A method for processing a quasi-periodic signal includes receiving a representation of a quasi-periodic signal and removing a DC component from the received signal to produce a purely time-varying signal. The time-varying signal is filtered to produce a pre-processed signal and at least a portion of a representation of the pre-processed signal is auto-correlated with itself. A corresponding auto-correlation output is stored in a memory device for the at least portion of the representation of the pre-processed signal. A biphasic tapering function is applied to the auto-correlation output, the tapering function including a time constant parameter that is a function of the quasi-periodic signal and producing a first maximum. A representation, based on the first maximum, is stored in the memory device as an indication of a rate or a frequency of the quasi-periodic signal.