Optical disk apparatus

An optical disk apparatus comprising:

• a first optical disk having a first recording density and a second optical disk having a second recording density lower than the first recording density, each of the first optical disk and the second optical disk recording information in a form of pit streams;
• a light source configured to emit a light beam;
• an optical system having an objective lens configured to focus the light beam emitted from the light source on the optical disk;
• a light detector configured to detect light reflected from the optical disk to output a detected light sight signal, the light detector including a quadrant photodetector having four light detecting regions arranged in a matrix;
• a tracking error signal generator configured to generate a tracking error signal from the detected light signal by a differential phase detection method;
• a driver configured to drive the optical system for moving the beam spot in accordance with the tracking error signal and shifting a center of the beam spot from a centerline of the pit stream formed in the second optical disk by a predetermined distance in a radial direction of the second optical disk, by delaying a first sum signal of a diagonal two of the four light detecting regions with respect to a second sum signal of a diagonal remaining two of the four light detecting regions;
• and
• an information readout device configured to read out the information from the optical disk.

A method of reading an optical disk, the method comprising:

• providing an optical disk mounting device configured to selectively mount first and second optical disks which have a relatively high recording density and a relatively low recording density, respectively, and in each of which information is recorded in a form of pit streams;
• providing a light source configured to emit a light beam;
• focusing the light beam emitted from the light source to form a beam spot on the optical disk;
• detecting light reflected from the optical disk with a tracking device, wherein the tracking device includes a quadrant photodetector having four light detecting regions arranged in matrix;
• generating a tracking error signal from the light;
• moving the beam spot in accordance with the tracking error signal;
• selectively delaying a first sum signal of a diagonal two of the four light detecting regions with respect to a second sum signal of a diagonal remaining two of the four light detecting regions;
• shifting a center of the beam spot from a centerline of the pit stream formed in the second optical disk by a predetermined distance in a radial direction of the second optical disk based on the selectively delaying;
• and
• reading out the information from the optical disk.