U.S. Patent No. 4,118,601, 10/03/1978 [ DjVu 77 KB ]
System and a method for equalizing an audio sound transducer system
Yeap, Arthur K. C. Assignee: Audio Developments International
Abstract: A system and a method of electronically equalizing the composite transfer function of a sound system and a room which receives the sound generated by the sound system. A test signal, such as white or pink noise, is applied to the sound system and a microphone for receiving the reference sound is placed in the room and has its output applied to an equalizer which comprises a plurality of contiguous narrow band filters covering the entire audio band. Each output signal from the filters is applied through an adjustable amplitude control means to a detector and each detected output signal is compared with a reference signal, such as the detected output signal from a selected mid-range filter and has its amplitude adjusted to provide a desired relationship with respect to the reference signal. After adjustment of the equalizer, the test signal and the microphone are disconnected from the system and the sound signal source is applied through the equalizer to the loudspeaker system.

U.S. Patent No. 4,138,594 , 02/06/1979 (app. 06/02/1977) [DjVu209KB]
Small dimension low frequency folded exponential horn loudspeaker with unitary sound path and loudspeaker system including same (class: 381/335; 181/147; 181/152; 381/99; 381/340; 381/341)
Klipsch; Paul W. (assignee: Klipsch and Associates, Inc.)
Abstract: A small dimension low frequency loudspeaker has a folded exponential horn which provides a unitary curved sound path from an electroacoustic transducer at the throat of the horn to a volume into which sound is radiated at the mouth of the horn. The length of the horn is such that, at an exponential rate of expansion between the throat and the mouth, the mouth, when it is bounded by at least one planar surface, such as a floor, a ceiling, and/or walls of a room, has adequate area to enable reproduction of low audible frequencies. An illustrative embodiment of the low frequency loudspeaker has an effective low end cut-off frequency of 55 Hz. A loudspeaker system, including a low frequency loudspeaker as well as midrange and high frequency loudspeakers and an LC crossover network, is also disclosed. The LC crossover network includes an autotransformer which not only serves as a component to determine a crossover frequency but which also boosts the electrical signal that is input to the electroacoustic transducer of a less efficient loudspeaker. The autotransformer increases the output of the less efficient loudspeaker and accommodates its use with more efficient loudspeakers so that the overall loudspeaker system operates at optimum efficiency. An illustrative embodiment of the loudspeaker system affords 108 dB SPL output at 1 meter with 1 watt input which corresponds to about 20% overall efficiency. The smoothness of amplitude response over the range of audible frequencies that is necessary for high fidelity sound reproduction is improved by inclusion of peaking circuits in the LC crossover network of the loudspeaker system to enhance amplitude response in the regions of crossover frequencies. Side wings are additionally provided to eliminate cavities at the sides of the loudspeaker system which would otherwise cause deterioration of smoothness of amplitude response.

U.S. Patent No. 4,155,047, 05/15/1979 [ DjVu 153 KB ]
Voltage controlled attenuator
Rubens, Harvey A. and Baskind, David L.
Abstract: A voltage controlled attenuator having extremely wide dynamic range and having low distortion is achieved by limiting the current density in each unit area of the transistors to minimize the current noise and by utilizing high current sources to minimize the noise voltage. A pair of compound gain cells having low current densities are each directly fed by a linearized current source. The gain cells are controlled by a single gain control voltage and feed a pair of linearized buffer amplifiers for minimizing loading on the gain cells. The output of the buffer amplifiers feed a differential output amplifier for generating an output signal that eliminates the effect of the control voltage in the output signal and any DC shift caused by changes in control voltage and nulls remaining distortion products.

U.S. Patent No. 4,198,540, 04/15/1980 [DjVu93KB]
Compensated crossover network
Cizek, Fred R. (assignee Cizek Audio Systems, Inc.)
Abstract: The signal from the crossover network of an audio output circuit is applied to certain of the audio system's low, middle and high frequency speakers through particular compensation circuits that are associated with the crossover network and the speaker driver coils. These compensation networks include: a resistor-inductor-capacitor sequence of selected values connected across the terminals of a high frequency or other driver whose resonant impedance peak must be compensated to a resistive impedance for optimum crossover; a resistor-capacitor sequence of selected values connected across the terminals of a low frequency or other driver whose inductance must be compensated to a resistive impedance for optimum crossover; and a variable inductor connected in series with one or more of the terminals of any driver where a "roll off" at higher frequency may be desired. The result is more faithful audio reproduction.

U.S. Patent No. 4,225,794, 09/30/1980 [ DjVu 102 KB ]
Voltage controlled amplifier
Buff, Paul C.
Abstract: A voltage controlled amplifier or multiplier circuit including a dual polarity rectifier for dividing the AC input signal into first and second unipolar signals, the first unipolar signal being the positive excursion of the AC input signal and the second unipolar signal being the inverted negative portion of the AC signal. Each unipolar signal is processed through a separate, conventional, unipolar log-antilog amplifier circuit, to each of which is applied a control voltage. The resultant unipolar signals are then subtracted in a differential amplifier to reconstitute the waveform of the original AC input signal, thereby producing the gain-controlled output signal.

U.S. Patent No. 4,234,804, 11/18/1980 [ DjVu 198 KB ]
Signal correction for electrical gain control systems
Bergstrom, Gary (assignee dbx, Inc.)
Abstract: The present invention relates to an electrical gain control system which provides a first signal logarithmically related to an input signal, sums a gain control signal with the first signal; and then provides an output signal which is an antilogarithmic function of the sum of the gain control signal and first signal. In accordance with the present invention in order to at least partially compensate for errors in the output signal arising from inherent characteristics of the components of the system, the system is improved by comparing the input signal and the output signal; generating an error correction signal in response to the comparison as a function of the errors in the output signal; and summing the error correction signal with the first signal and the gain control signal so as to reduce the error in the output signal.