Patent US 4,136,684

A linear electromyographic biofeedback system operates over a dynamic range of at least one thousand to one. That is, the system remains linear from input levels of approximately one microvolt to input levels in excess of one millivolt. The linear electromyographic biofeedback system includes a sensitive transducer which is followed by a protection circuit and connected to a differential amplifier for eliminating common mode noise. The output of the differential amplifier is filtered and amplified to further eliminate unwanted signals. This filtered signal is rectified and averaged in a third order averaging filter to obtain a close approximation of a time averaging without the necessity of discrete timing periods. The output signal from the averaging filter is then used to control a current controlled oscillator which provides a series of audible pulses at a rate which varies linearly with the value of the input voltage detected by the transducer, the repetition rate range being from approximately one hertz to greater than five thousand hertz.