1. A subject monitoring system for processing physiological information of a subject, comprising: a sensor configured to generate a physiological signal, wherein the sensor detects light attenuated by the subject; and processing equipment coupled to the sensor, wherein the processing equipment is configured to: calculate a correlation sequence corresponding to multiple lag values based on the physiological signal; for at least one lag value of the multiple lag values: compare the correlation sequence value corresponding to the at least one lag value to a threshold, wherein the threshold is predetermined and wherein the threshold varies as a function of lag, determine whether the correlation sequence value exceeds the threshold, and determine whether the correlation sequence value corresponds to a peak; and identify a particular lag value of the multiple lag values when the correlation sequence value corresponding to the particular lag value exceeds the threshold and corresponds to a peak.