Abstract: In order to prevent tap coefficient values from drifting toward excessively large values or divergence, an adaptive echo canceller is provided with a correction signal controller which (a) reduces the tap coefficient values upon the polarities of the tap coefficient signal and the tap coefficient correction signal coinciding and (b) increases the tap coefficient values upon the polarities differing.

Abstract: In a speech transmission or storage system using LPC parameters and a Baseband Signal derived from the prediction error signal, the synthesis excitation signal is formed from the baseband plus high-frequency regeneration, which is then spectrum-flattened for proper synthesis.

Abstract: In a speech analysis-synthesis system for the narrow band transmission of a speech signal, speech is separated into a plurality of spectrum information (K.sub.i), average (V.sub.0) of linear prediction error signal, pitch period (L), voiced/unvoiced decision signal (V/UV), a pseudo exciting signal (I) which is a part of a linear prediction error signal or an impulse response of the same, and a frame information which determines the interval and the duration of the spectrum analyzation, then, in a receive side, the product of said pseudo excitation signal (I) and said pitch period (L), a white noise is switched according to said voiced/unvoiced decision signal (V/UV), and the output of the switch is applied to a synthesis filter which attaches correlation components to an input signal according to spectrum information (K.sub.i) to provide synthesized speech.

Abstract: An echo control system for an adaptive echo canceller which forms a pseudo-echo or its residual signal by combining respective outputs from a tapped delay line holding an estimated characteristic of an echo path as a tap weight and receiving a received signal or its residual signal as an input signal, subtracts the pseudo-echo or its residual signal from an echo signal or its residual signal, and corrects the tap weight using an amount of correction obtained by the resulting residual echo signal or its residual signal and the received signal or its residual signal.

Abstract: A system for detecting the pitch of a voice signal, in which a plurality of pitch searching periods are determined so that pitch components of multiple relationship are not included in each of the pitch searching periods, and in which after detecting a pitch searching period including the pitch from the pitch searching periods, the pitch searching periods are adaptively shifted in a mannger to follow the change direction of the pitch predicted from the result of detection of the detected pitch.

Abstract: An echo control system for an adaptive echo canceller which cancels an echo signal while successively estimating the transmission characteristic of an echo path by the use of a received signal and the echo signal, comprising means using the received signal of a predetermined duration for obtaining an autoregression coefficient in a case of using the received signal as the output of an autoregressive model, means for obtaining a difference signal between a predicted value of the received signal produced by using the autoregression coefficient and the received signal, means for obtaining a difference signal between a predicted value of the echo signal produced by using the autoregression coefficient and the echo signal, a memory for storing the estimated transmission characteristic of the echo path, and a corrector for correcting the content of the memory following an identification method by learning.

Abstract: An echo canceller in a telephone circuit for minimizing a residual echo obtained by subtracting a pseudo-echo reproduced from an impulse response of an echo path from a true echo, in which the inpulse response is corrected by the use of the product of the residual echo and received input. A variable coefficient circuit is provided between a multiplier for obtaining the product and a circuit for correcting the impulse response. A control circuit is provided for reducing the coefficient of the variable coefficient circuit in accordance with the lapse of time of the duration of the received input exceeding a predetermined threshold level after receiving a circuit connection completion signal of the telephone circuit.

Abstract: An echo cancelling system for multiplex circuits, in which an impulse response of an echo-path is obtained from signals at respective ends of the transmission path and the receiving path connected to the echo-path to produce an echo-path model, and in which a pseudo echo signal is then provided to subtract it from an echo signal of the echo-path thereby to cancel the echo signal.