International Business Machines Corporation

Andre E Desblache

Claude R Galand

Thomas E Stern

Robert Vermot-Gauchy

Dominique N Godard

Philippe Emmanuel Thirion

Francois Gabriel Caron

Alain Croisier

Francis Paul Maddens

Speech signal presence is detected in a VAD (Voice Activity Detector) in two steps: (1) Signal energy above a threshold decides presence, below threshold decides ambiguity; (2) ambiguity is resolved by testing the rate of change of spectral parameters.

Voice activity detection process and means for implementing said process

A method of initializing a filter in an echo canceller as used in a telephone system that includes hybrid transformers for connecting unidirectional lines to bidirectional lines and vice versa. Said method includes the steps of successively transmitting sequences of real and imaginary components of a signal exhibiting an uniform amplitude (i.e. flat) spectrum; receiving and storing sequences of echoes of each of said sequences of components; calculating the fast Fourier transform of complex sequences obtained from the stored sequences of echoes; rotating the terms of the Fourier transform; calculating the inverse transform of the sequence obtained by rotating said terms; and sorting the terms of the inverse transform to derive therefrom the values of the initial coefficients of said filter in said echo canceller.

Method of initializing a filter in an echo canceller and communication

A method and device for measuring the slope of the envelope delay characteristic in a transmission channel. The slope is determined from a received measurement signal having a frequency spectrum that includes three rays at frequencies f.sub.o, f.sub.1 and f.sub.2, which are approximately located in the center and at the edges of the useful passband of the channel. At the output of the channel, the three components at frequencies f.sub.o, f.sub.1 and f.sub.2 are extracted from the received measurement signal, and the instantaneous phases .psi.0, .psi.1 and .psi.2 of these three components are measured. The value of the slope S of the envelope delay characteristic is obtained from the relation ##EQU1## where .phi..sub.o ', .phi..sub.1 ' and .phi..sub.2 ' are the phases of the three rays at frequencies f.sub.o, f.sub.1 and f.sub.2 of the measurement signal sent over the channel. When the data transmission system employs double sideband-quadrature carrier (DSB-QC) modulation, the measurement signal is generated by causing the carrier to undergo successive phase changes of +.pi./2 and -.pi./2 radians at the signaling rate. The receiver includes a set of fixed equalizers and the appropriate fixed equalizer is selected as a function of the measured slope.

Method and device for measuring the slope of the envelope delay

A method and device for measuring the slope of the envelope delay characteristic of a transmission channel and deriving the phase intercept value therefrom. A measurment signal having a frequency spectrum including three rays at frequencies f.sub.o, f.sub.1 and f.sub.2 is sent over the transmission channel. Frequency f.sub.o is the carrier frequency and frequencies f.sub.1 and f.sub.2 are defined as f.sub.1 =f.sub.o -1/2T and f.sub.2 =f.sub.o +1/2T, where 1/T is the signaling rate. At the output of the channel, the three components at frequencies f.sub.o, f.sub.1 and f.sub.2 are extracted from the received measurement signal, and the instantaneous phases .psi..sub.o, .psi..sub.1 and .psi..sub.2 of these three components are then measured. The value of the slope S of the envelope delay characteristic is obtained in accordance with the relation ##EQU1## where .phi..sub.o.sup.', .phi..sub.1' and .phi..sub.2' are the phases of the three rays at frequencies f.sub.o, f.sub.1 and f.sub.2 of the measurement signal sent over the channel. The value of the phase intercept .phi..sub.i for a given sampling phase .phi..sub.s is determined in accordance with the relation

Method and device for determining the phase intercept in a system

A new adaptive digital tuning filter for tracking a sinusoidal signal within a frequency band is described. The input signal representative of both said sinusoidal signal and noise is fed to a Hilbert transformer which provides the in-phase and quadrature components, x.sub.k and x.sub.k, respectively, of said sinusoidal signal. These components are applied to the input of a filter having a transfer function K where ##EQU1## WHERE .phi. = 2.pi.FT, f is the tuned frequency of the filter,

Self-adaptive digital filter for noise and phase jitter reduction

A clock setting circuit is provided at a receiving modem for adjusting the phase of a timing signal defining the signal sampling instants. The received signal is filtered in two filters to derive a first signal having a phase .phi..sub.1 and a frequency f.sub.1 equal to f.sub.c - 1/2T, f.sub.c being the carrier frequency and 1/T being the transmission baud rate, and a second signal having a phase .phi..sub.2 and a frequency f.sub.2 equal to f.sub.c + 1/2T. The first and second derived signals are combined to derive an error signal indicative of the phase difference .phi..sub.2 - .phi..sub.1 which difference is used for adjusting the phase of a phase locked oscillator which provides the timing signal.

Method and device for synchronizing the receiver clock in a data

A data transmitting and receiving system in which data is transmitted by varying the phase and amplitude of a carrier, including a receiver which detects the data in a received signal while reducing the effects of noise components which alter the phase and amplitude of the received signal. An in-phase component and a quadrature component are derived from the received signal and are then rotated an angular amount which is a function of an estimated value of the phase error in the received signal. The rotated components are then compared with two sets of reference coordinates for selecting a pair of reference coordinates which have values closest to the values of the rotated in-phase and quadrature components, respectively. The estimated value of the phase error is derived as a trigonometric function of the rotated components and selected reference coordinates. The selected reference coordinates are indicative of the transmitted data.

System for detecting digital data transmitted by modulating a carrier

A digital phase filter including two decision filters connected in cascade or parallel for minimizing the noise components in a received phase modulated digital signal. The first decision filter cancels the residual noise component representing the phase intercept component and the phase shift component introduced by frequency shift, and the second decision filter minimizes the random noise component representing phase jitter and white noise.

Phase filter for reducing the effects of the noise components altering

The feature of the invention is an adaptive equalizer for modulated carrier transmission systems in which a received signal is subjected to a Hilbert transformation to obtain a second signal having a 90.degree. phase shift for all components. The two signals are then passed through a pair of filters each and the outputs are cross-combined to generate the cartesian coordinate signals of an equalized signal. The coordinate signals are combined in a polar converter and are then decoded to detect the amplitude and phase data components of the received signal. The equalizers are made adaptive by determining at each sampling time, the phase and amplitude errors in the signal and using these errors to modify the coefficients of the equalizing filters. The phase and amplitude errors are recoded into Cartesian coordinates and multiplied by the values of the samples at each tap of the filters to generate four error signals for each tap. A cross combination of these signals is time averaged and used to adjust the corresponding coefficients at the associated taps of the equalizer.

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Nice, France

Andre E Desblache