112 



RADAR SCOPES 



the tube bias is high (the scope, dim), then 

 the addition of noise may actually improve 

 the signal visibihty because it simply adds 

 brightness to the screen. (See Fig. 2.) Of 

 course, the addition of certain frequencies of 

 noise may impair visibihty; for if the noise 

 pips are the same size as the signal pips, the 



video gain is turned up; and the optimal bias 

 should shift with gain. Garner and Ham- 

 burger (15) found this to be tme. 



Although bias and gain are critical factors, 

 most investigators have ignored them. 

 Payne-Scott (44), for example, merely says 

 she set the bias at cut-off (extinction of elec- 



ta 15 - 



♦ 2 -2 -4 -6 -8 -10 



CRT BIAS INVOLTS FROM VRI 



Fig. 2. Visibility, CRT bias, and video noise 

 Visibility is given in terms of the number of decibels by which a reference (8-volt) signal was at- 

 tenuated when the pip was at the threshold of visibility. Increased visibility is therefore indicated by 

 increased amounts of attenuation, i.e., by decreased signal voltages. Bias is a voltmeter reading taken 

 from a visual reference as zero, i.e., the minimum visible sweep line with 10 rpm rotation rate and 667 

 PRF. Noise level is 0.85 volts rms (thermocouple type meter); noise random within limits of video 

 band pass (100 cps to 5 MC) with i-f bandwidth greater than 10 MC. The large pip is 15 ais x 30°; the 

 small pip, 2 /xs X 2°; and the very small pip, ^ ^s x 1°. The optimal bias is about 5.5 volts less than the 

 visual reference without noise and less than this with noise, but the brightness of the screen is probably 

 the same for both optima. Being "grainy," the noise is detrimental to the discrimination of the very 

 smallest pip. Broken line indicates invisibility at —8 volts. 



probability of recognizing the signal is ob- 

 viously reduced. If the tube bias is already 

 at or below optimum, the addition of noise 

 adds not only confusion but also unwanted 

 phosphor excitation, and produces the equiv- 

 alent of an even lower bias. 



From this it must be evident that bias and 

 video gain are interdependent variables, each 

 tending to compensate for the other. Con- 

 sequently, the bias should be higher when the 



tron beam) and then allowed the observer to 

 adjust the gain as he pleased. No additional 

 information is given. 



Inasmuch as the optimal bias is dependent 

 on the response curve of the phosphor and 

 because phosphors change with use, there is 

 no known electrical way of setting the opti- 

 mal bias for a given tube by meters alone. 

 Consequently, it has been suggested that a 

 simple psychophysical method be employed 



