1. An oximeter for determining an estimate of the blood oxygen saturation level in the body of a patient, by processing red and infrared electrical signals acquired by a sensor arrangement upon application of red and infrared light towards a well perfused portion of the body of the patient, which electrical signals comprises a desired pulsatile component and an undesired noise component, comprising: a sensor arrangement for acquiring red and infrared electrical signals; a signal generator for operating independently of said acquired red and infrared electric signals, for providing a predetermined synthetic reference signal that is expected to exhibit a correlation with the desired pulsatile component of the acquired red and infrared electrical signals; an adaptive filter arrangement having a first input responsive to said acquired red and infrared electrical signals, and a second input responsive to said synthetic reference signal, said adaptive filter arrangement operating in a feedback manner so as to iteratively adjust the applied synthetic reference signal so as to progressively generate as an output of the filter arrangement more accurate approximations of the desired pulsatile component of the acquired red and infrared electrical signals; and a saturation estimate processor for processing said more accurate approximations of the desired pulsatile component of the acquired red and infrared electrical signals, to develop a saturation estimate of the blood oxygen saturation level in the body of the patient.