Abstract:
An arrangement for selectively playing back and recording data in which a rotating disk-shaped recording medium is provided with data storage tracks. An optical system reads the stored data by guiding a light beam along the data storage tracks. The data read by the optical pickup system is decoded by a decoder which has an error output that emits a pulse for every bit error. The light beam strikes the disk-shaped recording medium at an angle and has a vector component which is tangential to the recording medium or disk, so that when the angle is 90°, the vector component is zero. The bit errors are applied to a counter that provides the corresponding bit-error rate and provides a signal for controlling the adjustment for varying the vector component so as to maintain the bit-error rate below a predetermined threshold.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns equipment for playing back and optionally also recording data that are read by an optical pick-up system by means of guiding a beam of light along the data-storage tracks of a rotating disk-shaped recorded medium and that are then decoded in a decoder that emits a pulse from its error output terminal for every bit error. 
     SUMMARY OF THE INVENTION 
     Equipment of this type, compact-disk players, optico-magnetic equipment for playback and recording, DRAW-disk recorders and players, and videodisk players for example, are equipped with an optical pick-up system consisting of a laser diode, several lenses, a prism beam divider, and a photodetector. The design and function of an optical pick-up are described on pages 209 to 215 of Electronic Components and Applications, Vol. 6, No. 4, (1984). 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The lenses focus the beam of light emitted by the laser diode onto the compact disk, whence it is reflected onto the photodetector. The data stored on the compact disk are obtained along with actual values for the focusing circuit and for the tracking circuit from the output signal from the photodetector. The actual value for the focusing circuit is called the &#34;focusing error&#34; and the actual value for the tracking circuit the &#34;radial tracking error&#34; in the aforesaid reference. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The component that controls the focusing circuit is a coil, which generates a magnetic field that displaces an objective lens along the optical axis. The focusing circuit, in displacing the lens, keeps the beam of light emitted by the laser diode constantly focused on the compact disk. The tracking circuit, which is also often called the &#34;radial drive mechanism,&#34; displaces the optical pick-up system radially in relation to the compact disk, keeping the beam of light positioned on the spiral data-storage tracks on the disk. 
     The radial drive mechanism in some equipment comprises a &#34;coarse drive mechanism&#34; and a &#34;fine drive mechanism.&#34; The coarse drive mechanism can for example be a spindle that radially displaces the overall optical pick-up system, consisting of the laser diode, the lenses, the prism beam divider, and the photodetector. The fine drive mechanism tilts the beam of light radially at a small prescribed angle, allowing it to travel a short distance along the radius of the compact disk as the result of the tilting motion alone. In other words, the fine drive mechanism varies the radial vector component of the beam of light in relation to the compact disk. The tangential angle at which the beam strikes the disk, on the other hand, is always 90° C. The vector component of the beam of light tangential to the compact disk is accordingly zero. 
     The data signals emitted by the photodetector are, as already mentioned herein, decoded in a decoder that emits a pulse at its error output terminal, the C1F1 output terminal, at every bit error. 
     The number of bit errors depends, among other factors, on the angle at which the beam of light strikes the compact disk at an angle thereto. Since a compact disk, however, is not perfectly flat and circular but is subject to deviations in tolerance, the right angle at which the beam of light tangentially strikes the compact disk cannot be optimally established for all scanning points when the compact-disk player is manufactured. This situation increases the number of bit errors. 
     The object of the invention, accordingly, is to decrease the number of bit errors in equipment for playing back data. 
     This object is attained in accordance with the invention in that a signal for controlling an adjustment stage is obtained from the bit-error rate and is employed to vary the vector component of the beam of light that is tangential to the recorded medium. 
    
    
     In the drawings, 
     FIG. 1 illustrates one embodiment of the invention and 
     FIG. 2 illustrates another embodiment of the invention. 
    
    
     The embodiment illustrated in FIG. 1 will now be specified and its function explained. 
     What is called the error output terminal of the decoder D illustrated in FIG. 1 emits a pulse for every bit error and is connected to the input terminal of a counter Z, the output terminals of which are connected to the input terminals of a memory S. The output terminal of a synchronization generator TG is directly connected to the enable input terminal E of memory S and, by way of a delay stage VZ, to the reset input terminal R of counter Z. The output terminals of memory S are connected to the input terminals of a digital-to-analog converter DA1. The output terminal of the converter is connected to the input terminal of a differentiator DZ and to the first input terminal of a comparator VL, which has a reference voltage UR at its second input terminal. The output terminal of comparator VL is connected to the input terminal El and the output terminal of differentiator DZ to the input terminal E2 of controls PM, a microprocessor for example. One output terminal A of microprocessor PM is connected to the input terminal of a pulse-width modulator M1, the output terminal of which is connected to the input terminal of a digital-to-analog converter DA2. One output terminal A2 of microprocessor PM is connected to the input terminal of another pulse-width modulator M2, the output terminal of which is connected to the input terminal of a digital-to-analog converter DA3. The output terminal of digital-to-analog converter DA2 is connected to the first input terminal of a summation point S1, the output terminal of which is connected to the input terminal of an amplifier V1, whereas the output terminal of digital-to-analog converter DA3 is connected to the first input terminal of a summation point S2, the output terminal of which is connected to the input terminal of an amplifier V2. The output terminal of amplifier V1 is connected to a coil F1 and the output terminal of amplifier V2 to another coil F2. Coils F1 and F2 constitute an adjustment stage that varies the angle, to be called the tangential angle in what follows, at which the beam of light L of pickup P tangentially strikes the compact disk C. Since the stage that adjusts the focusing circuit is also a coil, it can, as illustrated in FIG. 1, also be exploited as a stage for adjusting the tangential angle if it is constructed of two symmetrical halves, for which purpose the actual value of the focusing circuit, the focusing error, is forwarded to the first input terminal of summation points S1 and S2. 
