Patent ID: 7349311

Claim:
A method of detecting a radial tilt of a disc, the method comprising: comparing phases of first summed signals obtained from a first plurality of signals that are generated when first light-receiving units of a photo diode receive light from a laser beam that is reflected from a surface of the disc to generate first phase comparison signal; comparing phases of second summed signals obtained from a second plurality of signals that are generated when second light-receiving units of the photo diode receive light from the laser beam that is reflected from the surface of the disc to generate a second phase comparison signal; and detecting the radial tilt based on a phase difference of the second and first phase comparison signals by reading a level value of the second phase comparison signal when a level value of the first phase comparison signal is substantially zero, wherein the zeroing of the level value of the first phase comparison signal occurs when the laser beam crosses a point signifying a ½ track pitch of a track on the disc.