Abstract: A telephone includes a transmit channel and a receive channel, each including a bank of sub-band filters having a VAD coupled one to each sub-band filter. Each VAD measures the spectral energy in a sub-band, compares the spectral energy to a first threshold, and produces an output signal representative of whether or not the first threshold is exceeded. The voice activity detector also includes a threshold circuit for calculating a dynamically adjustable noise threshold based upon averaged measured spectral energy. A wide band or system VAD monitors echo canceling circuitry to detect voice activity and double talk. A dynamic balance control circuit maintains the transmit and receive channels in a predetermined energy ratio during double talk.

Abstract: A telephone includes a first channel having a microphone input and a line output, a second channel having a line input and a speaker output, and each channel having a plurality of band pass filters with their outputs coupled to a multiplex circuit. The multiplex circuit selects subsets of the signals in each channel in accordance with the magnitudes of the signals in each band. Thus, full duplex operation is provided in accordance with the spectral content of each channel. In one embodiment of the invention, the multiplex circuits initially allocate alternate bands within each channel and complementary bands between the channels, thereby emulating a complementary comb filter. Thereafter, bands are allocated according to spectral content.

Abstract: A soft mute circuit includes a programmable amplifier controlled by a register. Data is stored in the register from an adder that combines the current data in the register with a second number for increasing or decreasing the gain of the amplifier. A summation circuit includes a plurality of inputs coupled by gates to a summation node and the summation node is coupled to an input of the programmable amplifier. The gates are controlled by suitable logic for selecting input signals in any combination. A control loop maintains the gain of the amplifier at a predetermined level.

Abstract: A bulk delay estimating circuit matches time intervals representing a signal with time intervals representing an echo of the signal to identify the echo and estimate bulk delay. Bulk delay is estimated by computing (1, 2, . . . n) intervals representing the signal, computing (1, 2, . . . n) intervals representing an echo of the signal, computing absolute differences between corresponding intervals to produce n absolute differences, summing the n absolute differences, and providing an output indicating whether or not the sum is less than a predetermined amount. The intervals are determined by defining a plurality of numbered frames, comparing the energy of a signal during each frame with at least one threshold, storing the numbers of the frames in which the threshold is exceeded, and defining an interval as the period from one frame in which the threshold is exceeded to the next frame in which the threshold is exceeded.

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: 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.