A method of attempting to determine a pitch period of an audio signal using a correlation-based signal derived from the audio signal. The correlation-based signal has known peaks, having been quadratically interpolated and filtered with coefficients that are a function of the interpolation ratio, each corresponding to a respective one of known time lags. The method comprises: identifying a time lag among the time lags; determining if there exists another time lag (i) within a time lag range of a respective one of one or more integer multiples of the identified time lag, and (ii) corresponding to a peak exceeding a peak threshold; and if the determination of step (a) passes, then returning the identified time lag as a time lag indicative of the pitch period.