A Unified Measure of Optical Disk Performance: Phase Margin Analysis

The characterization of optical disk performance has evolved from video measurements of flat-field signal-to-noise ratio and video dropouts to measurements of carrier-to-noise ratio (in a 30 kHz bandwidth) and bit error rate. Although these measurements are useful, they do not present a complete picture of system performance. For example, the carrier-to-noise ratio does not give information about intersymbol interference, and the bit error rate does not give an indication of how close to failure the system may be, i.e., how much phase margin remains. There exists a powerful method known as phase margin analysis that has been effectively used for the diagnosis of head-disk interfaces in magnetics technology.1 The technique essentially consists of measuring the bit error rate as a function of decoder window width relative to the bit cell. This information fully characterizes the channel performance, combining the effects of wide-band signal-to-noise ratio, intersymbol interference, and "hard" and "soft" defects in one easily read display.