Abstract: A scanning radio receiver that is electronically tuned for station selectivity over a given band of frequencies. Varactor diodes contained in the RF and local oscillator stages of the scanning receiver circuitry are voltage controlled to select a finite number of different station frequencies within said given frequency band. A scan control circuit composed of several sections each including a potentiometer applies discrete d.c. voltages corresponding to the selected station frequencies to said varactor diodes. Digital means are employed to sequentially actuate the scan control circuit sections under the control of a received signal.

Abstract: A detector for AM or FM signals is described. The detector employs a differentially connected emitter follower pair to which an in-phase and a phase-shifted signal are respectively applied. The emitter follower pair produces an output waveform with controllably spaced zero crossings which is the "greater of" the applied waveforms. For FM operation, the phase shift is made linearly frequency dependent, being in quadrature at resonance. In FM operation, the zero crossings are sensed to produce a succession of variable width unidirectional pulses whose widths are proportional to the frequency deviation. Amplitude variations in the variable width pulses may then be removed and integration in a filter with an audio frequency time constant recovers the FM modulation information. For AM operation, a phase shift of 180.degree. is used, producing full wave rectification. The AM information is then recovered from the rectified signal using the same integrating filter used for FM.

Abstract: Controllable bandwidth IF filter circuit for radio receivers generally, but having particular application to quadraphonic radio receivers intended for use in a quadraphonic system that is compatible with existing monophonic and biphonic equipment, said filter circuit being automatically adjusted in a digital manner to exhibit one of several different bandwidths in response to a pair of transmitted control signals present in the quadraphonic transmission so as to provide optimum receiver operation for the particular mode of broadcast being received, especially in terms of an improved protection ratio and signal to noise ratio for the received signal. The noted bandwidth adjustments are accomplished by modifying the values for certain of the component in the filter's tuned circuit and coupling sections through their selective connection into the circuit by means of semiconductor switching devices operated in response to said pair of control signals.

Abstract: A broadcast system capable of transmitting and receiving a broadcast signal containing four discrete stereophonically related audio frequency inputs in which there is produced within the transmitter four matrix outputs, each of which is a function of one or more of the inputs. A main carrier wave is then frequency modulated with the first matrix output, with the sidebands of a suppressed first and second subcarrier which has been amplitude modulated with the second and third matrix outputs in quadrature relationship with each other, and with the lower sideband and a relatively small portion of the upper sideband of a depressed third subcarrier that has been amplitude modulated with the fourth matrix output. The modulation of the third subcarrier is limited to a maximum voltage level substantially below the highest level otherwise possible. The first, second, and third subcarriers are regenerated in the receiver and the four matrix outputs are detected.