Abstract: A digital signalling process and apparatus employing multistage charge coupled devices provides a delay structure with a multiplicity of signal paths between an input signal terminal and an output signal terminal. The signal paths impose differing delays such that when the input signal is or contains a periodic waveform whose period is in a predetermined relation to the differences between the delays in the paths, there is reinforcement of the periodic waveform. The charge coupled devices are actuated by clocking pulses and operate so as effectively to sample the reinforced signal at a rate whose period exceeds the period of the periodic waveform. As a result there appears at the output of the apparatus an averaged and time stretched version of the periodic waveform.