1. A subject monitoring system for determining physiological information of a subject, comprising: a sensor configured to generate an intensity 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 based on a portion of the intensity signal and a stored reference waveform; determine physiological rate information based on the correlation sequence; select a new reference waveform that is derived from the intensity signal, wherein the new reference waveform is selected based on the physiological rate information; determine whether to qualify the new reference waveform; and when the new reference waveform is qualified, store the new reference waveform; calculate a subsequent correlation sequence based on a more recent portion of the intensity signal and the new reference waveform; and determine subsequent physiological rate information based on the subsequent correlation sequence.