Abstract: Sub-band filtering, adaptive echo cancellation, and residual echo suppression are selectively and separately applied in hierarchical order under the control of a circuit that monitors four separate signals to determine n machine states. The number of machine states is further divided among m levels of noise in determining which, and how much of, sub-band filtering, adaptive echo cancellation, and residual echo suppression to use at any given time. A voice activity detector (VAD) is used to monitor each signal and provides statistical information in addition to whether or not a voice signal is detected. Sub-band filter, echo cancelling, and non-linear processing are not permitted to provide maximum reduction simultaneously. Even so, the invention provides at least 15 dB more echo suppression than systems without a sub-band filter bank series coupled with a non-linear processor in each channel.

Abstract: Amplitude is corrected by subtracting a component signal from a composite signal to produce a remainder signal, correlating the remainder signal with the component signal to produce a product signal, averaging the product signal, and adjusting the magnitude of the component signal in accordance with the averaged product signal to minimize the product signal. The amplitude of the component signal is adjusted in a programmable gain amplifier controlled by an up-down counter. The up-down counter is part of a digital control loop including a pseudo-multiplier for multiplying the remainder signal with the component signal. The output of the multiplier controls the direction of the count, which is generally continuous except that it cannot roll over or roll under. The remainder signal is the received signal with the echo removed.

Abstract: A complementary comb filter includes a plurality of band pass filters and a plurality of notch filters, wherein the notch filters have the same center frequencies as the band pass filters or the same center frequencies as the dead bands defined by the band pass filters. Phase distortion by the notch filters occurs primarily in the notch, thereby eliminating distortion of the signal.

Abstract: Voice activity is detected by comparing a signal with two thresholds and producing data representing the energy of the signal. The data, in binary form, is compared with thresholds to determine voice activity. In accordance with another aspect of the invention, the thresholds are adjusted based upon statistical information. In accordance with another aspect of the invention, the data can be weighted to provide an indication of the quasi-RMS energy of an input signal. In accordance with another aspect of the invention, voice activity detectors, individually weighted, are provided at each input and each output of a telephone for reliably controlling echo cancelling circuitry within the telephone.

Abstract: Voice activity is detected by comparing an analog signal with two voltage thresholds and producing data representing the energy of the signal. The data, in binary form, is compared with thresholds to determine voice activity. In accordance with another aspect of the invention, the thresholds are adjusted based upon statistical information. In accordance with another aspect of the invention, the data can be weighted to provide an indication of the quasi-RMS energy of an input signal. The input signal itself is not converted into digital form yet the data derived from the input signal has high resolution.

Abstract: Voice activity is detected by comparing a signal with two thresholds and producing data representing the energy of the signal. The data, in binary form, is compared with thresholds to determine voice activity. In accordance with another aspect of the invention, the thresholds are adjusted based upon statistical information. In accordance with another aspect of the invention, the data can be weighted to provide an indication of the quasi-RMS energy of an input signal.

Abstract: An apparatus and electrical circuit utilizing a high power Class “D” type amplifier for delivering an audio signal to a acoustic transducer system. A PWM modulator, the output of which is inhibited during power fluctuations or in absence of an audio input, converts a suitably conditioned audio input signal into low power, high frequency positive and negative components 180 degrees out of phase. A PWM signal power amplifier stage and low pass filters amplify the signal components, remove the higher PWM switching carrier frequency and pass the results to a power audio transformer impedance matched to the output acoustic transducers. Feedback signals are taken from the power stage before the low pass filters, suitably conditioned and combined as negative feedback with the input analog audio signal to minimize output distortion and maintain correct audio output voltage with changing load impedance and power supply voltages.

Abstract: A band pass filter includes a pair of filters having different center frequencies and a difference amplifier coupled to the outputs of the filters. The filters preferably have response curves that intersect at −3 dB. The separation of the center frequencies determines, in part, the ripple in the response curve of the band pass filter.

