1. Field of the Invention
The present invention relates to an optical reproducing apparatus and more particularly to an optical reproducing apparatus suitable for use with an optical disc and which employs a laser beam for reproducing audio or video signals recorded in the form of pits or bumps suitably arranged along parallel, substantially circular tracks on a disc.
2. Description of the Prior Art
Optical disc players of the type described above typically include an optical pickup head having a focus servo mechanism and a tracking servo mechanism so as to properly trace or scan the parallel tracks which may be successive turns of a spiral. Generally, each track has a pit array which is not more than 1 m in width. The focus servo mechanism focuses the laser beam at the disc so as to form a beam spot thereon and the tracking servo mechanism shifts the laser beam spot in a radial direction of the disc. Additionally, the entire optical pickup head is moved in the radial direction through use of a motor-driven feed screw or a guide member.
In locating a particular or target track for reproducing the recorded information therein, hereinafter referred to as the access mode of operation of the player, the optical pickup head is moved rapidly to a desired position corresponding to a specific track address on the disc. In this access mode, a DC stepping voltage is applied to the motor for driving the feed screw or guide member of the optical pickup head for radially shifting the entire optical pickup head by one or more large steps. During such radial shifting of the head, data recorded on tracks within a control range (generally, .+-.0.2 to 0.4 mm) of the tracking servo mechanism is intermittently read out. As the optical pickup head approaches the desired or target track, the head is shifting by relatively smaller steps (e.g., track by track). As a result, the optical pickup head can accurately access the target track at a relatively high speed.
In the above described access mode, operation of the tracking servo mechanism follows the motor driven radial shifting of the optical pickup head. The tracking servo mechanism is reset every time the optical pickup head reaches the farthest point in the tracking range with the result that the laser beam spot is shifted or moved in a step-by-step manner. However, when the optical pickup head is radially shifted by a large step, it may "overrun" that is, move beyond the desired or target track, due to any one or more of the following reasons: play between the gears of the motor driven feed mechanism for shifting the optical pickup head; fluctuation in servo characteristics such as secondary resonance of the tracking servo mechanism; and delays in the operation of an arithmetic operational device for controlling the overall operation of the apparatus. Therefore, the access time required for reaching the target track may be prolonged during the subsequent smaller-step shifting from the overrun position.
Another proposed access mode employs jump pulses which are supplied to the tracking servo mechanism and by which the latter causes the laser spot beam to jump a substantial number of tracks, generally, 100 to 200 tracks, for each jump. Simultaneously, a jump pulse voltage is applied to the driving or feed motor of the optical pickup head, thereby to shift the entire optical pickup head after the laser spot beam has completed each jump.
In the last described access mode, the operation of the tracking servo mechanism does not follow the movement of the optical pickup head so that tracking can be performed without "overrunning" the target track. Therefore, the optical pickup head can be made to travel a relatively short access distance between the present track and the target track at a relatively high speed. However, since the jump range of the laser beam spot for each step is limited, a relatively long period of time is required for the laser beam spot to reach the target track when there is a large access distance, that is, a large distance between the present track and the target track. To enlarge the jump range of each step, a high loop gain of the tracking servo mechanism is required to compensate for the damping force of the tracking servo mechanism which occurs immediately after the beam jump. However, as the loop gain is increased, there is a corresponding increase in the servo phase angle, that is, the angle between the phase of the voltage from the photodetector of the tracking servo corresponding to a detected tracking error and the phase of the corresponding correcting voltage applied by the tracking servo, for example, to the coil of the galvano-mirror for correcting the tracking error. Further, if the servo phase angle is increased beyond a predetermined value, known as the servo phase allowance, the servo loop oscillates and stable operation of the tracking servo mechanism cannot be achieved.