Motorola Corporation

Matsushita Electric Company of America

Francis H Hilbert

Norman W Parker

Theodore S Rzeszewski

The signal transmitted is a compatible amplitude modulated carrier with amplitude directly proportional to the monaural information or 1+L+R up to a frequency near the upper end of the normal program spectrum. Above the transition region, the amplitude becomes a pure quadrature function. The instantaneous phase varies as a function of the quadrature phase relationship. Transmitter and receiver embodiments are disclosed.

Stereo broadcast system

The circuit determines whether an incoming frequency is within the normal 'lock-in' range of a phase locked loop VCO or not, and whether it is higher or lower than the VCO frequency. If not within normal range, the beats are rectified and gated to provide a DC control voltage of the proper value and polarity for locking the oscillator on the incoming frequency. Rectification may be half or full wave as needed, and the circuit may include filter bandwidth control as well. This circuit is suitable for signals of any waveform, quadrature signals, and AM stereophonic signals.

Phase locked loop with improved lock-in

A decoder for use with a compatible AM stereophonic signal utilizes a amplifier and a feedback loop from the amplifier output to control the output of a multiplier circuit which is one input to the amplifier. The amplifier output is thus forced to become the required correction factor used in restoring the stereo program signals.

AM Stereophonic decoder

The signal as transmitted is a compatible AM stereophonic signal with amplitude directly proportional to the monophonic signal 1+L+R. The instantaneous phase angle is the quadrature phase angle modified in that high level, high frequencies in the L-R channel are reduced before carrier modulation. The gain in the high frequency L - R channel may be reduced in proportion to the signal level only, or to both level and frequency, or to the percent of distortion introduced into the high frequencies of the L - R signal by RF filtering of the modulated carrier.

Compatible AM stereophonic system

A compatible AM stereo broadcasting system is disclosed in which the signal is a carrier having an amplitude directly variable with monoaural or sum (L+R) information, and having an instantaneous phase .phi. varying as a function of the resultant amplitude of the sum information (L+R) and difference information (L-R) which are established in a preselected phase relationship (quadrature). In a stereo receiver, L and R or the sum and difference signals may be restored by dividing the signal by the cosine of the angle .phi., and in a monaural receiver, the sum signal alone is detected.

Compatible AM stereo broadcast system

An improved AM stereo receiver for receiving broadcast signal having the form (1+L+R)cos(.omega..sub.c t+.phi.) where .phi. is arc tan(L-R)/(1+L+R). The sum signal (L+R) is demodulated in an envelope detector and coupled to a matrix. The uncorrected difference signal is demodulated in a synchronous detector, a corrector circuit derives the correction factor, and a divider circuit provides the corrected difference signal. During tuning, and until a PLL is locked on the carrier frequency, the receiver is locked into the monophonic mode and tuned on the monophonic signal. When the PLL locks, the difference signal is coupled through to the matrix for stereophonic mode operation.

Receiver for compatible AM stereo signals

An AM receiver for utilizing a broadcast signal of the form (1+L+R) cos (.omega..sub.c t+.phi.) separates the L and R signals by utilizing phase detection and matrixing. Distortion is either eliminated with non-linear amplification or reduced with partial matrixing.

AM stereophonic receiver

An AM receiver receiving a compatible stereo signal and utilizing a cosine correction signal for restoring the original stereo information limits the amount of correction at very large modulation angles to prevent excessive noise from being introduced by the increased gain.

AM stereo receiver with correction limiting

An AM receiver for receiving a compatible stereo signal and requiring a dynamic correction factor prevents the introduction of a false phase reference signal by limiting and filtering the signal going into the oscillator of the PLL which provides the correction reference signal.

AM stereo receiver with improved correction signals

A signal level controlled switching circuit removes the stereo correction factor in a compatible AM stereo receiver when the received signal level is low enough to allow signal degradation due to noise affecting the correction factor. The switching circuit can operate on either an instantaneous or an average signal level.

AM stereo receiver having signal-controlled corrector

In an AM broadcast transmitter, an infrasonic tone is added to the difference channel of a compatible AM stereo system to provide a stereo presence signal. In a stereophonic receiver, the corrected output of a synchronous detector in the difference signal channel is coupled to an infrasonic detector for control of a mono/stereo mode switch and a stereo presence indicator. When the received signal is weak, the infrasonic tone is effectively stronger, because the phase angle is increased. Since the s/n ratio is proportional to the phase angle, the stereo presence signal is self-adjusting. The mode switch and indicator are enabled for all values of L and R. The infrasonic tone could also be removed from the transmitted signal whenever the difference signal goes above a predetermined level during stereo broadcasting, in which case a comparator at the receiver would enable the mode switch and indicator upon detection of either a difference signal or the infrasonic tone.

Stereo presence signal for an AM stereo system

A low level video auxiliary signal is transmitted in addition to the conventional composite black and white or color television signal. The auxiliary signal is in the form of redundant, opposite-polarity signal portions during pairs of time successive signal portions, such as horizontal lines or fields of the composite television signal. The auxiliary signal also may be transmitted in the form of a predetermined frequency with the auxiliary information on successive lines of the composite television signal being out of phase by one-half the period of the auxiliary signal frequency. When the composite signal is detected and displayed by the conventional circuitry in the receiver, the auxiliary information is visually cancelled in the image displayed on the cathode ray tube and the normal video information is displayed. Additional circuitry is employed in the receiver to effect cancellation of the conventional video information from the display and to reinforce successive portions of the auxiliary signal; so that the auxiliary signal is displayed on the cathode ray tube of the receiver when desired.

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Addison, IL, United States of America

Francis H Hilbert