WO2008096333A1

WO2008096333A1 - Noise reduction

Info

Publication number: WO2008096333A1
Application number: PCT/IB2008/050495
Authority: WO
Inventors: Marcel Rieck
Original assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips Electronics NV
Current assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips NV
Priority date: 2007-02-09
Filing date: 2008-02-12
Publication date: 2008-08-14
Anticipated expiration: 2009-08-09
Also published as: TW200849231A

Abstract

In order to reduce the noise produced by a radial movement of the reading spot of an optical pick-up unit for an optical storage medium it is suggested to provide a filtering of the focus error signal or any further signal derived thereof in order to prevent a modulated actuation of the focus actuator.

Description

Noise Reduction

Field of the Invention:

The invention relates to the field of optical pick-up units (OPU) for reading out an optical data storage medium, e.g. a compact disc (CD) or digital video disc (DVD).

Background of the Invention:

The US Patent Application No. US 2004/0208092 Al discloses a focus position adjustment method for adjusting the focus position of the optical beam by which an optical storage device reads an optical storage medium. The optical storage medium comprises a header and a recording area. The optical storage device generates a focus error signal as reading the medium and utilizes the focus error signal to control the focus of the optical pickup head. After the optical storage device finishes tracking closed loop, a method is performed according to the deviation value between the levels of focus error signals of the header and the recording area to adjust the focus position to make the deviation value fall in a predetermined range. The referenced system comprises a deviation value detection module for determining the deviation value and a focus control module for adjusting the focus position.

In an optical pick-up unit according to the reference, a light source in combination with a detector are used in order to illuminate the disc surface to be read out and detect the light reflected from the surface of the disc providing the information encoded on the disc. As the information is stored on a track, e.g. a spirally shaped track or a track arranged in concentric rings, a two-stage electromechanical system is provided in order to control both the focus position of the focusing element (lens) with respect to the data layer on the disc and the radial spot position of the optical pick-up unit on the disc. This system consists of a carriage or sledge, on which the optical pick-up unit including a radial electromechanical actuator for the focusing element is mounted. Furthermore it has a second actuator providing a motion in a direction perpendicular to the surface of the disc which focuses the reading spot on the data layer. During normal track following, i.e. the reading of data stored on the track of the disc, the electromechanical actuator operating in a radial direction is responsible for the fast positioning of the focused reading spot in a radial direction with respect to the target track and keeps it in the middle of the track. As the disc rotates, the spiral track slowly moves outwards. In order to keep the radial lens actuator in the middle of the working range of the optical pick-up unit, the carriage or sledge slowly moves outwards, following the path of the track.

Summary of the Invention:

It would be advantageous to provide an optical pick-up unit to read out a disc-shaped data medium reducing the noise during a jump of the optical pick-up unit from one location on the disc to another location.

To better address one or more of these concerns in a first aspect of the invention, an optical pick-up unit for a disc, comprising a light source, a focusing element, a focus actuator for providing a motion perpendicular with respect to a surface of a disc, wherein the focusing element is mounted on the focus actuator, a detector for detecting light emitted from the light source and reflected by the disc and a detector circuit in communication with a detector for determining a focus error signal, wherein a filter is provided for filtering the focus error signal.

In an embodiment the lens of the optical pick-up unit is positioned using two electromechanical actuators, one in focus direction (perpendicular to the disc) and the other in radial direction (along the radius of the disc). The former focuses the reading spot in the plane of the data layer of the disc whereas the later provides the fast short stroke tracking of the track in the data layer. The radial carriage/sledge on which the optical pick-up unit is mounted provides for the slow long stroke tracking of the track. Its main goal is to keep the radial actuator the in the middle of its operating range, i.e. in the middle of the optical pick-up unit. To control the focus and radial actuators two error signals are generated from the reflected light. Namely: (1) The focus error signal, which is a measure for the distance (perpendicular to the disc) between the focal point of the lens and the data layer of the disc. (2) The radial error signal is a measure for the distance between the centre of the reading spot and the middle of the track on which the reading spot is currently positioned (along the radius of the disc). The focus error is used by the focus controller to position the focus actuator such that the focal point of the lens coincides with the data layer of the disc, whereas the radial controller controls the radial actuator using the radial error signal in order to keep the centre of the reading spot in the middle of the track being read. The sledge controller controls the sledge/carriage, it moves the sledge/carriage in such a way that the DC value of the radial controller output becomes zero, in that case the radial actuator positions the lens radially in the middle of the operating range of the optical pick-up unit.

