FISHERY BULLETIN: VOL. 71, NO. 3 



MULTIPLEX 



ANALOG 

 TO DIGITAL 

 CONVERTER 



TELETYPE 



COMPUTERS 



DIGITAL 

 SQUARING 



CIRCUIT 



Figure 1.- 



Block diagram of Digital Data-Acquisition 

 and -Processing System (DDAPS). 



hardwired 8- x 8-bit multiplier. Squaring re- 

 sults in an output that is proportional to acous- 

 tic intensity (Thorne, 1971). Conversion to 

 digital form before squaring eliminates the 

 problems with dynamic range associated with 

 analog voltage squared integrators. The 12 

 most significant bits of the square are retained 

 and transmitted to the PDP/8L central pro- 

 cessing section. 



The processing section adds the squared data 

 words to the appropriate integrator. Ten con- 

 tiguous depth intervals are provided for each 

 sounder, so that a maximum of 20 integrations 

 may be operating simultaneously. 



Overall timing for the system comes from 

 the echo sounders. An initiation pulse signals 

 the computer to begin data processing. A bot- 

 tom signal indicates to the computer that the 

 sea floor has been reached. Thus the bottom can 

 be tracked and integration terminated a short 

 distance above it. The magnitude of this dis- 

 tance is an input value to the computer and 

 generally depends on the steepness of the 

 slopes. Typical values are 2 to 3 m. If occa- 

 sional bottom echos are too weak to activate 

 the bottom indicator signal, the computer auto- 



matically terminates integration just above the 

 last known bottom depth. The bottom can also 

 be tracked manually under circumstances, 

 such as extremely steep slopes, where the bot- 

 tom signal is consistently too weak to be fol- 

 lowed automatically. 



The program keeps count of the transmitted 

 signals, and when the count reaches the input 

 value for "number of pings," processing ter- 

 minates. Alternately, the operator may termin- 

 ate processing by setting a sense switch. 



In the output section, the integrator value is 

 teletyped for each depth interval for each 

 sounder. Using input constants relating inte- 

 gration to fish density, th-^ program then calcu- 

 lates the fish density from ,he integrator values, 

 automatically correcting for depth. An example 

 of the output is given in Figure 2. The mathe- 

 matical relationship between integration and 

 fish density is detailed in Thorne (1970), Moose 

 and Ehrenberg (1971), and Moose, Ehrenberg, 

 and Green (1971). The constant relating fish 

 density to integration from any echo sounder 

 can be calculated when the target strength of 

 the fish and absolute calibration of the sounder 

 and transducer are known. Alternately, the 

 constant may be determined indirectly by cali- 

 bration of the integration against other mea- 

 sures of fish density, such as net hauls or echo 

 counts (Thorne, 1971, 1972). 



The hardware of the DDAPS, with the ex- 

 ception of the teletype, is contained in a 4-foot- 

 high, 19-inch relay rack. The special circuitry 

 is packaged on 4- x 6-inch, plug-in, wire- 

 wrapped cards. All digital circuitry is inte- 

 grated circuit, TTL logic. The system is highly 

 portable and is easily moved from laboratory 

 to ship in several hours. It is intended for pro- 

 cessing data aboard ship in real time, and in 



Figure 2. — Sample DDAPS output for one echosounder and three depth intervals. 



838 



