APPARATUS FOR PROJECTING A LIGHT BEAM ONTO AN OPTICALLY READABLE RECORD CARRIER

Abstract: An apparatus for playing optical discs is described, in which for a smoother locking-in of the focus servo control the focusing error signal is initially derived from the amplitude of the h.f. signal, followed by a gradual change-over to a difference signal derived by means of subdetectors. In this way the focus control point with the servo loop closed is shifted towards the desired in-focus point.

Description: The invention relates to an apparatus for projecting a light beam onto an optically readable record carrier, which apparatus comprises an optical detector for detecting by means of at least two subdetectors the light beam which has been modulated by the record carrier, a focussing device for focussing the light beam on the record carrier, a circuit for generating a focussing error signal which comprises a first signal corresponding to the difference between the two signals supplied by the subdetectors, and a focussing correction device for adjusting the focussing device in response to the focussing error signal. Such apparatus is known from EP-A-0095852. In the known apparatus the focussing device is controlled to an in-focus point as long as the focussing device remains within a predetermined capture range. When such apparatus is switched on the focussing device will generally be outside this capture range. In order to move the focussing device into its capture range the apparatus is provided with out-of-­ range detector for detecting when the focussing device is outside the capture range and with a control cicuit for moving the focussing device within capture range when an out-of-range condition is detected. In order to obtain a correct servo control the response of the focussing error signal near the in-focus point should be high. However, this results in oscillations about the in-focus point when the known lock-in method is used It is the object of the invention to provide an oscillation-free lock-in method and to this end the invention is characterized in that the circuit is adapted to initially generate as the focussing error signal a second signal comprising the sum of the signals supplied by the subdetectors and being incremented to ensure that the objective remains clear of the record carrier to change over gradually from said second signal to a third signal corresponding to the sum of the signals supplied by the subdetectors minus a threshold signal, and subsequently to change over gradually from said third signal to said first signal. In this way it is achieved that the focussing device is controlled from the predetermined initial position to a lock-in point with a low response and this lock-in point is gradually shifted towards the desired in-focus point, so that the desired in-focus point is reached gradually in spite of the high response around this point. Embodiments of the invention will now be des­ cribed in more detail, by way of example, with reference to the accompanying drawings, in which Fig. 1 shows a known apparatus for reading an optical record carrier, Fig. 2 shows the focusing-error signal as a function of the distance F of the objective from the record carrier relative to a reference point o , Fig. 3 shows an alternative focusing-error signal as a function of the distance F to illustrate the operation of the apparatus in accordance with the invention, Fig. 4 is a diagram to explain the shift of the focus control point during locking-in in an apparatus in accordance with the invention, Fig. 5 shows schematically an embodiment of the invention, and Fig. 6 shows the circuit 60 in Fig. 5 in more detail. Fig. 1 shows a known apparatus for reading an optical record carrier 1 insofar as it is relevant to the focusing. The record carrier 1 is rotated by means of a drive motor 2. An objective 4 projects the light from a laser 3 onto the record carrier 1. A semi-transparent mir­ ror 5 projects the reflected light onto four diodes D₁, D₂, D₃ and D₄ in such a way that the diode signals S₁, S₂, S₃ and S₄ provide information on the location of the light spot formed by the beam relative to the track on the record carrier and information on the degree of focusing. The focusing information may be provided by the signal D v which is generated by means of a circuit 6 in conformity with the equation D v = (S₁+S₄)-(S₂+S₃). This signal D v is applied to a focusing correction device 7 by means of which the objective 4 can be moved. Fig. 2 shows the signal D v as a function of the distance F of the objective from the record carrier 1. If O < F < B this signal is negative and the objective is moved towards the focus B, and if F < B the signal is positive and the objective is braked and moved towards the record carrier. Thus, controlling is effected in such a way that the objective is situated at a distance F = B. This control system operates correctly only within the range A < F < C where the signal D v is very steep. In order to bring the objective within this range after starting of the record carrier, it is known to render the control loop inoperative by means of a switch 8 and to move the objec­ tive from position F = O towards the record carrier by applying an increasing voltage from a generator 9 which is started by a playback control device 10. In the meantime the comparator 11 compares the signal D v with a reference voltage V ref which is such that for D v = V ref the objec­ tive is situated at a distance F = D within the range A < F < C. At this instant the comparator 11 causes the switch 8 to change over to apply the signal D v to the objective control loop 7. The known system has the disadvantage that in the range A < F < C a large steepness, and consequently a high gain, is required for an accurate control to the in-­ focus point F = B, which may lead to oscillations about point F = B during locking-in. In accordance with the invention this can be avoided by moving the objective 4 to the in-focus point F = B in a controlled manner by creating a lock-in point F < B, which is subsequently shifted gradually towards point B. This is achieved by taking the diode signal D s = S₁ + S₂ + S ₃+ S₄ and sub­ tracting a value I s from this signal, which will be explained by means of Fig. 3. Fig. 3 shows the diode sum signal D s , the diode sum signal D s plus a value I s , and the signal D s minus the value I s . The signal D s + I s is always positive and ensures that the objective remains clear of the record carrier at position F = O. When after starting D s + I s is gradually caused to change to D s - I s , the objective will move from F = O to F = E, as will be apparent from the intersections of the dashed curves between the signals D s and D s - I s . Thus, the objective moves in the direction indicated by the arrow P from point F = O to point F = E when the control loop is closed. Subsequently, whilst the control loop is closed, the objective is moved towards point F = B by causing the signal D s - I s to change gradually to the signal D v (Fig. 2), which is illustrated by Fig. 4 which shows the intersections of the zero-axis with the dashed curves between the signals D s - I s and D v , in accordance with which the objective moves from position F = E to position F = B. Once the change to the signal D v is completed, the objective is locked-in at position F = B. By the use of the lock-in method in accordance with the invention the device shown in Fig. 1 is modified to a device as shown in Fig. 5, in which the switch 8, the comparator 11 and the sawtooth generator 9 have been dispensed with and the circuit 6 which generates the signal D v is replaced by a circuit 60 which generates a focus control signal F R by means of the method described with reference to Figs. 3 and 4, which circuit 60 is started by the device 10. During locking-in the control loop remains closed via the device 7. Fig. 6 shows an example of a circuit 60 for generating the control signal F R , the signals S₁ to S₄ being supplied by high-impedance sources, i.e . by current sources. The sum of the signals S₂ and S₃ is the tail cur­ rent applied to a long-tailed pair comprising transistors T₁ and T₂. The collector currents of the transistors T₁ and T₂ are subtracted from each other and applied to the focusing device 7 as the focusing error signal F R . The base of the transistor T₁ is connected to a source supplying a suitable reference voltage 2V A and the base of the transis­ tor T₂ is connected to a capacitor C which is at a voltage V c . The sum of the signals S₁ and S₄ is added to the collector current of the transistor T₁. The fixed current I s is supplied to the emitters of the transistors T₃ and T₄ by a current source 12. The base of the transistor T₃ is connected to a source of a suitable reference voltage V A and the base of the transistor T₄ is connected to the capacitor C. The collector current of the transistor T₃ is added to the collector current of the transistor T₁ and the collector current of the transistor T₄ is applied to the emitters of the transistors T₅ and T₆. The transistors T₅, T₆ and the transistors T₁, T₂ are driven by voltages 2V A and V C . The collector current of the transistor T₆ is drained and the collector current of the transistor T₅ is added to the collector current of the transistor T2. The capacitor C is charged by means of a current source 13, whose value in combination with the capacitance of the capacitor C determines the rise time of the voltage V C and hence the speed of the lock-in process. The capacitor C is short-­ circuited by a switch 14 which is opened by the control device 10 to start the lock-in process. When the switch 14 is closed the capacitor C is short-circuited and the voltage V C = O. Assuming that the voltages V A and 2V A are sufficiently high, transistors T₁, T₃ and T₅ will be conductive and transistors T₂, T₄ and T₆ will be cut off. The signal I s + S₁ + S₂ + S₃ + S₄ = I s + D s (upper curve in Fig. 3) will then appear on the output F R . When the switch 14 is opened the voltage V C increases, which initially results in the current I s being gradually taken over from the transistor T₃ by the transistor T₄ and the sign of the output signal F R being inverted. Consequently, F R changes from D s + I s to D s - I s (in conformity with Fig. 3). A further increase of V C results in current being taken over from the transistors T₁ and T₅ by the transistors T₂ and T₆. For the transistors T₅ and T₆ this means that the current I s is gradually drained to the power supply instead of to the output F R , whilst the tail current S₂ + S₃ is taken over by the transistor T₂ and appears inverted in the output signal F R . The signal F R then changes gradually from D s - I s to D V (situation shown in Fig. 4). It is to be noted that the system described here is a system comprising four subdetectors arranged in line. However, the invention also relates to other systems. For example, a known system is to arrange four detectors in the four quadrants of a rectangular array. The difference signal D V is then formed by the sum of the signals from the subdetectors disposed on one of the diagonals minus the sum of the signals from the subdetectors disposed on the other diagonal. Another known system is a first detector surround­ ded by an annular second detector. The signal D V is then the difference between the two signals from these detectors and the signal D s is the sum of these signals.

