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 CL4 stepped by extracted clock pulses establishes a recurrent incoming frame of normally N symbol periods while a dead-end counter CC2 registers the first k modulable symbols of such frame and thus determines a recovery interval for the demodulation of their supplemental bits, followed by a redundancy interval in which no supermodulated waveforms are expected to occur in steady-state operation. A logic network PP counts the number of supermodulated waveforms detected in various sections of the redundancy interval and compares that number with the number of "1" bits carried by the first and kth modulable waveforms of the recovery interval; if this comparison reveals a disalignment between the outgoing and incoming frames, the logic network temporarily modifies the operating cycle of the cyclic counter to realign the two frames.