Fig. 2. Test film and prints: top-original negative, center-high-contrast negative print, 

 bottom-high- contrast positive print. 



lasts slightly more than 7 days. When recording 

 intermittently, approximately 10,000 l/8-inch 

 blocks of data may be recorded. 



THE COMPUTER 



The staggering task of manual reading is 

 obvious. For example, using a large-screen 16 mm 

 film viewer with motor-driven advance and a pre- 

 pared grid, it took one person more than one day 

 to read k2 hours of recording sampling at hourly 

 intervals. The approach taken to mechanize 

 reading of the tape involved adaptation of the 

 PDP-1 computer. One of the main reasons for the 

 selection was that the computer is eauipped with 

 an X-Y CRT point plotter feature for both read- in 

 and read-out. A specially designed computer- 

 controlled reader has been attached that contains 

 a film advance mechanism and a photomultiplier . 

 Initially the photomultiplier sensed the absolute 

 image density as seen through the film. A later 

 and superior version compared the image density 

 with the background on a differential basis, 

 thereby enhancing the sensitivity and overall 

 stability of the system. 



A test computer printout is shown in Fig. k. 

 The computer was programmed to type out ones and 



zeroes for validation by comparison with visual 

 reading. Agreement was found, as it was also 

 between repeat readings of the same film, and 

 corresponding scans of the same block of data 

 read both 16 and 128 scans per block. Fig. 5 

 shows the present printout. The binary numbers 

 are converted to degrees, the compass and vane 

 directions combined and corrected for the mag- 

 netic variation (in this case 15°W) to give the 

 true current direction, the rotor revolutions 

 converted to current speed in mm/sec and the day, 

 hour and minute of the recording computed. The 

 reading rate, while not yet fully optimized, is 

 approximately 10 blocks of data per second, each 

 block spanning 2.5 minutes. This provides read- 

 in of k-2 hours of data in approximately 1 l/2 

 minutes when the original recording was made at 

 one sample per minute. Of course, there are 

 other steps such as setup and splicing on an 

 instruction leader. Also, the time quoted is for 

 transferring the raw data to magnetic tape in the 

 standard IBM low-density format . One roll of 

 magnetic tape will hold about 90 current meter 

 records. 



Among the obvious advantages of so closely 

 integrating the transducer and computer is the 

 capability of performing in the computer many of 

 the operations normally done by the transducer. 



82 



