Signal processing apparatus and method
View Patent ↗A method and an apparatus to analyze two measured signals that are modeled as containing desired and undesired portions such as noise, FM and AM modulation. Coefficients relate the two signals according to a model defined in accordance with the present invention. In one embodiment, a transformation is used to evaluate a ratio of the two measured signals in order to find appropriate coefficients. The measured signals are then fed into a signal scrubber which uses the coefficients to remove the unwanted portions. The signal scrubbing is performed in either the time domain or in the frequency domain. The method and apparatus are particularly advantageous to blood oximetry and pulserate measurements. In another embodiment, an estimate of the pulserate is obtained by applying a set of rules to a spectral transform of the scrubbed signal. In another embodiment, an estimate of the pulserate is obtained by transforming the scrubbed signal from a first spectral domain into a second spectral domain. The pulserate is found by identifying the largest spectral peak in the second spectral domain.
1. In a signal processor for processing at least two measured signals M 1 and M 2 , where said signal M 1 comprises a physiological signal portion S 1 and may comprise a noise portion N 1 , and where said signal M 2 comprises a physiological signal portion S 2 and may comprise a noise portion N 2 , a method comprising:
determining a value for a coefficient c used in a combination of M 1 and M 2 such that the combination of M 1 and M 2 approximates S 1 , wherein determining the coefficient c comprises comparing frequency domain representations of the two measured signals M 1 and M 2 ;
using said coefficient c to remove at least some of the noise portion N 1 from the measured signal M 1 , thereby producing an approximation A 1 to said physiological signal S 1 ; and
determining a measured output value for one or more physiological parameters based at least in part on said approximation A 1 .
2. The method of claim 1 , where A 1 , M 1 and M 2 comprise frequency domain signals.
3. The method of claim 2 , wherein S 1 is indicative of a heart plethysmograph, further comprising calculating a pulse rate of the heart.
4. The method of claim 1 , further comprising displaying A 1 on a display.
5. The method of claim 1 , wherein said physiological parameter comprises arterial oxygen saturation.
6. The method of claim 1 , further comprising determining, based upon the comparison of M 1 and M 2 , whether M 1 comprises N 1 .
7. The method of claim 6 , wherein said value for the coefficient c is selected to reduce its effect if M 1 does not comprise N 1 .
8. The method of claim 1 , wherein the combination of M 1 and M 2 comprises a linear combination of M 1 and M 2 .
9. The method of claim 8 , wherein the linear combination of M 1 and M 2 comprises cM 1 −M 2 such that A 1 =cM 1 −M 2 .
10. The method of claim 9 , wherein an error value, e, given by the relation e=S 1 −(cM 1 −M 2 ) is reduced.
11. The method of claim 1 , wherein comparing the two measured signals M 1 and M 2 comprises calculating ratios of values of a frequency domain representation of M 1 to corresponding values of a frequency domain representation of M 2 .
12. The method of claim 11 , wherein said ratios are calculated using values that correspond to peaks in said frequency domain representations of M 1 and M 2 .
13. The method of claim 11 , further comprising determining whether the ratios have a predetermined degree of similarity.
14. The method of claim 1 , comprising displaying the measured output value on a display.