Modelling the characteristics of a photodetector in a DVD player

This paper presents a new method for modelling the non-linear characteristics of a photodetector (so-called the S-curve) in a DVD player. The photodetector is used in the DVD player to generate the focus and tracking error signals, which in turn have to be controlled at the minimum levels. For focus error signal generation, an analytical and a numerical photodetector model are developed here, based on the astigmatic method and on opto-geometrical analysis. The influence of model parameters on the focus error signal is discussed. To estimate the unknown model parameters, a curve fitting method is applied, using measured data from an industrial DVD-video player developed in STMicroelectronics laboratories. Model quality is illustrated by a comparison with the real focus error signal.