An echo controller comprises a parallel circuit of an echo suppressor and a self-adaptive echo canceller near an echo path of a long-distance telephone network. The echo controller further comprises a mode switch responsive to levels of a received signal, an unprocessed signal, and an output signal of the echo canceller and operable during absence of double talk for suspending operation of the echo canceller and putting instead the echo suppressor into operation only when characteristics of the echo path and operation of the echo canceller are objectionable.
A speech detector uses a signal classifier (19) to identify portions of a representation of the average magnitude of a group of signal samples indicative of either speech or noise. A controller (33) in the signal classifier follows a four state sequence using appropriate time constants for signal measures in a variety of signal conditions in defining the speech and noise portions of the representation. A level estimator (21) uses selectively obtained signal measures from the defined portions of the representation to provide adaptively variable decision levels. A speech definer (16) compares the representation to a first decision level and the signal samples to a higher decision level to indicate the occurrence of speech signal activity when either decision level is exceeded. In a two way transmission arrangement, a receive trunk speech detector uses a stretcher (133) to prevent adaptation of the transmit speech detector thresholds when echo signals are present.
A method for providing and for testing adaptive echo cancellation in an apparatus including transmission equipment for data communication. The apparatus includes a 2-wire to 4-wire hybrid circuit which receives a signal from a remote end which is cut off for purposes of the testing method. A signal consisting of randomly occurring ones and zeros is applied to the send end of the apparatus. Also included in the apparatus is an adaptive echo canceller which cooperates with the send end and with a filtered signal received from the hybrid circuit to produce a correction signal which is fed back to the adaptive echo canceller and which is also compared with threshold values for purposes of testing the function of the apparatus.
A residual echo suppressor for an echo canceller compares the levels of the uncancelled signal and the residual signal. If the level of the residual signal has a selected proportionality to the level of the uncancelled signal, it is assumed that the residual signal consists solely of echo and it is suppressed to zero. If the residual signal fails the proportionality test, which indicates that it may contain speech, it is transmitted in its entirety. To prevent sporadic transmissions of residual signal which fails the proportionality test, but which might not contain speech, a hysteresis circuit blocks transmission of the residual signal upon the same continuously failing the proportionality test for a selected time, at the end of which time the residual signal is assumed to contain speech and is transmitted.
Three double talk detectors (DTD) are used in combination with an echo canceller having an adaptive filter and a center clipper. First and second double talk detectors are used in the presence of double talk to selectively freeze the adaptive filter correction loop and to disable the center clipper. The third double talk detector is used to detect the initial adaptive period of the echo canceller. Control of the adaptive filter and clipper is effectively transferred from the first to the second double talk detector upon termination of the initial adaptive period as determined by the third double talk detector. The third double talk detector also detects a false double talk condition (where the distant talker pauses in speech) and overrides the second double talk detector in order to maintain the clipper in an active state.
An echo suppressor (21, 22) monitors signal levels associated with the working environment of an echo canceler (16, 17) to prevent spurious transient responses from the canceler. These spurious responses typically occur before the echo canceler has had sufficient time to appropriately adapt which is also the time when a call is first established. At such times, signaling and maintenance testing are performed in a telecommunications network and the echo canceler tends to produce an output when there is no actual transmit input signal. The echo suppressor includes a comparator (26, 27) which detects the presence of this output and the absence of an actual transmit input signal and operates a switch. The switch prevents the transient echo from propagating on the transmit path.
