Apparatus and methods are described for monitoring a subject. A sensor monitors the subject and generates a sensor signal in response thereto and a plurality of filters are used to filter the sensor signal using respective filter parameters. A computer processor receives the sensor signal, filters the signal with each of two or more of the filters, and in response to a quality of each of the filtered signals, selects one of the plurality of filters to filter the sensor signal. Subsequently, the computer processor detects that the subject has undergone motion, by analyzing the sensor signal, and in response thereto, filters the signal with each of two or more of the filters, and in response to a quality of each of the filtered signals, selects one of the plurality of filters to filter the sensor signal. Other applications are also described.