Margins of disk-tilt and lens-tilt for a phase-change optical head were examined digital recording at a bit length of half of a laser wavelength. The margins for an objective lens of numerical aperture 0.6 for 0.6 mm in disk-thickness were more than three times that of the lens with a 1.2mm disk-thickness according to characteristics of bit-error-rate using partial response maximum likelihood of three-level Viterbi decoding.
