An analoque nonlinear dynamic filter

An analogue non-linear dynamic filter having a substantially semisoidal input-output transfer characteristic with a trough at a first constant level and a peak at a second constant level, and a continuously variable transfer function between the first and second levels. The first level, which is substantially zero, is defined by a small area of the input signal in the vicinity of the zero crossings thereof, and is obtained by subtracting the output of a first clipper circuit (12) from the input signal. The second constant level is obtained by applying the signal to a second clipper circuit (14) and the signal is then applied to a rectifier (16) before being multiplied by the input signal to attain a variable gain function