1. Field of the Invention
The present invention relates to a method and an apparatus in which a bias of a focus servo circuit is adjusted so that a signal surface of an optical disc may be placed within a focus depth of a laser beam.
2. Description of the Prior Art
Conventionally, after irradiating a laser beam onto a signal surface of an optical disc, an optical disc player converts light reflected therefrom into an electric signal, and thereby reads out data which is recorded on the optical disc.
In the optical disc player known heretofore, it is customary to execute the following servo control.
For example, a light beam emitted from a light source is irradiated to the surface of an optical disc, and a return light beam from the optical disc is received by a photo detector.
A focus error signal and a tracking error signal are detected on the basis of the outputs of focus sensors which are divisions of the photo detector.
The focus error signal and the tracking error signal thus detected are supplied to a focus servo circuit and a tracking servo circuit, respectively and thereby execute focus servo control and tracking servo control.
FIG. 1 is a block diagram for showing a circuit configuration of a conventional optical disc player. As shown in FIG. 1, an optical disc player 1 comprises an optical pickup 2, a tracking servo circuit 5 and a focus servo circuit 6 which receive, via amplifier 3 and 4 respectively, a tracking error signal and a focus error signal obtained on the basis of the difference between the light quantities of individual light receiving elements of a photo detector in optical pickup 2.
Optical disc player 1 further comprises a tracking driver 7 for driving an actuator of optical pickup 2 under control on the basis of a servo signal from tracking servo circuit 5 to thereby move an objective lens in the tracking direction, a focus driver 8 for driving the actuator of optical pickup 2 under control on the basis of a servo signal from focus servo circuit 6 to thereby move the objective lens in the focusing direction, and an adding circuit 9A for applying a focus bias, which is obtained from the focus bias generator circuit 9, to the focus error signal outputted from amplifier 4.
Optical pickup 2 has a known structure wherein an objective lens (not shown) is held to be movable biaxially, so that when a tracking coil and a focus coil provided in the actuator are fed with current, the objective lens can be driven biaxially under control in both of the tracking and focusing directions.
Tracking servo circuit 6 is applied with the tracking error signal obtained from optical pickup 2 and amplified by amplifier 3, and then outputs a tracking control signal to driver 7 in accordance with the tracking error signal so as to minimize the tracking error.
Focus servo circuit 6 is supplied with the focus error signal obtained from optical pickup 2 and amplified by amplifier 4, and then outputs a focus control signal to driver 8 in accordance with the focus error signal so as to minimize the focus error.
Driver 7 serves to drive the actuator of optical pickup 2 in response to the tracking control signal received from tracking servo circuit 5 and thereby moves the objective lens in the tracking direction to minimize the tracking error.
Meanwhile, driver 8 serves to drive the actuator of optical pickup 2 in response to the focus control signal received from focus servo circuit 6 and thereby moves the objective lens in the focusing direction to minimize the focus error.
Since the minimum point of the focus error signal may sometimes fail to coincide with the least jitter point of a reproduced signal, a focus bias obtained from the focus bias generator circuit 9 is applied to the focus error signal for causing the minimum point of the focus error signal to coincide with the least jitter point of a reproduced signal.
In optical disc player 1 of the structure mentioned above, the focus error bias is adjusted in the following manner.
At the time of assembling optical disc player 1, focusing is performed in an on-state of the focus servo, and an adjusting rheostat 9B incorporated in the focus bias generator circuit 9 is manually operated while observing the RF signal from optical pickup 2 and monitoring the value of the jitter, whereby an optimal focus bias is determined with respect to the individual optical disc player 1.
However, in the optical disc player of the above structure, a time required for determining the focus bias is long with another disadvantage of necessitating the adjusting rheostat 9B, hence raising a problem of higher cost with regard to the component parts.
Further, in an operation of recording data on and/or reproducing the same from the optical disc, the refractive index of the optical disc to an incident light beam is rendered different if the material of the disc is different, so that the incident light quantity of the return light beam to the photo detector may also be changed. In addition, occurrence of an ambient temperature fluctuation brings about some harmful influences inclusive of a positional deviation of the objective lens due to the resultant temperature fluctuation in the apparatus.
Also, after the focus bias is determined by an experiment and the like, a determined focus bias is stored in a memory and the like which is included in the optical disc player. However, when the bias is thus pre-set, the focus bias is changed due to the aging of the optical disc player and thereby may incur the malfunction of the optical disc player.
Consequently, it become difficult to adjust the focus bias exactly to its optimal value in the reproduction of data from the optical disc.