Digital servo system with inverse non-linearity compensation
A servo system with in an optical disk drive with an inverse non-linearity compensation is disclosed. An error signal calculated from signals received from an optical pick-up unit can be offset by an offset value and amplified by a gain in the early stages of a calculation of a control signal for controlling a position of the optical pick-up unit. The non-linearity compensation adjusts the gain based on the offset value so that the response of the servo system to changes in the error signal is substantially linear. The error signal can be the tracking error signal in a tracking servo system and can be the focus error signal in a focus servo system.
1. A method of performing an inverse non-linearity compensation, comprising:
providing a plurality of photodetectors, each photodetector configured to provide a photodetector signal in response to receiving reflected light from an optical disk, the plurality of photodetectors thereby providing a plurality of photodetector signals;
receiving at a digital signal processor a digitized version of each of the photodetector signals in the plurality of photodetector signals;
in the digital signal processor, forming a digital focus error signal and a tracking error signal from the digitized versions;
adjusting the digital focus error signal with an offset value to form a digital offset focus error signal;
providing a gain to amplify the digital offset focus error signal so as to produce an amplified digital offset focus error signal, wherein the gain is based on the offset value such that the amplified digital offset focus error signal is substantially linear; and
amplifying the digital offset focus error signal with the gain to produce the amplified digital offset focus error signal.
2. The method of claim 1 , wherein the photodetectors each include a center element and two outside elements and the focus error signal is calculated from the sum of the signals from the two outside elements less the signal from the center element.
3. The method of claim 1 , wherein providing the gain includes obtaining a gain value from a look-up table, the look-up table being based on the overall response of a servo system to changes in the digital offset focus error signal as a function of the offset value.