Recommendation for Measuring Digital Volume Pulse in Mobile Application: For Healthy Normal Subject

The aim of this study was to identify changes in photoplethysmogram waveform with decreasing sampling frequency and quantization bit depth and suggest appropriate criteria for measurements. We performed down-sampling and re-quantization of phoplethysmogram measured at high resolution. Changes of waveform were quantitatively observed. Photoplethysmograms recorded at a 1-kHz sampling frequency and 16-bit quantization bit depth were converted to signals with sampling frequencies of 500-, 250-, 100-, 50-, 25-, and 10-Hz by means of down-sampling. We then performed re-quantization to convert the quantization bit depth into 16, 14, 12, 10, 8, and 6 bits for each down-sampled signal. Degradation of signal was quantified in terms of morphological change and feature-point deviation. Morphological analysis revealed that the correlation coefficient was decreased while normalized root mean square error was increased with decreasing sampling frequency and quantization bit depth. Feature-point analysis revealed that the mean absolute time error and amplitude error of feature points tended to increase with decreasing sampling frequency and quantization bit depth. Although there were differences according to pulse onset and systolic peak, sampling frequency ≥ 250-Hz and 16-bit quantization bits were required in order to have ≤ 1 ms of timing error and a normalized amplitude error ≤1%. In addition, sampling frequency ≥ 100-Hz and 12-bit quantization bits are recommended to have feature-point time errors and amplitude errors < 10 ms and a normalized amplitude error < 10%.