Abstract: A method of reconstructing, at the receiving end, digital data defining an encoded voice signal segment lost in transmission between the transmitter and the receiver in a transmission system wherein a low-frequency signal such as a residual baseband signal is derived at the transmitting end from the signal to be encoded and is then distributed among several sub-bands whose sampled contents are quantized separately. The reconstruction method includes a step of analyzing the received signal to detect any missing segment thereof and, as the case may be, to initiate an analysis, within each sub-band, of the segment(s) adjacent to the lost segment, so as to generate a term relating to the period T.sub.(k) of the signal so analyzed and to reconstruct a segment of signal of period T.sub.(k) intended to be substituted for the lost segment. Cyclic redundancy is used to detect errors, and modulo sequence numbering identification is used to detect loss of packet.

Abstract: The process is made to detect a received tone including at least one single predetermined frequency, within a signal encoded using block coding and Linear Prediction Coding techniques providing auto-correlation coefficients R(i) and direct form coefficients a(i). The process is based upon Fourier Transform computations over the successive blocks of a(i)'s, with table look up tone identification, further confirmed upon R(i)'s stationarity detecting.

Abstract: 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.

Abstract: 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.