Knowles Electronics, Inc.

Industrial Research Products, Inc.

Industrial Products, Inc.

Richard W Peters

John G Christopher

John W Staples

John Stuhr French

Martin Dippel

An adjustable equalizer network has a series chain of N all pass amplifier stages driving N+1 scaling circuits connected to a final summing circuit that includes circuitry to adjust the frequency response at M+1 frequency control points to obtain a predetermined ripple free minimum phase shift frequency response over a given frequency range. The scaling circuits have scaling coefficients, of magnitude and sign determined by Fourier analysis and an appropriate truncation function, such that the filter has a pass band representative of M+1 contiguous bands conjointly encompassing the frequency range. Calculation of the scaling coefficients and control of the scaling circuits is accomplished through digital computation and manipulation. Some embodiments may use passive circuits or linear analogue circuits as scaling circuits, controlled manually or by a computing device.

Computer controlled transversal equalizer

A tiny pre-amplifier for a small, low voltage, high impedance signal source, usually an electret microphone, exhibiting near unity gain, has an input cascode stage connected to the microphone and a matched balancing cascode stage; both cascode stages are energized from the same power supply circuit. The two cascode stages supply intermediate signals to the inputs of a differential amplifier to generate an output signal having a greatly reduced noise content, independent of power supply variations. The pre-amplifier is wholly integratable, has low power drain, and fits into the microphone housing.

Pre-amplifier

An adjustable equalizer network has a series chain of N all pass amplifier stages driving N+1 scaling circuits connected to a final summing circuit that adjusts the frequency response at M+1 frequency control points to obtain a predetermined ripple free minimum phase shift frequency response over a given frequency range. The scaling circuits have scaling coefficients, of magnitude and sign determined by Fourier analysis and an appropriate truncation function, such that the filter has a pass band representative of M+1 contiguous bands conjointly encompassing the frequency range. Calculation of the scaling coefficients and control of the scaling circuits is accomplished through digital computation and manipulation. Some embodiments may use passive circuits or linear analogue circuits as scaling circuits, controlled manually or by a computing device.

An automatic microphone mixer for a multi-microphone audio system in which each microphone audio channel is provided with a control channel including a comparator that compares the microphone output, as an A.C. signal, with a D.C. threshold signal, and that switches the microphone channel to 'on' condition when the microphone signal exceeds the threshold; the threshold signal starts at a maximum level for a time T1 and then decreases as a function of time. A D.C. offset is added to the threshold signal in order to prevent low-level extraneous noise sources from activating a channel. All of the control channels are coupled to a threshold signal restoration circuit that drives the threshold signal back to its maximum level each time an audio channel is switched 'on.' An audio channel that has been switched 'on' remains 'on' for a time T2 substantially longer than the time T1. The number of channels currently in 'on' condition is continuously monitored and the output gain is reduced whenever two or more channels are 'on'. The monitoring circuit is also connected to the control channels by a logic circuit and a selector switch. The logic circuit applies control signals to the control channels resulting in all channels either being 'off' in the rest state, one channel being 'on' in the rest state, or two channels being 'on' in the rest state, depending on the setting of the selector switch.

Automatic microphone mixer

An adjustable equalizer network comprises a series chain of N all pass amplifier stages driving M minimum phase shift transversal filters connected to a final summing circuit that includes means to adjust each filter output to obtain a ripple free predetermined frequency response over a given frequency range. Each filter includes N+1 scaling circuits connected to an intermediate summing circuit; the scaling circuits have scaling coefficients, of magnitude and sign determined by Fourier analysis and an appropriate truncation function, such that the filter has a pass band representative of one of M contiguous bands conjointly encompassing the frequency range. The adjustable equalizer, once adjusted for a desired frequency response, may be supplied with a DC reference input and used to determine the contribution (magnitude and sign) to the frequency response from each tap on the all pass chain, thus developing a set of N+1 scaling coefficients for a simplified equalizer network, comprising the all pass chain with just one set of N+1 scaling circuits, that duplicates the response characteristic of the more complex adjustable equalizer. Some embodiments incorporate digital programming and control of the frequency response characteristic.

Equalizer networks and methods of developing scaling coefficients

An audio loudness control system for processing a broad dynamic range of audio input signals (e.g. 70 to 100 dB) comprises a gain-controlled amplifier, a feed forward control signal generator having an audio signal input that develops a feed forward gain control signal having an amplitude representative of long-term input excursions of the audio signal above a predetermined loudness threshold, a feedback control signal generator having an input from the audio output of the amplifier that develops a feedback gain control signal having an amplitude representative of transient output signal excursions above the loudness threshold, and a selector gate that selects the gain control signal of instantaneously higher effective magnitude and applies the selected gain control signal to the gain control input of the amplifier.

Audio loudness control system

A priority mixer control for a multi-microphone audio system comprises a threshold signal generator that develops a scanning D.C. threshold signal starting at a maximum level for a time T1 and then decreasing in amplitude as a function of time. Each microphone channel is provided with a control channel including a comparator that compares the microphone signal, as an A.C. signal, with the threshold signal, and that switches the microphone channel to 'on' condition when the microphone signal exceeds the threshold; all of the control channels are coupled to a threshold signal restoration means that drives the threshold signal back to its maximum level each time an audio channel is switched 'on'. An audio channel that has been switched 'on' remains 'on' for a time T2 substantially longer than the time T1. The number of channels currently in 'on' condition is continuously monitored and the output gain is reduced whenever two or more channels are in 'on' condition.

Priority mixer control

An audio system comprising a record player, tape deck, microphones, or other signal source which generates stereo signals supplied to two main speakers for reproduction; the stereo signals are added together to afford an intermediate signal which is subjected to at least two successive equal delays to generate a plurality of delayed signals which are summed with the intermediate signal in predetermined amplitude and phase relation to generate a spatial effect signal that is essentially free of noticeable reverberations or echoes. This spatial effect signal is reproduced by a separate speaker in the same listening space as the main speakers to add ambience and diffuseness to the total sound field. In the preferred construction two delayed signals are employed, delay is effected with the signals in digital form, and the summing ratio for the intermediate, first delayed, and second delayed signals is approximately 1:2:1 with the intermediate signal and the second delayed signal in phase opposition. A corresponding monophonic system is also disclosed.

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Algonquin, IL, United States of America

Richard W Peters