GB2235808A - Optically readable data carriers - Google Patents

Abstract

Carriers of optically readable data are made as replicas from a master having a storage area onto which data are programmed and separate sets of data are arranged to appear on the data storage area in a plurality of predetermined separate physical locations so that screening of selected locations can be effected to result in only certain data on the carrier being readable. The screening may be accomplished by the application of a printed marking layer on the disc surface, which allows only intended areas of the disc to be read. Windows may be provided in the marked area to allow positional (tracking) information to be read but which otherwise corrupt the data marked off. Alternatively the master disc may be masked so that only the desired section of data are transferred to the copy. <IMAGE>

Description

OPTICALLY READABLE DATA CARRIERS This invention relates to optically readable data carriers and particularly, though not exclusively, to carriers which are in the form of compact discs" which are also known as "CD ROMS".

In the production of CD ROMS, a master is produced and then replicas are made from the master.

The cost of making original masters is extremely high compared with the cost of making the replicas. The data carrying capacity of optical data carriers is usually much greater than the storage requirements of the data providers who use this medium to store and transmit information.

The present invention is in recognition of the possibility of making significant saving in costs of producing optically readable data carriers such as CD ROMS and the invention provides a method of production of carriers of optically readable data of the kind in which the carriers are made as replicas from a master where the master has a storage area onto which data are programmed, which method includes the steps of providing for separate sets of data to be arranged on the storage area in a plurality of predetermined separate physical locations, and providing screening means for selectively masking over the data storage area to leave at least one of the locations unmasked, and hence its associated set of data readable, whilst the other locations are masked, and hence their associated sets of data not readable.

Using this method, a master can be made with several separate sets of data, say ten sets of data.

CD ROMS produced from this master could then have ten different replicated versions, each version allowing for only one of the ten sets of data to be readable.

In this way the data storage capacity of the master can be shared, thus providing more efficient use of the master and enabling the high cost of producing the single common master to be shared.

The invention also provides a carrier of optically readable data produced by a method as defined herein.

By way. of example, embodiments of the invention will now be described with reference to the accompanying drawing, which diagramatically illustrates an optically readable data carrier produced by a method in accordance with the invention.

The optically readable data carrier seen in the drawing is in the form of a disc 10, known as a "compact disc" or "CD ROM". Data are stored on the disc 10 by means of surface discontinuities in a spiral track. The data are read off the disc 10 by an optical reading head, using e.g. laser light, as the disc rotates.

The disc 10 is made as a replica from a master. The master is produced by laying down the data onto a storage area, usually using a laser device etching onto glass. Discs are replicated from the master, for example, by deposition of a thin layer of metal, usually aluminium, onto the glass and then sandwiching the thin metal layer between two protective layers, usually of clear plastics material.

In the method according to the present invention, it is arranged for separate sets of data to be programmed onto the storage area. The separate sets of data are arranged to appear in a plurality of predetermined separate physical locations on the storage area. This enables screening to be provided for selectively masking off certain of the locations storing data so that only a selected set of data is accessible.

Using this method, therefore, a master having a data storage capacity of 600 megabytes, for example, could be split up into ten individual sets of data, each of approximately 60 megabytes. The separate sets of data would appear as ten concentric rings on the master. Discs replicated from the master would have nine of their data sets masked off with a selected data set unmasked. Thus, using just one master, ten different replicated versions of disc can be produced.

In the compact disc 10 seen in the drawing, the separate physical locations storing the separate sets of data take the form of concentric annuli. This is achieved by suitable programming when the original master is produced. Screening is achieved by means of a printed masking layer on the surface of the disc.

The shape and position of the masking layer is made to correspond with the positions of the separate data locations on the storage area.

In the drawing, 11 and 12 represent masked locations of the data storage area, whilst 13 represents an unmasked location. The set of data in location 13 is readable whilst those of locations 11 and 12 are not. Annulus 14 contains the usual reading head directing instructions for initiating the program.

It is preferable to allow for the sacrifice of some lengths of track in order to ensure that the physical separation of the data locations is sufficient to allow for manufacturing tolerances and printing registration tolerances, i.e. in this case, to allow sufficient gap between the annuli.

Preferably, the masking layer does not completely prevent reading of the data, but instead incorporates "windows". By this means, sufficient information can be picked up by the optical reading head to enable it to track properly over the whole disc. However, the information thus provided is merely for positional purposes and not enough to enable the data from the masked off locations to be read meaningfully, i.e. the masking is sufficient to corrupt the masked data.

As a precaution to safeguard against the reading of masked off data, there may be provided a blocking filter medium which is effectively not possible to remove, e.g. special infra-red light blocking inks which chemically infuse indelibly into the plastics material of the replicated discs.

Alternatively, the disc could be coated in such a way as to prevent the masking off layer from being removed without irretrievably damaging the surface and thus impairing the ability of the laser beam to read through to the underlying data. As the masking off of data only requires for light and infra-red radiation to be effectively blocked or scattered, there could be many alternative methods used to provide a suitable screening means.

As an alternative to providing the screening means on the disc itself, it would equally be possible to provide a screening means for the master. Thus, in replicating from the master using this alternative method, only the unmasked data would be transferred from the master to the carrier.

Claims (7)

1. A method of production of carriers of optically readable data of the kind in which the carriers are made as replicas from a master where the master has a storage area onto which data are programmed, which method includes the steps of providing for separate sets of data to be arranged on the storage area in a plurality of predetermined separate physical locations, and providing screening means for selectively masking over the data storage area to leave at least one of the locations unmasked, and hence its associated set of data readable, whilst the other locations are masked, and hence their associated sets of data not readable.

2. A method as claimed in claim in which the masking step is performed after the carriers are made from the master.

3. A method as claimed in claim 2 wherein the screening means allows reading of only sufficient of the data of the masked locations to ensure normal tracking of the optical reading device over the data storage area in use of the carrier.

4. A method as claimed in claim 2 or claim 3 in which the masking step comprises overlaying on the carrier a first layer of light reflecting, diffusing or absorbing material.

5. A method as claimed in claim 4 and comprising the further step of overlaying on the carrier after said masking step a second layer of protective material to prevent tampering with the first layer.

6. A method substantially as herein described with reference to the accompanying drawing.

7. A carrier of optically readable data made in accordance with a method as claimed in any preceding claim.