Chong, K. S., K. B. Gan, and M. A. Mohd. Alauddin. “Development of a Two-Channel Simultaneous Photoplethysmography Recording System”. Journal of ICT Research and Applications, vol. 6, no. 2, Sept. 2013, pp. 171-82, doi:10.5614/itbj.ict.2012.6.2.5.