Abstract: Correlation is improved by comparing the output of a multiplier and low pass filter to a first threshold and incrementing a counter when the output from the filter exceeds the first threshold; comparing the output of the filter to a second threshold and decrementing the counter when the output is below a second threshold. Correlation is indicated by the count in the counter exceeding a predetermined amount.

Abstract: The amplitudes of a signal on the microphone input of a telephone and a signal on the line input of the telephone are compared with each other and with two thresholds. The comparisons are sampled and the samples control an up-down counter. The count in the counter is compared with two thresholds and the results of the comparisons are used to control at least two accumulators. A first accumulator counts consecutive comparisons having the same result and a second accumulator counts up or down in accordance with the comparisons. The counts in the first and second accumulators are logically analyzed to determine whether to receive or transmit. A plurality of samples defines a window, at the end of which the up-down counter is reset. The accumulators store data from a plurality of windows.

Abstract: An audio signal is divided into a plurality of bands prior to generating a shadow. The shadow is created by delaying the portion of the audio signal in at least one band by less than fifty milliseconds and combining the shadow signal with the portion of the audio signal. The portions, with any shadows, are combined to produce a reconstructed audio signal. The presence or absence of a shadow signal represents data or the data is represented by two or more shadow signals, in one or more bands. The period of delay of a portion in a band should not equal the period of the center frequency of that band. Preferably, the period of delay of a portion in a band does not equal the period of any frequency within the pass band.

Abstract: A transmitted signal is coupled through a coarse delay and a fine delay to one input of a correlator. Another input of the correlator is coupled to a received signal. The delays are adjusted for maximum correlation and the output of the fine delay is subtracted from the received signal to reduce or eliminate an echo. The delays operate by sampling the signal and storing the samples at successive storage sites. The storage sites are read a predetermined time later, producing the delay. The read operation for the coarse echo includes three readings from separate sites and using the correlation products to indicate the magnitude and direction for changing the delay.

Abstract: The separation of a adjacent band pass filters is improved, without changing the filters, by inverting the output signals from alternate filters and not inverting the remaining output signals. All the output signals are then summed. The result is a deeper notch in the frequency response of adjacent filters.

Abstract: An electrical signal is applied to a band pass filter, a first notch filter, and a second notch filter in any order. The center frequencies of the notch filters straddle the pass band of the band pass filter. The notch filters improve group delay and steepen the skirts of the response curve of the band pass filter.

Abstract: A bio erodable myringotomy tube which is bio erodable and does not require removal.

Abstract: A low pass limited, low frequency pseudo-random noise signal is added to the input signal for tagging the input signal in order to facilitate correlation with an echo.

Abstract: An echo is cancelled from a composite signal by providing a coarse phase adjustment, a fine phase adjustment, an amplitude correction, and then subtracting the phase shifted signal from the composite signal. In the coarse adjustment, the phase is slightly under-corrected, thereby allowing the fine adjustment to proceed in a single direction and decreasing convergence time. Correlation is provided by an amplitude inverting multiplying circuit and phase adjustment is provided by an all-pass phase shift network combined with an impedance multiplier coupled to the phase shift network.

Abstract: A band pass filter includes a pair of notch filters having different notch frequencies and a difference amplifier coupled to the outputs of the notch filters. The notch filters preferably have response curves that intersect at −3 dB. The separation of the notch frequencies determines, in part, the ripple in the response curve of the band pass filter.

Abstract: A filter includes an FET and a capacitor in a phase shift network wherein the FET operates as a variable resistor. An impedance multiplier is coupled to the FET for increasing the range of resistance of the FET.

Abstract: Sound is converted into an electrical signal by a microphone and is converted into an inaudible, digitally modulated signal that is combined with the electrical signal from the microphone, amplified, and converted into sound waves by a speaker. Any sound traveling from the speaker back to the microphone includes the inaudible component representing the original sound. The inaudible component is separated from the audible components, and the original sound is reconstructed in a digital demodulator. The reconstructed original sound is subtracted from the signal from the microphone, thereby reducing any echo and canceling feedback. Digital modulation includes any form of shift keying, including coherent and noncoherent techniques for modulating frequency, phase, or amplitude.