ELECTRONIC LINE CALLING
An electronic line judge is a device used in tennis to automatically detect where a ball has landed on the court. Attempts to revolutionize tennis officiating and the judging of calls in the sport began in the early 1970s and has resulted in the design, development and prototyping of several computerized, electronic line-judge devices. The methods have been based upon the use of pressure sensors, sensors to detect magnetized or electrically conductive tennis balls, infrared laser beams, and most recently video cameras.
This inaugural electronic line-judge device worked on a principle of large, thin mylar conductive plastic pressure sensors beneath the surface of a carpet court. It was able to distinguish between a foot and the tennis ball; the computer detected the sharp, millisecond pulse of the ball bounce striking the ground, as compared to the slowness and electronic print of player’s foot movements. Even when a foot was present on the sensor a dynamic load circuit was able to detect and distinguish between the signals. Beneath the court surface on each boundary line of the court were both and "in" and "out" sensors so that if a ball hit the outside of the line and also hit the "out" sensor the computer would correctly overrule the "out" call allowing play to continue.
This first electronic device made decisions not only as to whether the ball landed within the boundaries of the playing zones, but also was wired to make foot fault and service net-cord legal serve decisions. The foot fault judge used directional microphones to detect the striking of the ball by the player’s racquet, when serving the ball, that functioned in conjunction with a timing circuit to detect if the players foot had activated the baseline line ‘IN’ sensor immediately prior to, or during, the striking of the ball. The net cord ‘Let’ sensor was a simple piezoelectric device, initially a guitar pickup, to detect if the tennis ball touched the net during the service delivery. The service line sensors, net-cord sensor, legal serve and foot fault devices were turned "on" in unison during the process of a player serving the ball at the beginning of each point and then turned "off" as the opponent returned the ball.