Alpha envelope detection and distortion. A polyphase rectifier circuit

Abstract

The extraction from a complex EEG signal of a wave form which follows the variations of alpha amplitude may be accomplished by filtering and detection. To avoid distortion of the original envelope the filter should have a low Q value and the detector smoothing circuit should not attenuate the components of the envelope wave form. A detector circuit that does not necessitate the use of a smoothing circuit is described. For the operation of this circuit the original EEG signal is phase shifted in identical successive steps. Each phase shifted signal is then rectified and combined with the others to give an output following closely the peaks of the signal (polyphase rectification). In this way the only distortion of the envelope is that due to the band pass filter.