Combined encoder and decoder circuit

Abstract

A combined encoder and decoder circuit which uses a single frequency selective resonant reed in a closed loop oscillator configuraion permitting simultaneous encoder and decoder operation. A closed loop oscillator has a resonant reed connected in series between an input and output terminal and produces a signal at the resonant frequency of the reed. Positive signal feedback from the output to input terminal is provided by an amplifier, operated between saturation and cut-off, and a controllable attenuator. A level detector and the attenuator effectively form a limited range negative feedback loop which effectively maintains the signal magnitude at the output terminal at a substantially constant level. A decode detector monitors the signal level at the output terminal and produces a decode signal when an input signal, having the same frequency as the resonant frequency of the reed, has a magnitude which is so large that the negative feedback circuitry cannot maintain a constant signal level at the oscillator output terminal. The output of the positive feedback amplifier always provides a constant amplitude output signal which can be used for encoding purposes. Because of the use of both positive and negative feedback, the circuit as a fast decoding response to large input signals while preventing responses to small amplitude input signals.