A direct mark edge detection scheme for readout in phase-change optical disk systems is described. The medium for edge detetection must be optimized to have a 90° phase difference between the amorphous mark and the crystalline space. Theoretical analysis and numerical simulation have shown that the readout signal using mark edge detection is as good as that using conventional detection of reflectivity for long marks and superior for short marks. Noise level at the output of differential edge detection is lower than that at the output of conventional detection. We also show experimental results that confirm these predictions. ©1997 American Institute of Physics.