Abstract: An apparatus for the recovery of an amplitude-modulated and phase-modulated signal wherein the signal is multiplied by two correction and adjustment signals in phase-quadrature in a pair of multipliers connected in low-pass filters to the sample and decision circuit. The output of the latter is an error signal which represents the phase error of the recovered carrier obtained by the comparison made between the baseband components of the demodulated signal and suitable decision thresholds at a decision instant estimating transmitted digital symbols. A logic circuit is connected to the output of the sample and decision circuit and delivers its output to a pulse generator whose correction train of pulses is filtered before being applied to a voltage-controlled oscillator whose output is applied directly to one of the multipliers and through a quadrature-phase shifter to the other.

Abstract: A stream of digital symbols with three or more distinct amplitude levels, organized in a recurrent frame of N symbol periods, carries supplemental information in the form of a binary word of k bits per frame, with k substantially less than the probable minimum number of modulable waveforms, i.e. symbols of the lowest or the highest normal level. At a transmitting end of a signal path, the binary word is superimposed upon a group of k consecutive modulable waveforms at the beginning of each outgoing frame, with bits of logical value "1" indicated by a supermodulation of corresponding modulable waveforms to raise or lower their amplitudes to an extra-high or an extra-low level; the first and the last bit of this word have invariably the value "1". At a receiving end, a cyclic counter CL.sub.4 stepped by extracted clock pulses establishes a recurrent incoming frame of normally N symbol periods while a dead-end counter CC.sub.

Abstract: A multilevel mainstream of digital signals, organized in a recurrent frame, is monitored at a transmitting end by a pulse generator GP1 which emits a marking pulse whenever that mainstream has a modulable waveform, i.e. a symbol represented by either the lowest or the highest amplitude level. A waveform generator GF controlled by pulse generator GP1 produces a bipolar signal 19 whose pulses are to be superimposed upon the modulable waveforms coinciding with the occurrences of the marking pulses, the superimposition being carried out in an analog adder S1 which extends the amplitude excursion of the mainstream signal beyond its limiting level.