1. A signal processor which determines an output oxygen saturation (SpO 2 ) value for a set of input SpO 2 values, the signal processor comprising: a buffer which stores a window of input SpO 2 values determined using a signal representative of one or more signals attenuated after emission into tissue at a measurement site; a signal confidence indicator; and a processor which receives the signal confidence indicator and which accesses the buffer to receive the input SpO 2 values, wherein the processor is configured to determine output SpO 2 values for the window of input SpO 2 values by adjusting the output SpO 2 values between smoothing characteristics and tracking characteristics, the tracking characteristics indicating a trend of the input SpO 2 values, wherein the adjustment is based on the signal confidence indicator.