A detector signal in a pulse oximeter is prefiltered, prior to processing so as to determine a physiological parameter value, so as to reduce the effects of artifact in the signal. An exemplary prefilter multiplies signal amplitude values by a quantitude that is inversely proportional to a substantially current amplitude so as to reduce the amplitude of large amplitude sections typically associated with artifact. In one implementation, the quantity is a ratio of an amplitude associated with a clean section of the signal by some exponential factor of a substantially current amplitude. In this manner, artifact affected portions of the signal are deemphasized in subsequent analyses.