Claims: 1. An apparatus for projecting a light beam onto an optically readable record carrier (1), which apparatus comprises an optical detector (D1, D2, D3, D4) for detecting means of at least two subdetectors (D1, D4; D2, D3) the light beam which has been modulated by the record carrier (1), a focussing device (4) for focussing the light beam on the record carrier (1), a circuit (60) for generating a focussing error signal (F R ) which comprises a first signal (D v ) corresponding to the difference between the two signals (S1, S4; S2, S3) supplied by the sub-detectors (D1, D4; D2, D3) and a focussing correction device (7) for adjusting the focussing device (4) in response to the focussing error signal (F R ), characterized in that the circuit (60) is adapted to initially generate as the focussing error signal (F R ) a second signal comprising the sum (DS) of the signals (S1, S4; S2, S3) supplied by the subdetectores (D1, D4; D2, D3) and being incremented to ensure that the objective remains clear of the record carrier to change over gradually from said second signal to a third signal corresponding to the sum (D s ) of the signals (S1, S4; S2, S3) supplied by the subdetectors (D1, D4; D2, D3) minus a threshold signal (I s ), and subsequently to change over gradually from said third signal to said first signal (D v ). 2. An apparatus as claimed in Claim 1, characterized in that said second signal corresponds to the sum (D s ) of signals (S1, S4; S2, S3) plus a threshold signal (I s ). 3. An apparatus as claimed in Claim 2, characterized in that both threshold signals (I s ) are equal.

Inventors: BIERHOFF, MARTINUS P. M., LONIJ, HUBERTUS M. M.
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: G11B7
IPC codes: G11B7, G02B7