Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Patent
1994-11-07
1997-03-04
Nelms, David C.
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
3692753, 369273, G06K 1906
Patent
active
056087187
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a disk-shaped optical storage medium for binary information, stored serially as a sequence of pits and lands along a spiral track in a smooth optical reference surface, the pit and land lengths, which are independent from each other, representing the stored binary values which are readable by means of a tracked focussed laser beam according to the principle of interference. The invention further relates to a method of fabricating a so-called glass master, from which dies can be copied, which in turn serve the mass production of such a disk-shaped storage medium.
One of the most well-known storage media of the aforementioned kind is the compact disc, the salient properties of which are defined by the Standard IEC 908 and by the specification "Compact Disc Digital Audio System Description", June 1989, published by Sony Corp. and N. V. Philips. Among other things the compact disc has the following features: the information is recorded serially in the form of a spiral track on a smooth reference surface and readable in the reflection mode by means of a focussed laser beam. In the plane of the smooth reference surface the focussing diameter is equivalent to the track width of approx. 1 .mu.m. Within the track a sequence of troughshaped recesses, the so-called pits are provided which are separated from each other in the track direction and in the radial direction by regions of the smooth reference surface. The regions of the reference surface located in the track direction between neighboring pits are called lands. The width of the pits amounts to less than half the track width. The lengths of the pits and lands represent the stored binary values such that, as viewed in the track direction, each transition between a pit and a land or between a land and a pit is detected as logic one, the remaining regions of both the pits and the lands as logic zero, however. For detecting the transitions use is made of the principle of destructive interfence known from German patent 2,208,379. This is at an optimum for a disk surface of consistent reflection when, on the one hand, the depth of the pits corresponds precisely to a quarter of the wavelength of the reading light bundle, on the other, when one half of the light incident is reflected by the regions of the smooth reference surface surrounding the corresponding pit (including the lands), the other half by the bottom of the pits. The sequence of pits and lands would then furnish maximum amplitude modulation of the reflected light bundle. The mandatory requirement for non-contact reading an optical storage medium is, however, also obtaining a tracking signal from the reflected portion of the reading light bundle. The tracking signal is, however, for a pit depth of .lambda./4=0, while passing through a maximum at a pit depth of .lambda./8, however.
Thus, neither with the compact disc nor with any other optical storage medium working according to this same reading principle can use be made of the optimum pit depth of .alpha./4 for retrieving the information signal. Instead, the pit depth amounts to approx. 0.225 .lambda.. For a compact disc having n=1.46, which, as is known, is read through the transparent polycarbonate, and at the usual wavelength of 780 nm of the laser light, i.e. at a wavelength of 534 nm in polycarbonate, the pit depth thus amounts to 120 nm.
From the German laid-open patent application 4,121,505 a further development of the optical storage medium of the aforementioned kind is known which has the objective of simplifying the method of fabrication by allowing greater tolerances in pit depth and particularly to permit speeding up clocking the injection molding machines used to mold the storage medium without endangering the compatibility of the storage medium with a storage medium of the kind as stated at the outset. For this purpose, in the further-developed storage medium instead of the sequence of pits and lands a full-length groove is provided with is modulated in depth, namely exhibiting first regions of greater depth, corre
REFERENCES:
patent: 4967286 (1990-10-01), Nomula
patent: 5398231 (1995-03-01), Shin et al.
Patent Abstracts of Japan vol. 10. 312 (P-509), Oct. 23, 1986.
Feiereisen Henry M.
Le Vu A.
Nelms David C.
Sonopress Produktionsgesellschaft fur Ton- und Informationstrage
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