 



ANALOG VS DIGITAL 



If it is necessary to choose between instruments giving analog or 

 digital records, several things should be considered. Digital data may 

 reduce the length of the record but sacrific details and flexibility. 

 Digital data may be sampled at temporal or spatial intervals, or at 

 intervals of some other variable such as salinity, light intensity, 

 pressure, etc. Digital data also may be obtained from analog data at 

 any point in the data sampling, relaying, processing, or storing opera- 

 tion; the reverse is not necessarily true. 



The ideal system is one that permits selection of analog data or a 

 variety of digital information at any point in the system. Initially, it 

 appears that a pulse width or FM system most nearly approaches such 

 an ideal. 



One such system is that designed at the Scripps Institution of Oceano- 

 graphy for telemetering temperature and depth data over a single, 

 insulated, steel cable. The transmitted signals fall within the audible 

 frequency band. Separate segments within this band can be used for 

 transmitting data from several sensing elements simultaneously, such 

 as temperature and depth. Such data can be recorded directly on magnetic 

 tape, possibly with some preamplification necessary. It is believed that 

 a simple counter could be built into the magnetic tape recorder to be 

 used for monitoring to insure that the signal is being recorded properly. 

 This system would be a simple, portable one to which components and 

 modifications could be added for recording data directly on paper tapes, 



A- 5 



