Am stereo decoder for multiple coding systems

Abstract

Decoding or demodulating circuitry can produce stereo right channel and left channel signals from AM stereo signals that have been coded according to any of the well known methods. A stereo sum signal made up of the left and right channel stereo signals added together, and a stereo difference signal made up of the difference between the stereo left and right channel signals are developed and fed to a matrix to interconnect these signals to extract or separate the left and right channel stereo signals. At least one of the known methods of stereo coding requires phase shifting of these two developed signals and suitable phase shift circuitry, along with a switching circuit provides the matrix with the appropriate signals when that method of stereo coding is employed at the broadcast side. The stereo sum signal is developed using envelope detection and the stereo difference signal is developed using synchronous detection, with the synchronous detector employing signals developed in the envelope detection process. When the stereo broadcasting wave belongs to the other coding systems, the stereo sum and difference signals are respectively supplied directly to the matrix circuit through the switches.