the optical pick up head is then derived wherein the value of pi po is a target for the quality of the reproduced signal please refer to fig1 a diagram showing the relative value of pi po to the reference values for dac is illustrated wherein the first parameters of the horizontal axis are the reference values for dac which are converted to the analog voltages for controlling the optical pick up head to compensate the tilt the second parameters of the vertical axis are the corresponding value of pi po which is a target for the quality of the reproduced signal referred to fig1 each of the first parameters indicates a predetermined voltage applied to the optical pick up head for compensating the tilt each of the first parameters can also be derived in accordance with the pi threshold 110 referred to fig1 one part of the first parameters can be derived by a first sample rate when the corresponding values of pi po are larger than pi threshold 110 accordingly the other part of the first parameters can be derived by a second sample rate when the corresponding values of pi po are smaller than pi threshold 110 wherein the first sample rate is smaller than the second sample rate as a result a parabolic curve as illustrated in fig1 can be derived by each of the first parameters and the corresponding values of pi po in the above first sample rate and second sample rate the larger of the values of pi po means the worse of the quality of the reproduced signal when the values of pi po are large as the point a.