1. FIELD OF THE INVENTION
The present invention relates to an optical information medium, a transparent substrate and a stamper for use in the manufacture thereof, and a recording method therefor, on which can be recorded information that is optically reproducible, and in particular, relates to an optical information medium having a sector address information detection means therein, and is suitable for a high density recording medium on which the information can be recorded and reproduced with a red laser beam of a short wavelength of from 630 nm to 670 nm.
2. DESCRIPTION OF RELATED ART
With the recent advances in development and application of a short wavelength laser, standardization has proceeded on the standard for DVDs (Digital Versatile Discs) which enables recording and reproducing at a high density, as well as the practical application thereof. On the DVD is provided a data recording area on at least one surface thereof, in which pits are formed as a means for recording information, and a reflection layer made of a metal film is formed on the data recording area.
For the DVDs, a different standard for high density is determined, compared to the standard for the optical information medium being the most general or popular now, such as a CD (Compact Disc). For example, with the optical pickup, it is determined to use the red laser of short wavelength from 630 nm to 670 nm, and to use an objective lens of a high numerical aperture, such as of 0.6 in NA, etc.
Also, accompanying with this, for dealing with a curve or bend of the disc, the thickness thereof is determined to be 0.6 mm, about half (xc2xd) of that of the CD. However, for maintaining a compatibility or interchangeability in sizes between a CD having a thickness of 1.2 mm, the disc of DVDs applies such structure that two discs are aligned and pasted together. According to the DVD standard, it is standardized that one disc have a recording capacity of 4.7 GB at the maximum, in average, i.e., video and audio information for approximately 133 minutes.
Up to now, with a recordable CD, such as CD-R, etc., address information, i.e., ATIP (Absolute Time in Pregroove) is obtained through FM modulation of wobbling of the spiral groove tracking guide. In contrast to this, with a recordable DVD, such as a DVD-R, etc., the address information, including position information on the optical information medium or the like, is given by means of pre-pits which are provided in advance on a land portion between the tracking guides, and the wobbling as well, in place of the ATIP.
The pre-pits which are applied to the DVD-R as the high density recording medium mentioned above can be read out by the optical pickup together with the pits of the recorded signal, however, in this instance, the optical pickup is controlled by a tracking servo along with the tracking guide. Therefore, under a normal condition, the pre-pits are read out distinct from the pits for the recording of the data signals which are formed on the groove. However, depending upon the shapes, sizes and so on thereof, the pre-pits may be read out mixed with RF signals which are obtained from the pits for the recording of the data signals. In this case, it may be a cause of the occurrence of errors in reading of the data signals.
In the DVD-R and/or DVD-RW, the pre-pits are necessary for obtaining address information such as the position information on the optical information medium when recording. However, though they are not used when reproducing, the fact that the signals obtained from the pre-pits are mixed into the RF signals indicative of the recorded data may be the cause of the occurrence of the errors in the reading of the RF data signals.
On the other hand, if it is difficult to read out the pre-pits when recording, information, such as the address, cannot be read out correctly during the recording, therefore the recording comes to be unstable, and there remains a problem that the reproduction itself of the recorded signals is unstable.
In accordance with the present invention, taking into consideration those problems accompanying with the reading of the pre-pits for reading the address information of the optical information medium in conformity, such as a high density recording, an object thereof is to provide an optical information medium, a substrate for use of the optical information medium and a stamper for forming the substrate, with which the address information can be read out from the pre-pits with certainty when recording, including information on shifting when the pickup is shifted to a starting position of the recording after accessing a non-recorded area, and the error is removed or dissolved in reading due to interference between the signal obtained from the pre-pits indicative of the address information and the recorded data read out from the pits on the tracking guide, thereby enabling the prevention of the mixing of the signal obtained from the pre-pits into the data signal when being reproduced.
For accomplishing the object mentioned above, according to the present invention, the pre-pits 6 provided on the land for indicating the address information are intentionally shifted from the land 8, thereby increasing the intensity of the tracking error signal, as well as weakening the leakage into the RF signal, so as to discriminate between the tracking error signal and the RF signal which is obtained from the pits on the grooves 3, and preventing them from being erroneously read out with each other.
According to the present invention, there is provided a stamper, comprising concave and convex-like steps formed in a concentric or a spiral shape thereof, wherein in the concave portion of said stamper is formed a projection portion in such a manner that the projection portion is connected to at least one of the convex portions adjoining said concave portion.
Also, according to the present invention, there is provided a substrate for use in an optical information medium, formed from a resin by a stamper comprising concave and convex-like steps formed in concentric or a spiral shape thereof, wherein on said substrate is provided pre-pits indicating address information, formed by means of a projection portion, which is provided on the concave portion of said stamper in such manner that the projection portion is connected to at least one of the convex portions adjoining the concave portion.
