Stop on station circuit including center channel detector

Abstract

A Stop On Station (SOS) circuit for a AM and FM radio receivers evaluates field strength of a received station signal and in addition measures and evaluates the frequency of the IF signal so as to assure that the received signal is essentially centered at the IF frequency to identify candidate stations signals which are suitable for listening. The IF frequency for a received candidate station signal is evaluated for an evaluation period of time (e.g., for 40 milliseconds). During each 40 millisecond period, fifteen samples of the frequency of the IF signal are counted and evaluated. If the frequency of the measured signal falls within acceptable limits, a four stage binary persistence counter is incriminated. If a count of 12 or more out of the 15 possible counts is accumulated therein during a 40 ms evaluation period, a station signal is considered to be suitable from a standpoint of received frequency and a 'frequency pass' signal is generated. The SOS circuit, in turn, generates a 'Stop' signal if the AGC signal is above a predetermined threshold at the time the pass signal occurs. In this case of FM signals, the magnitude of ultrasonic noise (noise in the range of 100 kHz to 200 kHz) in the output of the FM detector is measured and used to identify acceptable station signals. Noise above a threshold value eliminates a station signal from consideration.