1. Apparatus for analysing statistical characteristics of an input signal, the apparatus comprising: a signal input for receiving the signal; means coupled to the input for detecting events at which the signal level crosses a predetermined level with a predetermined slope; means for combining multiple versions of the signal, the versions being shifted with respect to each other by amounts corresponding to the spacing of said events, to form a representation of the signal; and means for measuring a parameter dependent upon the shape of said representation and indicative of a statistical characteristic of said signal.