Digital Filter for Photoplethysmography Ambient Light Cancellation
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Solution Overview
Problem
Existing digital filtering methods for photoplethysmography (PPG) devices face challenges in effectively eliminating ambient light, particularly in implementing multi-order filters using analog circuits or hardware.
Innovation Solution
A digital filtering method that samples mixed-light and ambient-light signals multiple times to generate digital values, which are then processed using a digital filter of order (M+N−1) to separate and eliminate ambient light components, utilizing coefficients to generate output values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If analog circuits or hardware are used to filter ambient light, then ambient light can be filtered out, but multi-order filters cannot be implemented
Solution Approach 1:
The patent replaces analog mechanical filter circuits with a digital filtering system. The digital filter processes sampled light signals through software algorithms, enabling multi-order filtering that cannot be achieved with analog hardware while maintaining ambient light rejection capability and improving measurement precision.
Solution Approach 2:
The patent changes the filtering approach from fixed analog filter parameters to adjustable digital filter coefficients. By modifying the digital filter's order and coefficients based on signal characteristics, the system achieves superior ambient light filtering without the hardware complexity constraints of analog implementations.
2Measurement precision
If digital filtering is used to eliminate ambient light, then multi-order filtering can be implemented, but processing time and computational complexity increase
Solution Approach 1:
The patent performs preliminary sampling of the light signal at multiple time points before applying the digital filter. By pre-acquiring M mixed-light samples and N ambient-light samples, the system prepares data in advance that reduces the computational burden during the actual filtering process, achieving high precision ambient light cancellation while controlling processing time.
Solution Approach 2:
The patent segments the filtering process into distinct stages: separate sampling of mixed-light and ambient-light signals, independent digital filtering of each component, and final combination. This segmentation allows optimized processing of each stage and reduces overall computational complexity compared to a single monolithic filter.
3Measurement precision
If multiple sampling points are used for filtering, then ambient light can be effectively eliminated, but the number of processing steps increases
Solution Approach 1:
The patent designs a universal digital filter that handles multiple functions: it processes both mixed-light signals and ambient-light signals, applies multi-order filtering, and combines the results to extract pure optical absorption data. This multi-functional approach achieves high measurement precision without proportionally increasing process complexity.
Solution Approach 2:
The patent uses the sampled ambient-light signals as feedback to correct the mixed-light measurements. By continuously comparing and adjusting based on the ambient-light samples, the digital filter accurately compensates for ambient light effects, improving optical absorption measurement accuracy while maintaining manageable processing complexity through iterative refinement.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method achieves excellent ambient light cancellation, with an improvement from conventional methods' -70 dB to -76 dB to -82 dB, ensuring accurate optical absorption measurements by effectively filtering out ambient light.
Implementation Method 1
measuring the consequent variation in optical absorption
Implementation Method 2
using the digital filter to multiply the M mixed-light digital value(s) by M coefficient(s) respectively
Data Source
AI summary
A digital filtering method is applicable to a photoplethysmography (PPG) device. The PPG device samples a mixed-light signal M time(s) to obtain M mixed-light digital value(s), and samples an ambient-light signal N time(s) to obtain N ambient-light digital value(s), wherein each mixed-light digital value includes a controllable-light component and an ambient-light component. The method includes: preparing a digital filter whose filter order is (M+N−1); using the digital filter to multiply the M mixed-light digital value(s) by M coefficient(s) respectively and thereby generate M value(s); using the digital filter to multiply N ambient-light digital value(s) by N coefficient(s) respectively and thereby generate N value(s); and using the digital filter to add up the M value(s) and the N value(s) and thereby generate an output value.