Evaluation of the Effect of Step Size on Delta Modulation for Photoplethysmogram Compression

Abstract

In this paper, delta modulation (DM) is employed as a compression technique for a high-resolution photoplethysmogram (PPG) signal. To accommodate both clean PPG and signals affected by motion artifacts, the effect of step size is evaluated on the performance of DM in order to optimize this technique before it can be deployed in a wireless data acquisition system. To this end, the PPG was recorded using 16-bit analog-to-digital converter (ADC) at a 1000 Hz sampling rate. In order to take into consideration the effect of the DC and AC of the PPG during the performance evaluation, both the PRMSAC+DC (with DC component) and PRMSAC (without DC component) were estimated. As expected, results show that the PRMSAC+DC was lowerthan PRMSAC at all step sizes. Simulation results show that for clean PPG free of motion artifact the optimum step size required to decompress the PPG is V/(2-15-1) with a PRMSAC = 4.7%; PRMSAC+DC = 0.1%, where V is the dynamic range of the ADC. For PPG affected by motion artifact, the optimum step size required to decompress the PPG is V/(2-14-1) with a PRMSAC = 4.9% and PRMSAC+DC = 0.12%. The closeness of these values to the finest possible step size V/(2-16-1) can be explained by the relatively high sampling rate compared to the Nyquist frequency of the PPG.