Abstract: A notch filter as disclosed which is adaptable to notching out a background siren from voice communications from an emergency vehicle. Two embodiments are disclosed. In the first embodiment, a frequency lock loop tracks the fundamental frequency of an electronic siren. The siren fundamental frequency is then notched out from voice transmissions using a notch filter. In the preferred transmissions using a notch filter. In the preferred embodiment, a digital ramp is generated by a first digital signal processor. This signal is input to an amplifier and a speaker thus producing an electronic siren. The digital ramp is also input to a second digital signal processor which notches out the fundamental frequency and odd harmonics thereof from the digital ramp thus acting as a comb notch filter. Analog voice is digitized by way of an analog to digital converter and input to the second digital signal processor.

Abstract: An inverse control system is disclosed, which comprises FIR filters provided between transmitting elements at n (n=2, 3, . . . ) input points of a linear FIR system and a common signal source, for an inverse control such as to provide desired impulse responses between the signal source and m (n>m) output points of the linear FIR system. A j-th (j=1, 2, . . . , n) one of the FIR filters has a number L.sub.j of taps which satisfies the relationships represented by ##EQU1## for all i=1, 2, . . . , m and j=1, 2 . . . , n where w.sub.ij is the number of discrete signals representing the impulse response g.sub.ij (k) between the j-th output point and i-output point and P.sub.i is the number of discrete signals representing the desired impulse response r.sub.i (k) between the signal source and i-th output point. The j-th FIR filters has a filter coefficient h.sub.j (k) satisfying the relationship ##EQU2## where .circle.* is a discrete convolution.

Abstract: An automatic equalizer and oscillation suppressor system utilizing phase lock loop circuits to detect oscillation candidates at identified frequencies and a low cost microprocessor to process corrective band gain action. Oscillation candidates are initially suppressed by incremental adjustment of band gains to a maximum point after which notch filters are assigned to the oscillating frequency and the band gain returned to its normal equalized value. Automatic equalization is achieved initially with a pink noise source and on an on-going real time basis with program content by use of a frequency addressable narrow band boost circuit which is selectively inserted at frequencies across the bandwidth and band gains adjusted until some oscillation has occurred in all bands or maximum master gain has been reached.

Abstract: A sound system for use in government council chambers, corporate board rooms, and the like employs low level switching of the audio signal to selectively attenuate the audio output of a given speaker or group of speakers as a function of the source of that audio signal.

Abstract: An amplifier for acoustic signals provided with means for suppressing undersired acoustic feedback signals in a rapid manner. The amplifier includes a high-pass filter which separates a part of the signal to be amplified to obtain a control signal for reducing the gain of the amplifier when an acoustic feedback signal is detected. The output of the high-pass filter is connected to a periodicity detector, e.g. a single peak detector, which discriminates between an acoustic feedback signal having a regular periodicity and a signal having an irregular character, such as noise or speech. The control signal is derived from the output of the periodicity detector.

Abstract: Positive feedback of an acoustic signal from a loud speaker to a microphone of a loud speaker system, i.e., howling, is prevented by randomly phase shifting an electric signal containing audio information supplied to the speaker. The random phase shifting is provided by variably delaying the electric signal in a serial analog register having a shift rate determined by an output signal of a pseudo random code generator.