A processing apparatus for processing a physiological signal is presented. The processing apparatus is configured to perform the steps of: obtaining the physiological signal (6) containing at least two wave cycles originating from a physiological process (6b), obtaining a feature signal (12) descriptive of occurrences of a feature (P) in the physiological signal (6), determining an average waveform of the physiological signal (6) around said occurrences of the feature (P) in the physiological signal (6), and determining a model signal (18) comprising amplitude-scaled instances of the average waveform of the physiological signal (6) placed at the occurrences of the feature (P) in the physiological signal (6). Furthermore, a corresponding processing method, a system (1) for determining a vital sign of a subject comprising said processing apparatus and a computer program are presented.
