A digital signal processor is provided with the facility to range and converge on each of a plurality of echoes occurring at respective locations along a communications line. Specifically, the digital signal processor is arranged to adaptively process samples of a signal that has been transmitted to the communications line correlated with samples of a signal that have been received from the communications line. In the processing of the samples, the digital signal processor separates and measures the magnitude of each echo that it converges on, beginning with a so-called near-end echo. In addition, the digital signal processor calculates the propagation delay of each echo that it converges on using the number of sampling intervals that have been processed in order to position the processor on the dispersion of that echo.
A speaking apparatus having a handfree conversation function, provided with an echo canceler which can cancel the echo caused by direct acoustic coupling between a speaker and a microphone positioned in a system having a casing, a ground surface and a speech switching circuit. In the apparatus, the amount of insertion attenuation of the transmitted and received signal is set with an upper limit of the amount of attenuation sufficient for cancelling the echo caused by the indirect acoustic coupling determined by the location where the speaking apparatus is used. In addition, the echo canceler and the speech switching circuit share optimum functions to obtain an excellent speaking quality apparatus with an inexpensive and small sized processing circuit.
An echo cancellation apparatus is included in a variable rate modem. The echo cancellation apparatus includes a local echo canceler, a remote echo canceler and a quadrature echo canceler unit. The local and remote echo cancelers both operate to cancel echoes in a similar manner employing topped delay lines or transversal filters wherein the taps of each delay line are adjusted to accommodate both local and remote echo cancellation. The local echo canceler and the remote echo canceler share many programs in common. However, the remote echo canceler operates with a phase lock loop which essentially derives a signal obtained from the quadrature and local echo cancelers which signal is further correlated with the output residuals from the local echo canceler to develop samples applied to the remote echo canceler and to derive a frequency which is indicative of a offset carrier frequency to enable the remote echo canceler to cancel remote echoes. Remote echoes are due to frequency shifts occurring remote from the modem, such as those caused by changes in satellite positions or changes in frequency sources. The remote echo canceler operates to cancel echoes which are greater than 15 milliseconds in delay, as compared to local echoes, which are canceled by the local echo canceler responding to echoes which are delayed 15 milliseconds or less.
A sign-based decorrelation detection and adaptive control arrangement includes structure for detecting cross-correlation between a far-end signal and an echo residual following a balance filter. During the adaptive process, if the detected correlation value is below a certain threshold, indicating that the two signals are decorrelated, the adaptation of the balancing filter is stopped. At such a point, proper echo cancellation has been achieved. Conversely, when the detected correlation value exceeds a threshold, the adaptation is ,continued until the correlation value falls below the threshold again. In any event, such decorrelation controllers are able to detect signal decorrelation and to control adaptation even in the presence of a double-talker condition.
A terminal apparatus for full duplex transmission of digital data comprising an echo canceler having FIR and IIR type filters and also comprising a filter convergence controller. The filter convergence controller performs its converging operation over both filters in a first duration of a communication starting sequence from the start of transmission from a local terminal to a remote terminal to the detection of the convergence of the echo canceller to a certain extent, stops the converging operation of the IIR type filter in a second duration of the communication starting sequence from the detection of the convergence to the detection of synchronization with the remote terminal, and again performs the converging operation of the both filters in a third duration after the detection of the synchronization.
A filter for filtering a speech signal to reduce acoustic noise is disclosed. In accordance with the inventive filter, the parameters of an all-pole vocal tract model are first estimated from the noisy signal using a least mean square algorithm as if no noise were present, and then the speech signal is filtered using an approximate limiting Kalman filter constructed according to the estimated parameters.
