Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Patent
1993-11-03
1995-12-12
Epps, Georgia Y.
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
369 48, 369 54, 369 58, 369 59, 369116, 369124, G11B 7007
Patent
active
054756723
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a movable information medium, in particular in the form of a disk intended for recording/reading of information along tracks.
The media currently used are equipped with a structure having preformats produced during a pre-etching phase.
These preformats are used to generate service signals allowing the recorder/reader to operate: physical representation of the axis of the track, focussing, synchronization, addresses, etc.
Numerous pre-etching methods have been proposed. Among these may be mentioned the method as described in French patent application No. 82 04218 filed on 12 Mar. 1982 and published under U.S. Pat. No. 2,523,347 on 16 Sep. 1983.
The method described consists in providing, along virtual tracks of an information medium, discrete, non-contiguous elements physically representing the mean axis of the said tracks and framing areas intended for the recording of information. These discrete elements, called "flags", must, naturally, be able to be discriminated by some means from the elements representing so-called "useful" information.
This method is, moreover, in accordance with one of the solutions currently adopted at the present time for producing optical or magneto-optical disks.
In reality, two main formats exist, which have been described in ISO standards: ISO 9171, 1 and 2, in the case of media of the read-only, or "Write Once" type according to the terminology; or in the ISO 10089 standard in the case of magneto-optical disks.
They are referred to as the A format and the B format.
The first format (A format) is known as continuous-composite. This format is characterized by continuous, pre-etched grooves, possibly interrupted from place to place. These grooves serve for guiding the beam during writing or reading of information. The pre-etched grooves may be arranged centred between two information tracks, or, in contrast, be coincident with the information tracks.
The second format (B format) based on a method of track following with sampling. The abovementioned French patent application belongs to this format class.
According to the ISO standard, each circular track includes 32 sectors numbered from 0 to 31. Each sector itself includes 43 segments each of 18 bytes.
The first segment is called "Header" according to the terminology, and may be compared with the flags described in the abovementioned French patent application.
In effect, it includes a certain number of pre-etched elements the functions of which are detailed by the standard.
Among the main functions fulfilled by these pre-etched elements may be mentioned, in particular, track following, focussing and synchronization. They must also carry out the abovementioned discrimination function, also called recognition function.
One of the problems encountered during the phase of recording data relates to exerting control over the form factor of the etched elements.
In fact, use is made of a thermal effect, produced by a laser beam, for recording an information element. Physically, this action translates into an alteration in the sensitive thin layer of the medium. This alteration may take different forms: ablation, that is to say disappearance of the layer, deformation, for example in the form of a bubble, or also a crystallographic phase change. The heat may also, with the aid of a magnetic field generator, bring about a modification of the magnetization condition of the said layer.
In reading, a laser beam is also used, of lower power, for reading these alterations. It can thus be envisaged that use is made of non-linear effects. Below a certain heating threshold, there is no alteration in the layer.
For various reasons related, for example, to the quality of the material constituting the layer, to the conditions of setting up the etching laser beam and especially to the power setting as well as to the focussing, the dimensions of the alteration will be more or less well controlled.
To these causes the so-called thermal effect must also be added.
Since it is necessary to exceed a temperature threshold so
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patent: 4873680 (1989-10-01), Chung et al.
patent: 4932017 (1990-06-01), Van Uijen
patent: 5161145 (1992-11-01), Ogawa et al.
patent: 5339303 (1994-08-01), Yoshimaru
I.E.E.E.: "Topical Meeting on Optical Data Storage", Technical Digest Series 1987, vol. 10, (Optical Society of America, Washington, D.C. 1987) pp. 64-67, Saito et al.: High Storage Density Optical Disks using Pit Edge Recording on Pb-Te-Se Thin Films, *voir p. 66, lignes 4-11*.
"Thomson-CSF"
Chu Kim-Kwok
Epps Georgia Y.
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