D. G. Tucker 43 
oO \ 2 3 4 
oY) 
X= We 
Fig. 2.13. Graph showing the shape of the delay/frequency characteristics for an m-derived 
low-pass filter section with m= 3. The curve is a reasonable approximation to a straight 
line over the range x= 0.1 to 0.4. A filter section with m> 1 can be realized by the use of 
mutual inductance as shown, 
If then the difference frequency is obtained, this will be a measure of target 
bearing. (The sum frequency will be ameasure of mean range.) Thus, two trans- 
ducers will give a full sector search with no secondary-lobe effect, although in 
the simplest case there will be a left-right ambiguity. Even this latter can be 
removed by using a polyphase circuit for obtaining the difference frequency. 
The block schematic of the system is shown in Fig. 2.14. Kay suggests that 
this is the way bats obtain accurate obstacle- and prey-location with only two 
small ears. 
2.4. DISPLAYS 
The theoretical investigation of displays has always been bedeviled by the 
difficulty of finding a criterion of performance which is both realistic and suit- 
able for mathematical analysis. Experimental work has been hindered by the 
subjective nature of the measures of performance. Thus, progress has been 
slow, as far as any fundamental improvement in detection thresholds is con- 
cerned. One contribution from Birmingham is the improved integrator described 
below. It is also thought that the work on pattern recognition to be described 
