A system for determining the frequency of a periodic physiological process of a subject, comprising: a device comprising: a data receiving unit configured to receive plural time windows of data representing physiological measurements on the subject; a data processing unit; and a sensing system configured to perform the physiological measurements on the subject; and a data processing station connectable via a network to the device, wherein: the data processing unit is configured to: identify reference features corresponding to one or more modulation modes of the physiological measurements in each of the time windows; for each modulation mode, extract a modulation of the corresponding reference features in each time window; process each extracted modulation to obtain a quality parameter for each combination of modulation mode and time window, the quality parameter representing how strongly the extracted modulation exhibits a waveform of the periodic physiological process; and process the extracted modulations to calculate the frequency of the periodic physiological process of the subject, wherein: the frequency of the periodic physiological process is calculated using only a subset of the extracted modulations, the subset being selected using the quality parameters, the frequency of the periodic physiological process is calculated using a combination of the extracted modulations, each extracted modulation having a weighting defined by the quality parameter of the extracted modulation, or the frequency of the periodic physiological process is calculated using only a subset of the extracted modulations, the subset being selected using the quality parameters, and the frequency of the periodic physiological process is calculated using a combination of the selected subset of extracted modulations, each one of the selected subset of extracted modulations having a weighting defined by the quality parameter of the extracted modulation; and wherein: the system is configured to distribute processing of the extracted modulations to calculate the frequency of a periodic physiological process of the subject between the device and the data processing station according to the quality parameters of the extracted modulations obtained by the device. a device comprising: a data receiving unit configured to receive plural time windows of data representing physiological measurements on the subject; a data processing unit; and a sensing system configured to perform the physiological measurements on the subject; and a data receiving unit configured to receive plural time windows of data representing physiological measurements on the subject; a data processing unit; and a sensing system configured to perform the physiological measurements on the subject; and a data processing station connectable via a network to the device, identify reference features corresponding to one or more modulation modes of the physiological measurements in each of the time windows; for each modulation mode, extract a modulation of the corresponding reference features in each time window; process each extracted modulation to obtain a quality parameter for each combination of modulation mode and time window, the quality parameter representing how strongly the extracted modulation exhibits a waveform of the periodic physiological process; and process the extracted modulations to calculate the frequency of the periodic physiological process of the subject, wherein: the frequency of the periodic physiological process is calculated using only a subset of the extracted modulations, the subset being selected using the quality parameters, the frequency of the periodic physiological process is calculated using a combination of the extracted modulations, each extracted modulation having a weighting defined by the quality parameter of the extracted modulation, or the frequency of the periodic physiological process is calculated using only a subset of the extracted modulations, the subset being selected using the quality parameters, and the frequency of the periodic physiological process is calculated using a combination of the selected subset of extracted modulations, each one of the selected subset of extracted modulations having a weighting defined by the quality parameter of the extracted modulation; the frequency of the periodic physiological process is calculated using only a subset of the extracted modulations, the subset being selected using the quality parameters, the frequency of the periodic physiological process is calculated using a combination of the extracted modulations, each extracted modulation having a weighting defined by the quality parameter of the extracted modulation, or the frequency of the periodic physiological process is calculated using only a subset of the extracted modulations, the subset being selected using the quality parameters, and the frequency of the periodic physiological process is calculated using a combination of the selected subset of extracted modulations, each one of the selected subset of extracted modulations having a weighting defined by the quality parameter of the extracted modulation;