Abstract: Apparatus and a technique for echo cancellation is described in which the real and imaginary parts of an echo replica signal can evolve independently of each other. By removing the coupling heretofore found in cross coupled echo canceller structures, the echo replica obtained in conjunction with carrier-phase tracking circuitry can be a more exact duplicate of the real and imaginary components of a received echo containing signal. The echo canceller may include a plurality of subcancellers, each including at least four adaptive filters. Alternatively the filters within each subcanceller can be implemented by a suitably programmed digital signal processor.

Abstract: An echo canceller for passband data signals is comprised of a pair of real passband filters which generate samples of the echo replica in response to respective streams of rotated data symbol components.

Abstract: In a data receiver (40) for a quadrature-amplitude-modulated signal s(t) having symbol interval T, line samples (s.sub.k) are formed at a sampling frequency n/T which is at least equal to the Nyquist frequency. With the sampling being performed at this rate, the functions conventionally provided by individual bandpass filter (12), phase splitter (14) and equalizer (22) circuits are performed by a single transversal filter equalizer (122). Tap coefficients for the transversal filter equalizer are derived in a decision-directed manner. The tendency of the tap coefficient values to drift is suppressed by adding energy to the received signal at frequencies within the band (0-n/2T) Hz at which the received signal has negligible energy.

Abstract: Combined amplitude and frequency-shift-keying (AM-FSK) modulation of a carrier wave by a baseband data signal achieves significant suppression of one of the sidebands to ease the design requirements of filters generally used in the transmission of FSK signals. The significant suppression effect is observed when the ratio of minimum to maximum amplitude of the envelope of the amplitude modulated carrier wave is restrained to be approximately between 0.6 and 0.9.In one embodiment amplitude modulation is performed by a pair of amplifiers, each having a predetermined gain. In another embodiment, an all digital technique is used to synthesize the AM-FSK signal.