An interpolation filter (50) for interpolating a digital signal (x (n)) includes a cascade of template filters (40), each having an identical template transfer function A (z), which is arranged to receive and filter an input sequence representing the digital signal sampled at an input sampling rate. Ancillary circuitry is coupled to the cascade so as to produce first (y 0 (n)) and second phase outputs (y 1 (n)). A multiplexer (41) is arranged to multiplex the phase outputs in order to generate an output sequence having an output sampling rate equal to twice the input sampling rate.