a signal filter method and apparatus for removing portions of a signal indicative of erroneous data , an exemplary apparatus including means for receiving the signal , means for adjusting a threshold by increasing the threshold a first predetermined amount when the signal is in a predetermined relationship with the threshold and for decreasing the threshold a second predetermined amount when the signal is in a second predetermined relationship with the threshold , and means for comparing the signal with the adjusted threshold and for transmitting the signal if the signal bears a first predetermined relationship to the threshold . disclosed herein are digital and analog implementations of the signal filter method and apparatus , particularly suited for use with catheter - type oximeter apparatus .