1. In a signal processor, a method of analyzing a measurement condition of a blood parameter of pulsing blood, the method comprising: receiving first and second intensity signals from a light-sensitive detector which detects light of at least first and second wavelengths attenuated by body tissue carrying pulsing blood; electronically transforming the first and second intensity signals into the frequency domain; electronically representing the first and second intensity signals mathematically in a manner which corresponds to graphically representing values of the first intensity signal with values of the second intensity signal on axes of a graph, wherein the mathematical representation comprises a plurality of ratios of values of the transformed first intensity signal to values of the transformed second intensity signal; electronically determining, based at least in part upon the plurality of ratios, one or more values of the transformed first and second intensity signals at nonzero frequencies that correspond to patient motion-induced noise and one or more values that correspond to physiological information; electronically calculating a blood parameter estimate using the one or more values that correspond to physiological information; and electronically determining, using one or more physiologically-based rules, a confidence level representing a degree of confidence in the accuracy of the blood parameter estimate.