Frequently an access to the information stored on the disc involves a controlled jump from a first position on the disc to a second position on the disc, during which the track on the disc is crossed several times in a radial direction. During this crossing, the number of tracks crossed is counted. To achieve a controlled jump from a first position on the disc to a second position on the disc, the lens actuator providing a movement in a radial direction is actuated. Aside from relatively short acceleration and deceleration periods at the beginning and the end of the jump, respectively, the lens actuator moves radially over the disc, crossing the tracks at constant velocity. The second stage of the electromechanical system, i.e. the sledge, just follows and keeps the lens actuator in the middle of its working range.

If during a jump between two radial positions on the disc, the spot position of the optical pick-up unit is moved in a radial direction with respect to the disc, the track crossing frequency during the jump lies typically in the kHz range. The reflectivity of a disc is different when spot positions on the track and on all other parts of the disc are compared. Thus, a radial movement of the reading spot across the disc generates a repeated change in intensity of the light reflected from the optical disc. The changes in intensity of the reflected light are thus also in the kHz range.

The difference in the amount of reflected light between spot positions on the track and on other areas of the disc is used to control the spot position such that it follows the track on the disc. The light reflected from the disc is used to generate a focus error and a radial error signal, the former indicates the difference between the actual focal point position and the optimal focussing position, whereas the latter is a measure for the difference between the reading spot centre and the middle of the track to be followed . The focus error signal is used to provide a feedback to the focus controller controlling the electromechanical actuators of the Optical Pick-up Unit. In an embodiment of the invention the focus controller is a PID element (Proportional Integral Differential element).

The above changes in the amount of light reflected from the disc surface during a jump result in a distortion and modulation of the focus error signal. The gain of the focus error signal changes and an offset is introduced as well. The distortion introduced in the focus error signal by a crossing of the spot position over the disc with constant velocity is amplified by the focus controller and actuates the focus actuator with an additional oscillatory movement in the kHz range.

Due to the mechanical coupling of the focus actuator to the casing and further surroundings, this mechanical modulation of the focus actuator in the kHz range is translated into a whistling noise. This noise is hearable and disturbs the overall impression obtained by a user using a respective CD or DVD player employing the optical pick-up unit. Furthermore, the amplitude of the focus actuator voltage and therefore the current through it also becomes extensively large during the constant velocity part of the jump which might cause damage to the optical pick-up unit.

An embodiment of the present invention preferably suppresses the distortion in the focus error signal, thereby suppressing the high frequency actuation of the focus actuator which generates the whistling noise. Also, the high actuator current might be reduced.

In terms of the present application filtering of the focus error signal means the focus error signal as computed by a detector circuit or any signal derived thereof, containing the information carried by the focus error signal.

As the track-crossing frequency can be kept almost constant during a jump, the distortion suppression in the focus error signal in an embodiment of the present invention can be implemented by means of a band-stop filter.

In an embodiment of the invention, the band-stop filter operates at a fixed frequency corresponding to the track-crossing frequency. The track crossing frequency is defined as the number of tracks crossed per second during a movement of the spot in a radial direction. This embodiment is particularly useful once the track-crossing frequency, i.e. the constant velocity provided by the actuator during a jump is the same for each jump occurring in the system.

In a further embodiment the filter is arranged such that its filtering frequency can be adjusted to the track-crossing frequency, i.e. to the velocity provided by the actuator moving in a radial direction.

In an embodiment the filter provides a filtering in a range from 2 to 5 kHz, in particular at 3.7 kHz. In an embodiment of the invention the bandwidth of the stop-band should be wide enough to suppress deviations in the track-cross frequency over the complete jump and narrow enough not to make the focus control loop instable, i.e. not to lose too much phase at the bandwidth frequency of the focus control loop.

There is an embodiment of the present invention in which the optical pick-up unit comprises a focus controller to drive the electromechanical actuator, wherein the focus controller is designed to receive the focus error signal. In a further embodiment of the invention the filter is arranged between the detector circuit and the focus controller. As the output of the focus controller in an embodiment is amplified to drive the focus actuator, any distortions received with the focus error signal will be amplified. Therefore, a filtering according to an embodiment of the present invention before the focus error signal enters into the focus controller enables a more efficient filtering of the signal at the track-crossing frequency when compared to a signal filtering after an amplification of the distortions.

However, in an alternative embodiment of the present invention the filtering could also be provided on the output signal of the focus contr oiler as the distortion of this output signal corresponds to the distortion of the focus error signal.

