Abstract: A microphone output limiter which detects the amplitude of a microphone and produces a control signal which lowers the microphone's impedance when the amplitude of the audio signal exceeds a preselected threshold level. The output signal is detected at the output of the microphone or it may be detected at the output of a later pre-amplifier stage. Once a control signal is generated it is coupled to a variable impedance device which produces a shunt load impedance across the output of the microphone thereby lowering its impedance. The microphone, as well as the variable impedance device, may be coupled through impedance converters in order to facilitate component matching.

Abstract: The present invention phase coherent crossover incorporates a method and apparatus for providing phase coherent crossover (PCC) signals. The PCC method and apparatus involve the utilizing of an all-pass filter for receiving an input signal and producing an all-pass signal, and a partial-pass filter also receiving the input signal and producing a partial-pass signal at a partial-pass output. The PCC method and apparatus further involve the combining of the partial-pass signal produced by the partial-pass filter with the all-pass signal produced by the all-pass filter to produce an opposite partial-pass signal at an opposite partial-pass output.

Abstract: A drift canceling method and apparatus is used in conjunction with an analog to digital converter. The drift canceling method and apparatus analyzes the duty cycle of the target analog to digital converter's most significant output data bit to determine if the average numeric value of the analog to digital converter's output data stream is data centered upon the analog to digital converter's numeric data range or if it is ranging above or below the data range center. If the average data value is above or below center, a servo error signal is generated which is added to the analog to digital converter's input signal to cause the analog to digital converter's average output data magnitude to slew towards the data range center. In this way, the average analog to digital converter output data offset is continuously canceled without interfering with the normal and continuous operation of the target analog to digital converter.

Abstract: A sticky leveler utilizing a window detection technique to preserve the relatively small level changes of an audio signal. The sticky leveler includes a voltage controlled amplifier (VCA) and a sticky detector, where an input signal is received by the VCA for creating a variable gain path to the VCA output. The control output of the sticky detector is coupled to the VCA control input for controlling the VCA gain. The detection input of the sticky detector may be coupled to the input signal for generating a feedforward automatic gain control leveler. Alternatively the sticky detector may be coupled to the output signal of the VCA for generating a feedback automatic gain control leveler.

Abstract: A distributed pre-emphasis equalizer comprises a partial pre-emphasizer and a (50) microsecond pre-emphasizer. The partial pre-emphasizer receives an input signal and produces a partially pre-emphasized output signal. The (50) microsecond pre-emphasizer receives the partially pre-emphasized output signal from the partial pre-emphasizer and adds a (50) microsecond equalization to produce a pre-emphasized output signal equal to a standard (75) microsecond pre-emphasis curve. The distributed pre-emphasis equalizer allows an FM audio processor to produce only the "dulling" effect of a (50) microsecond pre-emphasis even when generating a (75) microsecond pre-emphasis curve. The distributed pre-emphasis equalizer also creates a pre-emphasizing method which is inherently compatible with both the (75) microsecond and (50) microsecond standards. Furthermore, the distributed pre-emphasis equalizer creates a pre-emphasizing method which is easily switchable between the (75) and (50) microsecond standards.

Abstract: A split-band clipper which generates less distortion, minimizes or reduces the occurrence of all IMD, or at least will minimize and reduce the occurrence of widely space IMD, and reduce the quantity of harmonic distortion caused by clipping of bass frequencies.

Abstract: A device for enhancing the bass frequency components in an audio signal while avoiding overload problems associated with fixed levels of bass frequency enhancement. The device divides the input signal into two parts, the first of which passes through to the output of the device. The second part of the input signal is input into a low pass filter which passes only the bass components of the input signal and imparts a frequency dependent phase shift to the bass components. The bass components then pass through a variable gain amplifier which may or may not invert the polarity of the bass components and then are recombined with the first part of the audio signal. The gain of the variable gain amplifier is varied in accord with the level of the bass components.

Abstract: A discrete parallel path modulation multiplexer for use in generating a composite baseband signal from separate left and right input signals. The composite baseband signal includes the sum of the left and right input signals and the product of the difference between the left and right input signals and a subcarrier. The composite baseband signal may also include a pilot subcarrier at one-half the frequency of the subcarrier. In one signal path, the multiplexer multiplies the right input signal by the function (1+sin (wt)) and in a second signal path the multiplexer multiplies the left input signal by the function (1-sin (wt)). The products are then combined to obtain the composite baseband signal. A pilot subcarrier may also be generated and included as part of the composite baseband signal.

Abstract: An impulse detection circuit for use in triggering circuits such as those used for detecting drum beats within background noise and then triggering music synthesizers in response to the drum beat. Differentiators and peak-rectifiers and filters are used to detect impulse like inputs over a wide dynamic range in a noisy background. The input signal is rectified and differentiated and then inputted into a peak-rectifier and filter having a fast charging and a slow discharging time constant. The output of this peak-rectifier and filter is differentiated and inputted into a pair-rectifier and filters operating in parallel, which filters have rapid charging and slow discharging time constants. The time constants of one of the two filters are approximately twice those of the other. The outputs of the two peak-rectifier and filters are compared in a comparator.

Abstract: A multiband audio frequency signal limiting apparatus suitable for use in a wide range of applications is disclosed. An input signal is divided into discrete frequency bands (any practical number is suitable) which are each passed through a separate limited and subsequently summed to obtain an output signal. The summed output signal is fed into an automatic limit threshold circuit which generates a third signal, the Automatic Limit Threshold (ALT) signal, which modifies a common limiting threshold reference for the bands. This causes the bands to limit at a higher or lower threshold level and the band output levels to increase or decrease in direct proportion to the threshold level of limiting. The ALT signal cause the bands to limit at a lower threshold when the energy content of the various bands would combine toward higher values, which could thus generate a larger than desired output signal level.

Abstract: An audio crossover filter which automatically tunes the crossover frequency according to the content of the program audio which is being filtered.

Abstract: A circuit for automatically controlling the operation of a variable gain control circuit in an audio signal processing system by verifying whether gain change is desirable and then gating the operation of the gain control circuit is disclosed. A pair of operational amplifiers along with a set of four diodes produce two equal absolute value signals representing a full wave rectified version of the audio input signal. A first filter circuit processes one of the absolute value signals to produce a third signal corresponding to average signal amplitude. A second filter circuit processes the other absolute value signal to produce a fourth signal corresponding to an average of peak values. A comparator compares the third and fourth signals and produces an output signal which is used as the control signal for controlling the variable gain control circuit.