Patent ID: 7006423

Claim:
An address mark detecting circuit for an optical disk used in detecting marks for address information from an optical disk that has tracks formed in a spiral shape with waviness having a fixed period comprising: a push-pull signal generator that differentially detects the returning light of an optical beam, which performs tracking for the tracks on said optical disk, by means of the push-pull method, and generates a push-pull signal; a first envelope detector that follows the bottom envelope or the top envelope of said push-pull signal, and generates a first envelope signal representing the waveform of said bottom envelope or top envelope; a threshold signal generator that provides a threshold signal for converting the level of said envelope signal to a binary form, comprising a second envelope detector that follows the bottom envelope or top envelope of said push-pull signal at a tracking speed lower than that in said first envelope detector and generates a second envelope signal that represents the waveform of said bottom envelope or top envelope, and a level shift circuit that performs level shifting for said second envelope signal by a prescribed value and takes the obtained signal as said threshold signal; and an address mark detecting signal generator that compares the level of said envelope signal with the level of said threshold signal, and generates an address mark detecting signal representing said address marks.