Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-09-25
2002-06-04
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044260, C369S044290, C369S053220, C369S053280
Reexamination Certificate
active
06400661
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical recording medium reproducing apparatus, and in particular, to an optical, recording medium reproducing apparatus adapted to reproduce optically recorded information recorded on an optical recording medium by irradiating the medium with a light spot of a laser beam.
BACKGROUND ART
Compact discs (CDs) have conventionally been used as optical recording medium reproducing apparatus of this kind, wherein the disc as an optical recording medium is irradiated via an optical system 0.45 in numerical aperture (NA) with a laser beam 780 [nm] in wave length which is generated by a laser diode.
In recent years, however, new laser beam sources with a wave length smaller than 780 [nm] (for example, red=680 [nm], and semiconductor lasers of green and blue) have been developed as light sources for optical recording medium reproducing apparatuses. These new laser beam sources serve to implement recording medium reproducing apparatuses that can reproduce recording media with a higher recording density than compact discs. These recording medium reproducing apparatuses that can reproduce recording media with a higher recording density than compact discs desirably have a compatible reproducing function that also enables conventional compact discs (CDs) to be reproduced.
The diameter of the light spot that can be formed by a laser beam of a large wave length is selected so as to be somewhat larger than the width W
1
of pits P
1
formed in the compact disc as recorded information as shown by reference L
1
in FIG.
1
(A). This allows the light spot L
1
to constantly lie across the width W
1
during movement when entering the pit P
1
from land to scan it.
A sum signal (hereafter referred to as an “RF signal”) can thus be obtained from an optical pickup based on light reflected from the compact disc because of the light spot L
1
in
FIG. 1
(A). The RF signal falls from a first signal level LV
11
to a second signal level LV
12
when the light spot L
1
passes through the end of the pit P
1
, and subsequently maintains the signal level LV
12
until the light spot L
1
has passed the pit P
1
with itself lying across the pit P
1
, as shown in FIG.
1
(B). This results in a sum signal in which the signal level changes in response to the lengths of the land and the pit P
1
because the light spot L
1
scans both the land and the pit P
1
.
Thus, the signal level of the RF signal RF
1
decreases during scanning due to the interference between light reflected from the pit P
1
and light reflected from a reflecting surface (the land) located around the pit P
1
. This also occurs when a light spot L
2
of a relatively small wave length scans a pit P
2
formed in an optical disc that has a higher recording density than the compact discs.
Each time the light spot L
2
formed by a laser beam of a relatively small wave length enters the pit P
2
, an RF signal RF
2
, the signal level of which changes from LV
21
to LV
22
in response to recorded information can be obtained as shown in FIG.
2
(B).
The light spot L
2
formed by a laser beam of a relatively small wave length can be converged on a small diameter compared to the light spot L
1
formed by a laser beam of a relatively large wave length, so the width W
2
of the pit P
2
may be smaller than the width W
1
of the pit P
1
. As a result, optical recording medium reproducing apparatuses using the light spot L
2
formed by a laser beam of a relatively small wave length deal with a high recording density, while optical recording medium reproducing apparatuses using the light spot L
1
formed by a laser beam of a relatively large wave length cope with a low recording density.
If a high recording density optical recording medium reproducing apparatus is used to directly reproduce a compact disc (CD) designed to be reproduced by a low recording density optical recording medium reproducing apparatus, the diameter of the light spot L
2
is smaller than or equal to the width W
1
of the pit P
1
, as shown in FIG.
3
(A). In this case, light reflected from the land and light reflected from the pit P
1
interfere with each other when the light spot L
2
enters and leaves the pit P
1
, whereas no interference occurs while the light spot L
2
is scanning the pit P
1
because it is totally included within the pit P
1
. The signal level of an RF signal RF
3
changes from LV
31
to LV
32
only at both ends of the pit P
1
, as shown in FIG.
3
(B).
Although the signals shown in FIGS.
1
(B) and
2
(B) can be detected by integral detection, the signal shown in FIG.
3
(B) cannot be detected by such detection but requires differential detection. The differential detection, however, has a higher error rate than the integral detection.
This invention is proposed in view of the above points, and its objective is to provide an optical recording medium reproducing apparatus that performs reproducing operations using a light spot formed by a laser beam of a relatively small wave length and which can compatibly reproduce low recording density optical recording media.
DISCLOSURE OF THE INVENTION
This invention provides an optical recording medium reproducing apparatus that reproduces information recorded on an optical recording medium with a plurality of pits formed along tracks based on the recorded information, comprising: a laser beam emitting means for emitting a laser beam; a focus control means for controlling the focusing of the laser beam on the optical recording medium; and a control means for controlling the focus control means so as to increase the spot diameter of the laser beam emitted onto the optical recording medium when the medium has a low recording density with pits relatively sparsely arranged compared to the case in which the medium has a high recording density with pits relatively densely arranged.
Thus, when the optical recording medium mounted in the optical recording medium reproducing apparatus has a low recording density, this invention enables the information recorded in the tracks of the mounted low recording density recording medium to be reliably reproduced as in high recording density recording media, by using the control means to control the focus control means so as to increase the spot diameter of the laser beam.
According to this invention, to provide control in such a way that the spot diameter of the laser beam will be increased, the control means supplies different focus bias values depending upon whether the optical recording medium has a high or a low recording density.
In addition, this invention comprises a light receiving means for receiving a laser beam reflected from the optical recording medium; a servo error signal generating means for generating a servo error signal based on an output signal from the light receiving means; a polarity detecting means for detecting the polarity of the servo error signal; and a polarity selecting means for selecting the polarity of the servo error signal based on an output signal from the polarity detecting means, thereby reliably enabling the optical recording medium to enter a servo operation state even when the polarity of the servo error signal from the optical recording medium differs from that of standard optical recording media.
In addition, this invention comprises a light receiving means for receiving a laser beam reflected from the optical recording medium; a read (RF) signal generating means for generating a read (RF) signal for the recorded information based on an output signal from the light receiving means; a tangential push-pull signal generating means for generating a tangential push-pull signal based on the output signal from the light receiving means; an error detecting means for detecting the error conditions of the output signal from the light receiving means; and a selecting means for selectively outputting the read (RF) or tangential push-pull signal, wherein the control means controls the selecting means based on an output signal from the error detecting means. This constitution ena
Iimura Shinichiro
Ogawa Hiroshi
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Sony Corporation
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