A high pass filter adapted for use in a signal processing device comprising a sampler adapted to sample a signal and transmit the samples to the high pass filter, the high pass filter comprising a differentiator and an integrator; a counter which, in use, counts the number of samples received by the high pass filter from the sampler; and a control device which receives a first signal from the counter when the high pass filter has received a first sample from the sampler and receives a second signal from the counter when the high pass filter has received a second or subsequent sample from the sampler; whereupon receipt of the first signal, the control device causes the integrator output signal to substantially equal zero and whereupon receipt of the second signal, the control device causes the integrator output to be a function of an input signal from the differentiator.