1. Apparatus for estimating a physiological parameter from a physiological signal, comprising: a source of the physiological signal; a characteristic identification circuit, coupled to the physiological signal source, for identifying respective characteristics in the physiological signal; a plurality of likelihood circuits, coupled to the characteristic identification circuit, each circuit producing a plurality of signals, respectively corresponding to the identified characteristics, representing the likelihood that the corresponding identified characteristic represents said physiological parameter; an arbitrator, responsive to the likelihood representative signals, for selecting one of the identified characteristics; and a calculating circuit, for estimating the physiological parameter responsive to the selected one of the identified characteristics.