12-10] SPECIAL DISPLAYS 677 



ends. Many people can hear only a narrower band. The ear has essentially 

 a logarithmic response. It is a sensitive comparator of differences or 

 variations in loudness and frequency but is a poor indicator of absolute 

 values. For this reason, absolute values cannot be given by audible signals. 

 Deviation from otherwise determined procedures, direction of flight, 

 approach, etc., may be given audibly, however. 



The use of the dynamics of loudness or frequency variation can be 

 adapted to define parameters such as range to a target or aid in steering 

 toward a tracked target. They have not been used to any degree to date 

 because of the above-mentioned shortcomings and the ease with which the 

 visual sensors and the electrical-to-light transducers accommodate the 

 transfer of this intelligence to the brain. 



Some particular radar system signals are readily handled by audible 

 methods. Most of these take the form of alarm or "go-no-go" signals. 

 Instead of lights blinking, buzzers can be made to sound. The disturbance 

 factor of such an audio signal, if loud enough, probably has more impact 

 than visual alarms. Such alarms can be used for engagement breakaway in 

 aircraft interceptors, guidance path directors in landing, and collision 

 avoidance in low-flying aircraft. In most cases, even though the alarm 

 is given in audible processes, the operator must revert to his visual percep- 

 tion to understand the cause of alarm and the best way to overcome his 

 danger. 



Newer radar systems which use doppler techniques to separate moving 

 targets of varying speed by frequency separation can readily make use of an 

 audio tone to locate and maintain track on a target. AMTI (airborne 

 moving target indicator) radars used for air-to-ground search and track can 

 be mechanized to sense the presence of moving targets by audible methods 

 if the antenna dwell time on an area is long enough to obtain an audio tone. 

 Manual track on such targets can be maintained (until the automatic 

 lock-up processes are complete) by the differentiation of the frequency of a 

 particular target from other audio tone. Again, in such a system the 

 operator reverts to his indicator scope to understand his hearing with 

 relationship to the acquired target. However, in this radar the hearing 

 sensor can serve as a valuable adjunct to the visual sensor rather than as a 

 second-best substitute. 



REFERENCES FOR CHAPTER 12 



T. Seller, M. A. Starr, and G. E. Valley, Cathode Ray Tube Displays, Mas- 

 sachusetts Institute of Technology Radiation Laboratory Series, Vol. 22, 

 McGraw-Hill Book Co, Inc., New York, 1948. 



F. E. Terman, Radio Engineer s Handbook, "Electron Optics and CRT's," 

 Sec. 4, Par. 15 and 16, McGraw-Hill Book Co., Inc., New York, 1943. 



