Method and apparatus for demodulating signals in a pulse oximetry system
View Patent ↗A method and an apparatus measure blood oxygenation in a subject. A light source is activated to cause a first emission at a first wavelength and a second emission at a second wavelength. A detector detects a composite signal indicative of an attenuation of the first and second wavelengths by tissue of a patient. The composite signal is demodulated into a first intensity signal and a second intensity signal. Blood oxygenation in the subject is determined from the first and second intensity signals. In one embodiment, demodulation is based at least in part on a period when at least one of the wavelengths is not activated. In one embodiment, a modulation of the first and second wavelengths is determined in order to avoid frequencies of ambient noise. In one embodiment, the composite signal's sampling rate is reduced before and/or after demodulation.
1. A method of demodulating a detected light signal representative of light attenuated by tissue of a patient, the method comprising:
receiving a composite signal representative of an attenuation of a first and second emissions by body tissue;
adaptively demodulating said composite signal into at least a first intensity signal representative of said attenuation of said first emission by body tissue and a second intensity signal representative of said attenuation of said second emission by body tissue;
and
determining a physiological measurement based on the first and second intensity signals.
2. The method of claim 1 , further comprising reducing a sampling rate of said first and second intensity signals.
3. The method of claim 2 , further comprising reducing a sample rate of said composite signal before demodulating said composite signal.
4. The method of claim 2 , wherein reducing a sample rate comprises decimating.
5. The method of claim 1 , wherein adaptively demodulating comprises adjusting demodulation parameters during operation.
6. The method of claim 5 , wherein the demodulation parameters comprise a decimation rate.
7. The method of claim 1 , wherein adaptively demodulating comprises using feedback.
8. The method of claim 7 , wherein adaptively demodulating using feedback comprises using feedback from previously demodulated signals.
9. The method of claim 1 , further comprising low pass filtering and sample rate compressing the composite signal.
10. The method of claim 1 , further comprising low pass filtering and sample rate compressing the first and second intensity signals.
11. A system for demodulating a detected light signal representative of light attenuated by tissue of a patient, the system comprising:
an input for receiving a composite signal representative of an attenuation of a first and second emissions by body tissue;
a demodulation module configured to adaptively demodulate said composite signal into at least a first intensity signal representative of said attenuation of said first emission by body tissue and a second intensity signal representative of said attenuation of said second emission by body tissue;
and
a processor configured to determine and output a physiological measurement based on the first and second intensity signals.
12. The system of claim 11 further comprising a first reducing module configured to reduce a sample rate of said composite signal.
13. The system of claim 12 , wherein said first reducing module is configured to decimate said composite output signal.
14. The system of claim 12 further comprising a second reducing module configured to reduce a sample rate of said first and second intensity signals.
15. The system of claim 14 , wherein said second reducing module is configured to decimate said first and second intensity signals.
16. The system of claim 11 , wherein the demodulation module adaptively demodulates by adjusting demodulation parameters during operation.
17. The system of claim 16 , wherein the demodulation parameters comprise a decimation rate.
18. The system of claim 11 , wherein the demodulation module adaptively demodulates by using feedback.
19. The system of claim 18 , wherein the demodulation module adaptively demodulates by using feedback from previously demodulated signals.
20. The system of claim 11 , further comprising a low pass filter module and a sample rate compression module.