Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-01-04
2001-08-28
Hindi, Nabil (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044280, C369S044320
Reexamination Certificate
active
06282158
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a reproducing apparatus for reproducing a variety of disc type recording media and particularly to a reproducing apparatus of which cabinet attitude is changed in various ways like a portable and mobile reproducing apparatus.
BACKGROUND OF THE INVENTION
As an optical disc recording medium, CD (compact disc) and MD (mini-disc) (these are registered as trademarks) are widely spreading and used in various application fields such as audio, etc.
In a disc reproducing apparatus for reproducing various types of recording media, in order to control the tracking of an optical spot, there are provided a double-axis mechanism for driving an objective lens of an optical head with a tracking error signal obtained from a reflected light beam indicating a track guide information such as pit train and group, etc. and a sled mechanism for displacing the relative position of the optical head as a whole and disc surface in the disc radius direction.
As the sled mechanism, those for moving the optical head as a whole against a disc and for moving a turn-table on which a disc is mounted against the optical head which is fixed to the predetermined position are known. Moreover, as a control system of the sled mechanism, it has been proposed that a sled error signal generated by extraction of a low frequency element with a low-pass filter from the tracking error signal is amplified and is then applied to a sled motor as a sled drive signal. The sled error signal is changed to the signal indicating an offset of an objective lens driven for tracking by the double-axis mechanism of the optical head as a whole and the optical head.
Here,
FIG. 1
schematically shows an example of the structure of an optical block of the related art which is installed in an optical head of a reproducing apparatus for a disc such as CD, etc.
In this figure, an optical block of the optical head is composed of a semiconductor laser
81
, a collimator lens
82
, a deflected beam splitter
83
, an objective lens
84
and a photo detector
85
. For example, the laser beam radiated from the semiconductor laser
81
is paralleled by the collimator lens
82
, and then reflected by the deflected beam splitter
83
to radiate the recording surface of a disc
1
via the objective lens
84
.
When the laser beam is focused, the light beam reflected by the pit train provided on the disc
1
passes through the deflected beam splitter
83
and is then supplied to the photo detector
85
to provide the pit information of the disc
1
in the photo detector
85
.
FIG. 2
shows the concept of distribution of intensity of the reflected light beam in the relative positional relationship between the pits formed on the disc
1
and spot beam. In the so-called just tracking condition where the pit train of disc
1
is relatively matched with the position of the spot beam, the reflected beam indicated as (a) can be obtained on the photo detector
84
. Namely, pit information having an equal intensity distribution in the right and left sides can be obtained on the photo detector
85
.
Moreover, if the pit of the disc
1
is relatively displaced in position from the spot beam, for example, when the spot beam position is relatively deviated in the left side from the pit train (de-track condition), the pit information having intensity distribution as shown in (b) can be obtained on the photo detector
85
, while when the spot beam position is relatively deviated in the right side from the pit train (de-track condition), the pit information having intensity distribution as shown in (c) can be obtained on the photo detector
85
.
Namely, when positions of the pit provided on the disc
1
and spot beam are relatively deviated in the tracking direction, the pit information having different intensity distribution in the right and left sides can be obtained on the photo detector
85
.
A difference voltage obtained from intensity distribution difference in the right and left sides of the pit information obtained on the photo detector
85
is supplied to the double-axis mechanism to drive the objective lens
84
as a tracking error signal, a low frequency element of this tracking error signal is extracted to generate a sled error signal and it is then supplied to the sled mechanism for moving the optical head as a whole in order to control the tracking of the optical head to the ON-track condition. A method of detecting such tracking error signal is generally called a push-pull method and a tracking error signal obtained by this push-pull method is called a push-pull error signal in this specification of the present invention.
When the tracking of the reproducing apparatus is controlled by the push-pull method shown in
FIG. 1
, if the objective lens
84
of the optical head is shifted in the lateral direction (tracking direction) indicated by a broken line in
FIG. 1
due to the tracking servo operation, etc., the reflected beam obtained on the photo detector
85
is also shifted in the tracking direction as indicated by the broken line.
Therefore, the push-pull error signal generated from the pit information obtained on the photo detector
85
includes, even under the just tracking condition, a DC offset voltage depending on the shift of the objective lens
84
, resulting in the problem that this DC offset voltage disables accurate tracking control.
In view of solving such a problem, a detecting method called the top hold push-pull method has been proposed. In this method, the tracking error signal is detected after removing the DC offset voltage for detecting the tracking error signal. Such Top Hold Push-Pull (TPP) method has been disclosed in the Japanese Laid-Open Patent Number HEI 4-23234 presented (filed May
18, 1990)
by the same applicant of the present invention. In this case, a DC offset removing circuit for removing a DC offset voltage included in the push-pull error signal is provided to obtain the tracking error signal after having removed the DC offset voltage. Thereby, accurate tracking control can be realized by eliminating influence of the shift operation in the tracking direction of the objective lens
84
of the optical head. In the related art, the sled error signal has been generated by extracting a low frequency element of the tracking error signal obtained by the top hole push-pull method explained above.
In general, since a reproducing apparatus for reproducing a disc such as CD or MD is restricted in installation space, it is expected that the apparatus may be installed vertically or horizontally and is reduced in size. The reproducing apparatus is required, even if it is installed vertically or horizontally, that the optical block in the optical head (at least the objective lens) may be held reliably and an arm forming the double-axis mechanism is never shifted in the tracking direction (horizontal direction) due to its gravity. Namely, if the reproducing apparatus is installed, for example, vertically, it is just preferable as shown in
FIG. 3A
that the disc
1
is arranged in vertical, the sled axis
91
of the optical head
90
is arranged in vertical and the arm
85
holding the objective lens
84
of the optical head
90
is arranged to cross diagonally the sled axis
91
so that the position in the tracking direction of the objective lens
84
is never influenced by gravity.
However, in this case, since the sled mechanism for moving the optical head
90
in the radius direction of the disc
1
becomes larger than the external shape of the disc
1
as indicated as the shaded area of
FIG. 3A
, it has impeded reduction in size of the reproducing apparatus.
Therefore, when the sled mechanism is inclined for arrangement, for example, as shown in
FIG. 3B
, the sled mechanism can be accommodated in the internal side as much as &Dgr;1 in comparison with
FIG. 3A
to realize reduction in size of the reproducing apparatus. However, since the sled axis
91
holding the objective lens
84
is also inclined, the arm
85
is deviated with its self weight and the objective lens
84
Hindi Nabil
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Sony Corporation
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