Methods and systems are disclosed for defining a physiological parameter. A first physiological signal is transformed into in a complex transform space, the transformed signal having a magnitude and a phase. The transformed signal is rotated by altering its phase. The rotated signal is inverted, and the inverted signal is aligned in phase with a second physiological signal. The aligned inverted signal and the second physiological signal are combined to form a combined signal indicative of the physiological parameter.
