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, a method of determining a measurement value for a characteristic of arterial blood, the method comprising:
receiving a first signal from a light-sensitive detector, the first signal being indicative of light of a first wavelength that has been attenuated over a period of time by body tissue carrying arterial blood;
receiving a second signal from the light-sensitive detector, the second signal being indicative of light of a second wavelength that has been attenuated over the period of time by the body tissue carrying arterial blood;
electronically identifying peak frequency components of the first and second signals;
electronically determining a slope value corresponding to each of one or more of the peak frequency components, each slope value comprising a comparison of the respective amounts of the corresponding peak frequency component in the first and second signals;
electronically selecting a peak frequency component that exhibits physiological characteristics of arterial blood using, at least in part, the corresponding slope value; and
electronically determining the measurement value for the characteristic of arterial blood using the selected peak frequency.
2. The method of claim 1 , wherein the blood parameter comprises the pulse rate of the arterial blood.
3. The method of claim 1 , wherein the blood parameter comprises the oxygen saturation of the arterial blood.
4. The method of claim 1 , wherein each slope value comprises a ratio of the amount of the corresponding peak frequency component in the first signal to the amount of that frequency component in the second signal.
5. The method of claim 1 , further comprising determining a confidence factor associated with the selected peak frequency component.
6. The method of claim 1 , further comprising calculating the Fourier transform of the first and second signals.
7. The method of claim 1 , wherein selecting a peak frequency component that exhibits physiological characteristics of arterial blood comprises determining whether at least a subset of the peak frequency components of the first and second signals are harmonically related.
8. The method of claim 1 , wherein selecting a peak frequency component that exhibits physiological characteristics of arterial blood comprises determining whether a pulse rate corresponding to one or more of the peak frequency components is above a selected threshold.
9. The method of claim 1 , wherein selecting a peak frequency component that exhibits physiological characteristics of arterial blood comprises determining whether a pulse rate corresponding to one or more of the peak frequency components is below a selected threshold.
10. The method of claim 1 , wherein selecting a peak frequency component that exhibits physiological characteristics of arterial blood comprises determining whether a pulse rate corresponding to one or more of the peak frequency components is within a predetermined range.
11. The method of claim 1 , wherein selecting a peak frequency component that exhibits physiological characteristics of arterial blood comprises comparing the slope values of the peak frequency components.
12. A signal processor apparatus for determining a measurement value for a characteristic of arterial blood, the signal processor apparatus comprising:
an input port to receive a first signal from a light-sensitive detector, the first signal being indicative of light of a first wavelength that has been attenuated over a period of time by body tissue carrying arterial blood, and to receive a second signal from the light-sensitive detector, the second signal being indicative of light of a second wavelength that has been attenuated over the period of time by the body tissue carrying arterial blood;
a processor programmed to perform a method comprising,
electronically identifying peak frequency components of the first and second signals,
electronically determining a slope value corresponding to each of one or more of the peak frequency components, each slope value comprising a comparison of the respective amounts of the corresponding peak frequency component in the first and second signals,
electronically selecting a peak frequency component that exhibits physiological characteristics of arterial blood using, at least in part, the corresponding slope value, and
electronically determining the measurement value for the characteristic of arterial blood using the selected peak frequency; and. an output port for outputting the measurement value.
13. The signal processor apparatus of claim 12 , wherein the blood parameter comprises the pulse rate of the arterial blood.
14. The signal processor apparatus of claim 12 , wherein the blood parameter comprises the oxygen saturation of the arterial blood.
15. The signal processor apparatus of claim 12 , wherein each slope value comprises a ratio of the amount of the corresponding peak frequency component in the first signal to the amount of that frequency component in the second signal.
16. The signal processor apparatus of claim 12 , wherein the processor is further programmed to determine a confidence factor associated with the selected peak frequency component.
17. The signal processor apparatus of claim 12 , further comprising calculating the Fourier transform of the first and second signals.
18. The signal processor apparatus of claim 12 , wherein selecting a peak frequency component that exhibits physiological characteristics of arterial blood comprises determining whether at least a subset of the peak frequency components of the first and second signals are harmonically related.
19. The signal processor apparatus of claim 12 , wherein selecting a peak frequency component that exhibits physiological characteristics of arterial blood comprises determining whether a pulse rate corresponding to one or more of the peak frequency components is within a predetermined range.
20. The signal processor apparatus of claim 12 , wherein selecting a peak frequency component that exhibits physiological characteristics of arterial blood comprises comparing the slope values of the peak frequency components.