PHASE-LOCK-LOOP FM-STEREO DECODER INCLUDING STEREOPHONIC/MONOPHONIC BLEND SYSTEM FOR REDUCING AUDIO DISTORTION

摘要

An FM-stereo decoder includes a phase-lock-loop for providing an error voltage and an audio decoder having L and R output channels upon which L and R audio signals are developed, respectively. In response to an initial predetermined increase in the error voltage due to the initiation of distorted reception, the separation between the L and R output channels is substantially instantaneously decreased to less than full channel separation thereby to rapidly shift toward full monophonic operation for reducing the discernible audio distortion resulting from the distorted reception. Further, in response to a subsequent predetermined decrease in the error voltage due to the termination of distorted reception, the separation between the L and R output channels is relatively gradually increased to full channel separation thereby to slowly shift to full stereophonic operation for reducing the discernible audio disruption resulting from the shift to full stereophonic operation. Preferably, the separation between the L and R output channels of the audio decoder is varied during distorted reception from full channel separation to an intermediate channel separation which is greater than zero channel separation in proportion to the frequency of the L and R audio signals thereby to provide blended stereophonic/monophonic operation for reducing the discernible audio disruption resulting from the rather sudden shift toward full monophonic operation and for maximizing the amount of stereophonic operation over the lower frequencies of the L and R audio signals while minimizing the discernible audio distortion resulting from the higher frequencies of the L and R audio signals.