1. A physiological monitoring system for detecting arrhythmia in a subject, the system comprising: an interface configured to receive a plethysmograph signal; and processing equipment configured to: filter the plethysmograph signal based on a pulse rate of the subject to generate a filtered signal; perform an FM demodulation of the filtered signal to generate a demodulated signal; filter the demodulated signal based on respiration to generate a filtered demodulated signal; and subsample the filtered demodulated signal to generate a demodulated derived value signal; determine a plurality of arrhythmia features from the demodulated derived value signal; and generate an arrhythmia indicator based on the arrhythmia features.