Abstract: An electrical filter for extracting from an applied input signal a pseudo-random sequence signal contained in the input signal comprises: an analogue delay line; means for applying to the input of the delay line an input signal containing the pseudo-random sequence signal; feedback means for producing a feedback signal in response to the signals appearing at two or more points along the length of the delay line; and combining means for combining said feedback signal with said input signal; said feedback means being constructed in dependence on said pseudo-random sequence so that, at the input of the delay line, said pseudo-random signal in said input signal is reinforced by said feedback signal.