Dc offset correction circuit for an electronic musical instrument

Abstract

A DC offset correction circuit for use in an electronic organ for eliminating audible noise or thump produced by an instantaneous DC level shift in the standard organ keyer circuit at both key depression and release. A standard keyer circuit is responsive to the depression of a key on the manual by the organist and provides a square or stairstep waveform output at a frequency representative of the note key depressed. Each keyer output waveform includes a positive or negative polarity instantaneous DC level shift at both key depression and release which when coupled through a capacitive output circuit, such as a filter, provides an undesirable audible thump. The DC offset correction circuit comprises a monitoring circuit which is responsive to both key depression and release and a detector circuit responsive to the monitoring circuit to provide a DC level signal output of opposite polarity to the instantaneous DC level shift in the keyer circuit. The correction circuit further comprises a resistive combining circuit which receives the output signals from all the keyer circuits and the output signals from the monitoring circuit to provide a corrected output signal waveform without an instantaneous DC level shift. Standard organ output circuits receive the corrected output signal and provide corresponding audio output without the undesirable audible thump.