Constant linear velocity recording servo circuit for write-once type compact disk

Claim 1

A servo circuit for controlling a compact disk in a constant linear velocity, the compact disk having a spiral guide groove wobbled according to a wobbling signal of a predetermined carrier frequency which is frequency-modulated with a revolution synchronization signal, comprising: pick-up means for optically picking up the wobbling signal as a tracking error from the spiral guide groove on the disk; filter means for frequency-filtering the wobbling signal picked-up by said pick-up means; means for extracting the carrier frequency signal from the filtered wobbling signal in the form of an extracted carrier signal; means for demodulating the filtered wobbling signal to regenerate the revolution synchronization signal; control means for comparing the frequency of a control signal with a reference signal and controlling a spindle motor with the comparison results so that the control signal coincides with the reference signal in frequency, the spindle motor revolving the compact disk, and a lock signal being generated upon the coincidence in said comparison; and switch means for selectively applying one of the extracted carrier signal and the regenerated revolution synchronization signal as the control signal to said control means, wherein upon starting the spindle motor, said switch means applies the extracted carrier signal to said control signal and in response to the lock signal applies the revolution synchronization signal to said control signal in place of the extracted carrier signal.