The invention concerns a tracking circuit for an optical pick-up wherein the tracking error TE is obtained from the difference between two photovoltages.
The design and function of an optical pick-up are described in Electronic Components and Applications 6 (1984), 4, 209-15.
The beam of light emitted from the laser diode is focused on the compact disk by means of lenses and reflected onto a photodetector. The data stored on the disk and the actual values for the focusing and tracking circuits are obtained from the signal leaving the photodetector. The aforecited reference calls the actual value for the focusing circuit the "focusing error" and the actual value for the tracking circuit the "radial tracking error."
The positioning mechanism in the focusing circuit is a coil with an objective lens traveling along its optical axis. The focusing circuit displaces the objective lens in such a way that the beam of light emitted from the laser diode is always focused on the compact disk. The tracking circuit, which is often called a "radial drive mechanism," displaces the optical pick-up radially in relation to the compact disk. This measure makes it possible to guide the beam of light along the spiral data-storage tracks on the disk.
The radial drive mechanism in some equipment comprises what are called a coarse drive mechanism and a fine drive mechanism. The coarse drive mechanism is for example a spindle that radially displaces the overall optical pick-up comprising the laser diode, the lenses, the prism beam divider, and the photodetector. The fine drive mechanism tilts the beam of light radially at a slight prescribed angle, an action that in itself displaces the beam slightly along the radius of the disk.
The unobjectionable playback of the data--whether video and audio in a videodisk player or audio alone in a compact-disk player--requires not only that the beam of light be precisely focused on the videodisk or compact disk but also that it be precisely guided along the data-storage tracks.
FIGS. 1a-1e illustrate the photodetector PD in the optical pick-up of a compact-disk player wherein three laser beams L1, L2, and L3 are focused on a compact disk. Laser beams L1 and L2 are refraction beams of the +1st and -1st order. A pick-up of this type is called a "three-beam pick-up" in the aforecited reference.
The photodetector PD comprises four adjacent square photodiodes A, B, C, and D in the form of a larger square. Diagonally opposite the larger square consisting of the four photodiodes are two other square photodiodes E and F.
Central laser beam L1, which is focused on photodiodes A, B, C, and D, generates a data signal HF=AS+BS+CS+DS and a focusing error FE=(AS+CS)-(BS+DS). The two outer beams of light L2 and L3, the farther forward of which, L2, strikes photodiode E and the farther to the rear of which, L3, strikes photodiode F, generate a tracking error TE=ES-FS. AS, BS, CS, DS, ES, and FS represent the photovoltages of the photodiodes A, B, C, D, E, and F. Since there is an astigmatically active collimator lens in the path of the central laser beam L1 in the optical pick-up, the beam will strike the large square comprising photodiodes A, B, C, and D in the shape of a circle when it is precisely focused and in the shape of an ellipse when it is out of focus.
FIG. 1a illustrates the situation in which both focusing and tracking are precise. This situation will be discussed later herein. Since the spot of light projected onto the large square by central laser beam L1 is circular, focusing error FE=(AS+CS)-(BS+DS)=0. The zero indicates to the focusing circuit that the focus is precise.
FIG. 1b illustrates the out-of-focus situation in which the objective lens is too far from the compact disk. Focusing error FE=(AS+CS)-(BS+DS)&lt;0. The negative value indicates to the focusing circuit that the objective lens is too far away from the disk. The positioning mechanism in the focusing circuit accordingly displaces the lens toward the disk until focusing error FE becomes zero.
FIG. 1c illustrates the other out-of-focus situation in which the objective lens is too near the compact disk. Focusing error FE=(AS+CS)-(BS+DS)&gt;0. The positive value indicates to the focusing circuit that the objective lens is too near the disk. The positioning mechanism in the focusing circuit accordingly displaces the lens away from the disk until focusing error FE becomes zero.
How the tracking circuit carries out the tracking will now be described.
The laser beams L1, L2, and L3 in FIGS. 1a, 1b, and 1c are precisely on track. Tracking error TE=ES-FS=0.
FIG. 1d illustrates the situation in which laser beams L1, L2, and L3 are to the right of the track. The tracking error becomes negative: TE=ES-FS&lt;0. The positioning mechanism in the tracking circuit displaces the optical pick-up to the left until the tracking error becomes zero.
In the opposite situation, in which the laser beams are to the left of the track, the tracking error is positive: TE=ES-FS&gt;0. The positioning mechanism in the tracking circuit displaces the optical pick-up to the right until the tracking error becomes zero.
When laser beam L1 and its associated refraction beams L2 and L3 travel over several data-storage tracks, tracking error TE assumes the sinusoidal characteristic illustrated in FIG. 2. If for example the three laser beams L1, L2, and L3 are jolted away from the data-storage track being scanned by a mechanical vibration, they can only be shifted back to the correct data-storage track when the track deviation is no greater than 0.8 .mu.m, half the distance between two adjacent data-storage tracks. Otherwise, the laser beams will engage the adjacent track.
Since it is possible to employ the subcode in a compact disk to determine whether the beam has skipped over to an adjacent track, laser beam L1 can be deflected back onto the correct data-storage track. Since the information signal for at least one block unit or even for a complete revolution is interrupted during deviations of this type, which are caused by mechanical vibrations of or impacts against the compact-disk player, audible interruptions in the music or speech are the consequence. Another drawback is that a relatively expensive program is necessary to deflect the beam of light back to the correct data-storage track by means of the subcode.
This means of correction, however, is not even available with unformated optico-magnetic disks and DRAW-disks.
Japanese Exposure 60 10 429 discloses a tracking circuit wherein the positive or negative envelope of the high-frequency signal indicates whether the beam of light is traveling over data-storage tracks. Since however, only the high-frequency interventions are counted by a counter, there is no way of determining what direction the beam of light is moving in, whether in toward the center of the disk or out toward its edge. The counter counts only the high-frequency interventions and accordingly the number of data-storage tracks traveled.
When mechanical impacts or vibrations occur, however, the eccentricity of the disk can result in errors in counting the data-storage tracks traveled. When for example the pick-up is displaced outward by an impact and when the data-storage tracks below the pick-up simultaneously also move outward more rapidly than the pick-up as the result of the eccentricity, the pick-up will actually be moving inward in relation to the data-storage tracks. Since the counter will count the same data-storage tracks several times in such a situation, a much wider deviation from the correct data-storage track will be indicated than is actually the case. The correct data-storage track can accordingly no longer be located without additional corrective measures.
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