The present invention relates to receivers for demodulating broadcast signals having both amplitude and angle modulation components. Specifically, an apparatus is provided to minimize the distortion resulting from angle demodulating broadcast signals having marginal amplitude levels.
Recent interest has developed in generating stereophonic broadcast signals in the low frequency radio spectrum now reserved for amplitude modulation. Several proposals have been submitted to the FCC which suggest a new stereophonic broadcast service in that portion of the broadcast radio spectrum which utilizes amplitude modulation. The Magnavox Consumer Electronics Company has proposed a system for generating a broadcast signal which is amplitude modulated with the summation of two stereophonic related signals and linearly phase modulated with a difference signal produced by subtracting the stereophonic related signals. Specifics for this and other proposals for stereophonic broadcasting are to be found in FCC Docket 21313, "In The Matter of AM Stereophonic Broadcasting".
Difficulty in receiving and demodulating broadcast signals having both amplitude and angle modulation components is encountered during periods of high negative amplitude modulation of the broadcast signal. The angle demodulation process suffers from a loss of signal as the instantaneous amplitude level of the broadcast signal approaches zero. Under these conditions, conventional limiters and angle demodulators produced noise when demodulating a signal having a marginal amplitude. Therefore, the reception of such stereophonic broadcasts suffers during those periods of time that the negative amplitude modulation approaches 100% corresponding to a zero carrier level.