Compression and synthesis techniques and related apparatus for time domain signals, particularly signals whose information content resides in the power spectrum such as speech and more particularly signals whose amplitude is aperiodic, such as unvoiced speech sounds. Compression techniques include eliminating serially redundant segments of information. Synthesis, particularly of unvoiced sounds which are sensitive to injected artificial periodicity, involves repeating sequentially portions of the same segmentrepresentative of the source signal, including commencing and terminating at different points each repetition, varying the length of the portion and reproducing the portion forwards and backwards in time. The invention finds application in speech compression and compact speech synthesis devices.