     How the invention operates will now be explained. 
     Since, at every bit error, decoder D emits a counting pulse to counter Z, the initial setting of which is zero, every bit error increases the setting of the counter by one. Synchronization generator TG now periodically resets counter Z by forwarding a pulse to its resetting input terminal. Synchronization generator TG, however, simultaneously releases the pulse to the enable input terminal of memory S, causing it to release its contents to digital-to-analog converter DA1 in order to accommodate the state of counter Z. The pulse generated by synchronization generator TG is forwarded undelayed to the enable input terminal of memory S but, due to the intervention of delaying stage VZ, to the resetting input terminal R of counter Z subject to a delay, to allow memory S to accommodate the state of counter Z before it is reset. The state of counter Z, which is proportional to the bit-error rate, the number of bit errors between each pair of subsequent pulses from synchronization generator TG, is converted in digital-to-analog converter DA 1 into an analog voltage UF, which is accordingly also proportional to the bit-error rate. 
     As long as the analog voltage UF at the output terminal of digital-to-analog converter DA1, which will be called the &#34;test voltage&#34; in what follows, is lower than reference voltage UR, the tangential angle will not change. Once the bit-error rate increases to the extent that the test voltage is higher than the reference voltage, comparator VL will vary its output signal, signaling microprocessor PM that the bit-error rate is too high. Test voltage UF, however, does not make it possible to tell what direction the tangential angle will have to be varied in in order to decrease the bit-error rate. Microprocessor PM will accordingly generate a signal at each of its output terminals A1 and A2, causing pulse-width modulators M1 and M2 to emit pulse-shaped signals that are converted by digital-to-analog converters DA2 and DA3 and forwarded to coils F1 and F2. Coils F1 and F2, which constitute the component that adjusts the tangential angle, now initially vary the angle in one direction. If differentiator DZ determines during a prescribed interval of time, during the following two synchronization pulses from synchronization generator TG for example, that the bit-error rate is decreasing, the tangential angle will continue to be varied in the same direction until comparator VL signals microprocessor PM that test voltage UF has decreased to below reference voltage UR. 
     If differentiator DZ determines during the aforesaid interval of time that the bit-error rate is not decreasing, microprocessor PM will assume that it has adjusted the tangential angle in the wrong direction. It will accordingly emit signals at its output terminals A1 and A2 to vary the tangential angle in the other direction until test voltage UF decreases to below the reference voltage. 
     These measures will continue varying the tangential angle, the angle at which the beam of light strikes the compact disk at an angle, in such a way as to maintain the bit-error rate below a prescribed threshold. Irregularities in the surface of the compact disk will accordingly no longer entail the essential increase in the bit-error rate typical of a compact-disk player in which the tangential angle is adjusted constant. 
     Comparator VL, counter Z, memory S, differentiator DZ, and digital-to-analog converter can also be integrated into microprocessor PM. Microprocessor PM also makes it possible to vary the tangential angle until the bit-error rate is at a minimum. In this case the only purpose of the signal at the output terminal of comparator VL is to initiate adjustment of the tangential angle. The regulation process, however, does not terminate as soon as test voltage UF decreases to below reference voltage UR, but only once differentiator DZ determines that test voltage UF, and hence the bit-error rate is at a minimum and will stop decreasing. The result is a greater improvement in sound reproduction. 
     FIG. 2 illustrates another embodiment, which differs from the first in that one output terminal A3 of microprocessor PM is connected to the input terminal of a pulse-width modulator M3. The output terminal of the pulse-width modulator is connected to the input terminal of a digital-to-analog converter DA4, from the output terminal of which the reference voltage UR for the second input terminal of comparator VL can be obtained. The advantage of this embodiment is that the threshold at which the tangential angle begins to be adjusted can be set. In other words, the sensitivity of the circuit that controls the tangential angle can be adjusted. 
     The invention is appropriate not only for compact-disk players and videodisk players but can also be embodied in optico-magnetic recording and playback equipment wherein, as in audiotape equipment, data can be recorded, played back, and erased as often as desired. Equipment for recording and playing back DRAW-disks, which, like PROM equipment, allows data to be recorded only once and it is impossible to erase and re-record or to record over already recorded data, is also to be considered.