Abstract: An audio processor circuit for treating a digitally encoded incoming signal, the circuit comprising circuitry (3, 12) for sampling the encoded signal at two or more sample frequencies, circuitry (8, 13, 38, 40, 46) for calculating from the sampled values a cross-correlation function relevant to a particular time frame, the value of said function serving to control operation of an adaptive algorithm, in which the sampled signal portions are truncated (2, 5) such that only the arithmetic sign bit of each signal portion contributes to the function calculation.