Digital signature generation apparatus, comprising an envelope generator operable to generate an envelope representation, of only one polarity, of a sampled data segment, and operable for each of successive portions comprising a predetermined plurality of samples to provide a portion sum value as the sum the values of the samples in the portion, thereby to provide said envelope representation; a threshold value generator operable to determine a threshold value for each portion of the envelope representation; an event detector operable to detect, as an event, a transition of a portion sum value across the threshold value for the portion concerned; and a signature generator operable in response a detected event to generate a digital signature characteristic of the sampled data segment.