The optical pick-up unit according to the different embodiments of the present invention could advantageously be employed in a play back module for a data medium or a disc. A play back module in the sense of this application could be a simple play back unit and/or a recording equipment, preferably for a CD or DVD or any disc- shaped optical data medium.

A play back module according to an embodiment of the present invention could be employed in an audio or video system, advantageously in a portable or car audio or video system. However, the play back module according to the present invention could also be employed in any type of system for optically storing and reading data on or from an optical data storage medium.

The above objects could also be better addressed by providing a method for the reduction of noise generated by an Optical Pick-up Unit for a disc, wherein a focus error signal is generated for a detector output and wherein the focus error signal is filtered.

Furthermore, in an embodiment of the invention the method for the reduction of noise generated by the optical pick-up unit could be implemented as a computer program enabling a processor to carry out the method according to an embodiment of the present invention.

In an embodiment of the invention the computer program could be run on a DSP (Digital Signal Processing) board.

The respective computer program could advantageously be stored on a computer-readable medium such as an optical disc or a magnetic tape. However, in an alternative embodiment the information which represents a computer program corresponding to an embodiment of the present invention could also be provided as an electromagnetic signal, preferably via a computer network, e.g. the internet.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

Brief Description of Embodiments:

Figure 1 shows a cutaway top view of a disc and a spot location of an optical pick-up unit thereon. Figure 2 shows a schematic view of an optical pick-up unit according to an embodiment of the present invention.

Figure 3 shows the frequency characteristics of the band- stop filter according to an embodiment of the present invention.

Figure 4 shows a schematic view of an embodiment of the present invention implemented in a software run on a digital signal processing board.

Detailed Description of Embodiments

Figure 1 schematically shows a cutaway view of the surface of an optical compact disc. The different sections 1 belong to a single track provided for data storage in the surface of the disc. The track is spirally arranged around the center of the disc such that the distances between the sections 1 of the track when considered in a radial direction are equal.

During the readout operation to retrieve information stored in the track 1 of the disc, the optical spot 2 provided by the Optical Pick-up Unit moves along the track, following its spiral shape. However, in addition to the readout operation jumps of the spot position 2 over the surface 3 of the disc in a radial direction are necessary to access different parts of the track. A movement of the spot position 2 in a radial direction is provided by the lens actuator moving the lens or any other focusing element providing a focusing of the optical readout beam onto the disc. This movement occurs in essentially three phases 4, 5, 6. The three distinguishable phases 4, 5, 6 are schematically sketched in figure 1. A phase 4 of acceleration is followed by a phase 5 of constant radial velocity and a phase 6 of deceleration between a first spot position 2 and a second spot position 7 on the disc. During phase 5 with constant velocity, the sections 1 of the spirally shaped track are crossed at regular time intervals. Each crossing of a section 1 of the track leads to a change in the intensity of the light reflected from the surface 3 of the disc. Therefore the reflected intensity is modulated with a frequency depending on the velocity of the radial movement of the spot 2 as well as the radial distance between the different sections 1 of the track.

The light reflected from the surface 3 (data layer) of the disc is used to generate a focus error signal that is used to control the focus position of the lens such that the laser beam (reading spot) is kept focussed on the disc's data layer. Therefore the focus error signal during a movement of the spot in a radial direction is modulated corresponding to the crossing of the sections 1 of the track. The crossing of each radial section of the track leads to a variation in the reflected intensity.

In figure 2 a schematic view of an embodiment according to the present invention is shown. The data layer 11 below the surface of a disc 10 is illuminated by an optical pick-up unit 12 comprising a light source 13 and detector 14. The light generated by the light source 13 is focused by a lens 15 onto the data layer 11 of the disc 10. In order to enable a track following in radial direction, an electromechanical actuator (not shown) enables a movement of the lens 15 in a radial direction keeping the position of the spot 17 in the middle of the track 19. The light reflected from the data layer 11 of the disc 10 is detected by the detector 14. The signal generated by the detector 14 is further processed by a detector circuit 18 in order to determine a focus error signal providing an indication for the distance between the focal point of the lens 17 and the data layer 11 (perpendicular to the disc). The focus error signal 20 is communicated to a focus controller 21 which in the shown embodiment is a proportional plus integral plus derivative (PID) element. The PID element 21 computes the focus error signal 20 providing an output signal to control the focus actuator 16 in order to enable a correct positioning of the focal spot 17 on the data layer 11 of the disc 10. In order to drive the focus actuator 16, the output signal of the PID element 21 is further amplified by an amplifying element 22. As described with respect to figure 1, a radial movement of the spot position on the disc leads to a modulation of the overall intensity reflected from the data layer 11 of the disc 10. This also leads to a modulation or distortion of the focus error signal 20 and thus via the focus controller 21 and the driver circuit 22 to a modulated actuation of the focus actuator 16. In the typical example of the depicted embodiment, the modulation frequency and thus the modulated actuation occurs at approximately 3.7 kHz. Therefore the modulated actuation of the focus actuator 16 lies in the audible frequency range. The mounting of the focusing element 15 as well as the focus actuator 16 provide a resonance body and therefore the modulated actuation would clearly be audible for the user.

