The invention relates to AM stereo broadcast receivers. In the proposed systems L+R signal information is transmitted using a conventional amplitude modulated (AM) carrier wave. The L-R signal information is transmitted using phase modulation (PM) carrier information. Since a conventional radio receiver cannot react to the PM it will provide a compatible receiver response to the stereo broadcast. However, if the receiver is equipped with a PM decoder and matrix circuit, the L and R information can be reproduced to provide stereo audio.
U.S. Pat. No. 4,362,999 issued Dec. 7, 1982, to Don R. Sauer and is assigned to the assignee of the present invention. This patent relates to an AM STEREO PHASE MODULATION DECODER and its teaching is incorporated herein by reference. This patent discloses a combination FM detector and integrator to provide a PM decode. It is shown that such a combination reduces signal radiation, is linear, and has a reduced adverse response to noise and overmodulation. A simulated large value inductor is also disclosed for controlling the audio response and stabilizing the decoder performance.
In the typical AM stereo receiver the PM decoder is driven from a limiter that is in turn driven from the receiver IF amplifier. When the carrier is strongly modulated by the L+R signal, it is common for overmodulation to occur on occasional signal peaks. Normally this condition is minimized but does occur to some degree. During such overmodulation conditions the carrier goes to zero, or at least well below the limiting level, so that the PM decoder receives no signal for a brief interval. This can result in what appears to be noise at the decoder output and can be objectionable.