Composite Encoding Method for Spectral PPG Signal Acquisition Based on Color Camera

In the instrument system for noninvasive quantitative analysis of blood components by dynamic spectrum (DS), there are problems with low signal-to-noise ratio (SNR) and wide bandwidth occupied by carriers in the measurement of photoplethysmography (PPG) signals. To solve the above problems, this article uses a color camera as a sensor and proposes a composite coding modulation method that combines the color channels of the three primary colors and the frequency-division multiplexing (FDM) method of direct current plus co-frequency orthogonal square waves. After demodulation, multiple wavelengths of PPG signals with high SNR are obtained. In the experiment, this article uses the proposed composite coding method to obtain nine wavelengths of human PPG signals. Comparing the results of modulation and demodulation with those of unmodulated ones, the results after modulation and demodulation are better, which verifies the effectiveness of the method proposed in this article. This article conducts a simulation experiment of simulating human PPG signals and uses the DS of human serum extracted from simulated human PPG signals to establish models with the concentrations of total protein (TP), globulin (GLB), and albumin (ALB). The correlation coefficients (R) of the predictive performance of the TP model and the GLB model were above 0.80, and R of the predictive performance of the ALB model was above 0.70, which verified the feasibility of the method in quantitatively analyzing the components of human blood. The results show that this method can achieve the purpose of quickly acquiring high-precision PPG signals at multiple wavelengths with a lower carrier-occupied bandwidth, which has a reference role in the acquisition of multichannel signals. It also has great application value in the field of noninvasive measurement of human blood components.