In order to prevent the described modulated actuation of the focus actuator 16, a band-stop filter 23 is introduced between the detector circuit 18 and the PID element 21. In the depicted embodiment the band- stop filter provides a stop-band at 3.7 kHz, filtering out the modulation of the focus error signal 20 due to the track- crossing of the focal spot 17.

In summary Figure 2 describes the focus control loop of the system. I.e. a focus actuator 16 positioning a focussing element, i.e. lens 15, which focuses the laser beam radiating from laser 13 to a reading spot 17 on the track 19 on the data layer 11 (some distance below the surface) of the disc. The light reflected back from the disc (modulated by the data layer) to the detector 14, from which the focus error signal 20 is generated by a detector circuit 18. The focus error signal 20 filtered by the band-stop filter 23 and computed by the PID element 21 is used to actuate the focus actuator 16 via the driver circuit 22.

Figure 3 shows the Bode diagram for the band-stop filter 23 as used in the embodiment schematically shown in figure 2. The upper section of the diagram shows the magnitude in dB, whereas the lower part of the diagram shows the phase in degrees versus frequency.

Figure 4 is a schematic diagram of an embodiment of the PID element 50 as well as the band-stop filter 51, implemented in the embedded software of a digital signal processing board. The focus error signal 52 is first passed through a band-stop filter 51 before being communicated to the PID element 50 which drives a corresponding focus actuator providing a movement of the focusing element in a direction perpendicular to the disc to be read out.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative and exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that the combination of these measures can not be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

REFERENCE NUMBERS

1 track sections

2, 7 optical spot 3 surface of a disc

4 phase of acceleration

5 phase of constant velocity

6 phase of deceleration 10 disc 11 data layer

12 optical pick-up unit

13 light source

14 detector

15 lens 16 focus actuator

17 focal spot

18 detector circuit

19 track

20, 52 focus error signal 21, 50 PID element

22 driver circuit

23, 51 band- stop filter

Claims

CLAIMS:

1. Optical pick-up unit for a disc, comprising a light source (13), a focusing element (15), a focus actuator (16) for providing a motion perpendicular to a surface of a disc (10), wherein said focusing element (15) is mounted on said focus actuator (16), a detector (14) for detecting light emitted from said light source (13) and reflected by a disc (10), and a detector circuit (18) in communication with said detector (14) for determining a focus error signal (20, 52), wherein a filter (23, 51) is provided for filtering said focus error signal (20, 52).

2. Optical pick-up unit according to claim 1 , wherein said filter is a band- stop filter (23, 51).

3. Optical pick-up unit according to claim 2,wherein said band-stop filter (23, 51) comprises a stop-band in a range from 2 to 5 kHz.

4. Optical pick-up unit according to claim 1, comprising a focus controller (21, 50) to drive said focus actuator (16), wherein said focus controller (21, 50) is designed to receive said focus error signal (20, 52), and wherein said filter (23, 51) is arranged between said detector circuit (18) and said focus controller (21, 50).

5. Optical pick-up unit according to claim 1, comprising a focus controller (21, 50) to drive said focus actuator (16), wherein said focus controller (21, 50) is designed to receive said focus error signal (20, 52), and wherein said filter (23, 51) is arranged between said focus controller (21, 50) and said focus actuator (16).

6. Play back module for a disc comprising an optical pick-up unit according to claim 1.

7. Audio or video system or data storage system comprising a play back module according to claim 6.

8. Method for the reduction of noise generated by an optical pick-up unit for a disc, wherein a focus error signal is generated from a detector output, and wherein the focus error signal is filtered.

9. Computer program enabling a processor to carry out a method according to claim 8.

10. Computer readable medium comprising a computer program according to claim 9.