A Method of Determining the Frequency of a Periodic Physiological Process of a Subject, and a Device and System for Determining the Frequency of a Periodic Physiological Process of a Subject

Methods, devices and systems for determining the frequency of a periodic physiological process of a subject, particularly respiration rate or heart rate, are disclosed. In one arrangement plural time windows of physiological data are obtained. Reference features 5 corresponding to modulation modes are identified. Modulations of the reference features are extracted. Quality parameters are obtained by processing the extracted modulations. The quality parameters represent how strongly the extracted modulation exhibits a waveform of the periodic physiological process. The extracted modulations are processed to calculate the frequency of the periodic physiological process of the subject. The frequency of the periodic 10 physiological process may be calculated using only a subset of the extracted modulations, the subset being selected using the quality parameter, or using a combination of the extracted modulations wherein each extracted modulation has a weighting defined by the quality parameter of the extracted modulation. 15