A method of estimating a pitch period of a first portion of a signal wherein the first portion overlaps a previous portion. The method comprises computing a first autocorrelation value for part of the first portion not overlapping the previous portion. The method further comprises retrieving a stored second autocorrelation value for part of the first portion overlapping the previous portion, the second autocorrelation value having been computed during estimation of a pitch period of the previous portion. The method further comprises forming a combined autocorrelation value using the first and second autocorrelation values, and selecting the estimated pitch period in dependence on the combined autocorrelation value.