The present invention relates to a physiological monitor for determining blood oxygen saturation of a medical patient, comprising a sensor, wherein said sensor comprises at least three light emitting diodes, each light emitting diode adapted to emit light of a different wavelength and a detector, wherein said detector is adapted to receive light from said at least three light emitting diodes after being attenuated by tissue, and wherein said detector generates an output signal based at least in part upon said received light, a signal processor, wherein said signal processor determines blood oxygen saturation based at least upon said output signal, and a display, wherein said display provides an indication of said blood oxygen saturation, wherein the signal processor comprises a demodulation module (400), a sub-sampling module (402), a statistics module (404) and a saturation transform module (406), wherein the saturation transform module (406) is configured to receive packets for each wavelength of light from the sub-sampling module (402), wherein the saturation transform module (406) comprises a log module (537) configured to perform a logarithmic function on each of the packets and a bandpass filter module (538) configured to perform a bandpass filter on each of the packets, wherein the saturation transform module (406) further comprises a multi-variate process estimator (531) and a reference generator (530) configured to receive the outputs of the bandpass filtering module (538) and a plurality of possible saturation values and to generate a reference signal (N_ref) for use by the multi-variate process estimator (531).