According to the present invention, there is provided an optical information medium, comprising: a transparent substrate 1 for a recording laser beam to penetrate therethrough, a recording layer 12 formed on said transparent substrate; and a reflective layer 13 for reflecting reproducing laser beam thereupon, thereby enabling the recording of optically readable signals by means of an incident recording laser beam from said transparent substrate 1, wherein said transparent substrate has spiral-like tracking guides 3 on a surface thereof, on which the recording layer 12 is formed, and pre-pits 6 indicative of address information, formed on a land 8 defined between the tracking guides 3, and wherein the pre-pits 6 are formed by shifting from a center of the land 8. Thereby, the pre-pits 6 are opened to the side of the tracking guide 3 neighboring the land 8. On such an optical information medium, the optically readable signals are recorded by tracking the incident recording laser beam from said transparent substrate 1 along said tracking guide 3.
In the optical information medium using the substrate for an optical information medium, which is formed by the stamper, since the pre-pits 6 are formed shifted from the center of the land 8, in more detail, the pre-pits 6 are formed shifted from the center of the land 8, in more detail, the pre-pits 6 are formed shifted from the center of the land 8 in a direction of an inner periphery of the disc, then the pre-pits are formed in such a manner that they come nearer to the center of the beam spot when tracking, therefore it is possible to obtain an amplitude of the signal as the reproduced signal of the pre-pits, larger than that in a case where the pre-pits are formed at the center of the land, thereby performing the reading with certainty.
Also, with the construction mentioned above, it is possible to make the shape of the pre-pits 6 smaller than that in the case where the pre-pits are formed at the center of the land. This is caused by the fact that it is possible to obtain a larger signal amplitude than in the case where the pre-pits are formed at the center of the land, because the pre-pits 6 are off-set from the center of the land 8 in the inner periphery direction of the disc, as mentioned previously. In this case, since the pre-pits 6 can be made physically smaller in shape up to such a level that the signal amplitude thereof can be read out (for example, at the signal amplitude when the pre-pits 6 are formed at the center of the land 8), even in the case where the signal due to the pre-pits leaks into the RF signal, it comes to be sufficiently smaller than the time duration of the minimum signal as the data signal when it is decoded into an EFM signal, therefore, it is easy to discriminate it from the data signal, thereby preventing the erroneous reading of the signal due to the pre-pits.
For example, when reading the data pits formed on the tracking guide 3 while conducting the tracking servo on the optical pickup along with the tracking guide 3, the address signal obtained from the pre-pits 6 becomes weaker than the signal obtained from the data pits. Thereby, the signal of the pre-pits 6, which is mixed into the RF signal obtained from the data pits, becomes weak. Therefore, there hardly occurs the problem of mixing of noises into the data signal.
The pre-pits 6 are preferably shifted in the direction of the inner periphery of the transparent substrate 1, with respect to the center of the land. This is because, as was mentioned previously, the pre-pits are formed in such a manner that they come close to the center of the beam spot more when tracking, therefore, the reproduced signal of the pre-pits obtains the amplitude of a signal larger than that in the case where the pre-pits are formed at the center of the land.
The pitch p of the tracking guide 3 is p=0.74 xcexcm, and the half-width w thereof is preferably within a range, 0.23 xcexcmxe2x89xa6wxe2x89xa60.38 xcexcm. The half-width d of the pre-pit 6 is preferably within a range, 0.20 xcexcmxe2x89xa6dxe2x89xa60.33 xcexcm. From this relationship, the ratio w/d between the half-width w of the tracking guide 3 and the half-width d of the pre-pits 6 is within a range, 0.5 xe2x89xa6w/dxe2x89xa61.7. The off-set dimension xcex4 of the pre-pit 6 from the center of the land 8, depending upon the width of the land 8, is preferably within a range, 0 xcexcmxe2x89xa6xcex4xe2x89xa60.2 xcexcm, and the width 1 from the boundary portion between the tracking guide 3 and the land 8 to the pre-pit 6 preferably lies within a range, 0 xcexcmxe2x89xa61xe2x89xa60.2 xcexcm.
As mentioned above, with the optical information medium according to the present invention, wherein the signals can be recorded onto the optical information medium in conformity with the high density recording optically with the optical pickup, the address signal can be read out from the pre-pits with certainty when recording, and when reading out the recorded signals, errors are eliminated in reading the signals obtained from the land pre-pits indicative of the sector information and the recorded data read out from the pits on the tracking guide, thereby enabling the reading of the address information when recording and the data signals when reproducing, with accurate discrimination between them.