Abstract: An adaptive phase-jitter compensator, which is frequency selective, is disclosed for use in data receivers. The filter section (28) of the adaptive compensator receives a derived phase error signal and its own weighted sum output to provide the difference between them as selective noisy estimates of the actual phase via taps of a delay line (52-1 to 52-L). A multiplier (57) accepts each tap signal and the phase error to form a product used to update the coefficients or tap weights used in the filter section. An upmodulator (29) impresses the weighted sum output on a carrier signal whose output is used by a lookup table (31) to form components to be compatible with the received data signal. A complex multiplier, (23), multiplies the components and the data signal which has been previously split into components and equalized, to provide both demodulation and phase-jitter compensation.

Abstract: A quadrature amplitude-modulated (QAM) data signal receiver employs a phase compensation arrangement (16, 31, 33) before the equalizer (17). The arrangement utilizes the assumption that the frequency components typically present in the phase perturbance are power-line related. This enables an effective phase compensator to be of relatively low complexity compared to the equalizer (17). Since the compensator is "pretuned", only the phase and amplitude of the frequency components need be adaptively found. This makes for a relatively stable arrangement with a suitable convergence rate. In an alternative arrangement, a phase compensator (216, 233, 240) is provided for use after the equalizer (217).

Abstract: In a multipoint data communication system using quadrature-amplitude modulation, a master modem (20) and a plurality of tributary modems (11a, 11b . . . 11n) are interconnected via respective transmission channels (13a, 13b . . . 13n, 16). Adaptive equalizer circuitry (55, 56) in the master modem equalizes the channel from a particular tributary by multiplying samples of signals received from the tributary by an ensemble of tap coefficients associated with the tributary. The tap coefficient ensembles for each tributary are stored in a memory (91) from which they are retrieved at the start of transmission from that tributary. Timing-acquisition circuitry (29) within the master modem adjusts the phase of the latter's sampling circuitry (23, 27) at the start of transmission from a given tributary so that the received signals are sampled at the correct time points.

Abstract: A quadrature amplitude modulated (QAM) data signal transmitted at T symbols per second is sampled in a data receiver at a rate of 2/T samples per second and applied to a transversal-type equalizer structure (25, 46, 34, 35) having taps spaced at T/2 second intervals. A demodulated equalized signal (a.sub.j, b.sub.j), generated once every T seconds, is quantized to form a decision (a.sub.j *, b.sub.j *) as to the value of the original modulating data symbol. An error signal (e.sub.j, e.sub.j) is formed in response to the pre- and post-quantized values of the demodulated equalized signal. Tap coefficients (c.sub.i (j), c.sub.i (j)) used in generating the equalized signals are updated in response to (a) a correction term which is a function of the error signal and (b) a predetermined tap leakage term which has a constant magnitude. The introduction of the tap leakage term maintains the coefficient values at minimum levels.

Abstract: The echo path of an echo signal subjected to a time-varying phase shift is bridged by echo cancelling apparatus including a variable phase shifter and an adaptive echo canceller connected in series. The variable phase shifter includes an adaptive control loop whose operation is compatible with that of an adaptive echo canceller. The phase-shifter control loop derives a control signal from the output signal synthesized by the echo cancelling apparatus and the residual echo produced by subtracting this output signal from the echo path signal. The control signal is used to produce a variable phase shift on the signal passing through the variable phase shifter in the echo cancelling apparatus. The echo canceller adaptively synthesizes a replica of the echo signal produced by the linear dispersive portion of the echo channel, while the variable phase shifter is adapted to impart a time-varying phase shift corresponding to the time-varying phase shift encountered by the echo signal in the echo path.