Abstract: An automatic gain control circuit is disclosed which is direct coupled to phase opposed outputs of an IF amplifier and provides a DC control voltage for controlling the gain of the amplifier. The AGC circuit includes a peak-to-peak threshold detector and an integrating capacitor. The quiescent voltage across the capacitor is accurately established by a voltage divider network and a feedback network. Separate current sources are provided to establish dual time constants in the circuit so that distortion is minimized while accommodating a stop on station feature.

Abstract: A bridge audio amplifier is disclosed which includes an input stage for receiving an audio input signal and having a bias amplifier and a signal amplifier which provide a combined output current containing a substantially constant dc component and an ac component which is dependent on the input signal and the relative transconductance of the bias and signal amplifiers. The transconductance of the bias and signal amplifiers is reciprocably varied by a dc control voltage to vary the ac component of the current. The output current is converted to a voltage signal and applied to an output stage for driving a speaker. Two such audio amplifiers driving front and rear speakers may be employed to achieve a low level fade function.

Abstract: A local oscillator suitable for integrated circuit construction in a radio receiver maintains low level voltage oscillations across a tank circuit which may include a varactor diode. An emitter-coupled pair of transistors with a small switch-over voltage has the collector of the second transistor connected to the base of the first. Third and fourth matching transistors have emitters connected to the bases of the second and first transistors, respectively, and grounded collectors. The base of the third transistor is connected through a diode to ground; while the base of the fourth transistor is connected through a matching diode and the tank circuit to ground. A source of electric power having a voltage greater than two diode drops above ground is effective to supply current through resistors to the bases of the first and second transistors.

Abstract: A radio receiver includes apparatus for attenuating the audio signal derived from weak and noisy radio frequency signals to render the resulting noisy output audio signals less objectionable. Apparatus is provided to attenuate the AGC audio frequency signal in a predetermined constant attenuation, linearly mix the AGC regulated and attenuated audio frequency signals in a proportion controlled by an attenuation control signal and generate the attenuation control signal from the difference between the radio AGC signal and a predetermined constant reference.

Abstract: A radio receiver is effective to receive a radio frequency signal and derive therefrom a standard stereo composite signal comprising a sum of channels signal having components within a first band of frequencies in the audio range and a difference of channels signal having components within a second band of frequencies above the first band of frequencies. The radio receiver is provided with a low pass filter having a high frequency roll-off characteristic within the first frequency band and a substantially flat attenuated amplitude characteristic across the second frequency band. Linear mixer apparatus mixes filtered and unfiltered stereo composite signals in controlled relative proportion responsive to a control signal to produce a modified stereo composite signal for application to stereo decoder apparatus in order to simultaneously roll-off the high frequencies in each derived channel and to attenuate the difference of channels signal in degree controlled by the control signal.

Abstract: Control circuitry for a multi-channel audio system is disclosed which includes left and right signal processing channels and a DC control section. Each processing channel includes balance and volume sections which are connected with the DC control circuitry and controlled from respective balance and volume potentiometers connected with the DC control section. Each channel further includes a tone control section and more specifically treble and bass sections connected with the DC control circuitry and controlled from a tone potentiometer. The response of the bass section is also controlled as a function of the setting of the volume potentiometer to achieve a loudness function. The volume control circuitry is designed to produce a linear relationship between the DC control voltage and signal attenuation as measured in decibels. Current steering circuitry is employed in both amplitude and tone control.

Abstract: A protection circuit for a bridge audio power amplifier having two pairs of output transistors coupled between positive and negative terminals of a direct current supply and providing a pair of output terminals for connection across a load. The bias of the output transistors are controlled in response to excessive current flow, a short of the output terminals to ground, excessive DC supply voltage, a short of the output terminals to the positive terminal of the DC supply, and excessive temperature to prevent permanent damage to the bridge audio power amplifier.

Abstract: A circuit for generating a center tuned pulse for stopping the operation of a signal seeking FM receiver includes means for developing a bias current which is added to the output currents of the quadrature detector of the receiver. A stopping pulse is generated whenever the difference between the output currents is less than the bias current and a 10.7 mHz. signal is present in the IF amplifier.

Abstract: An audio processor is disclosed which includes circuitry for modifying pure left and right audio signals by blending the left audio signal with the right audio signal as a function of frequency and for rolling off the higher frequencies of the blended signal. The processor further includes circuitry for selecting complementary portions of a blended, rolled-off audio signal and a pure audio signal as a function of the RF carrier strength to improve the overall listenability of the audio.