This invention relates to data retrieval systems.

In known data retrieval systems, a central database is accessed by computer via modems and a telephone line. The user is charged for all the time he is connected to the database. This can prove to be expensive especially if the user is looking for an item but is uncertain as to its exact nature within that database. Many databases operate with billing systems which depend on the precise nature of the information displayed to the user. This leads to flexible operation but it is slow to operate, particularly as many telephone lines are of inadequate quality to transmit accurately at high data transmission rates.

An alternative approach to the access to large quantities of data is to place all of the data or some defined part of it on a single data carrier and to provide that data carrier to the user. One such known system of very large capacity uses a so-called CD-ROM, an optical data storage disc similar to a compact disc used for the recordal of music. A CD-ROM "player" unit is linked to a computer and appropriate software is provided to enable the user to access and display on screen material stored on the disc. Such a system can be used in connection with a "black box" which is a tamper-proof unit which records use of the

material on the disc, and which can therefore be used to generate billing data to enable the user to be billed at regular intervals by the data provider, with the CD-ROM provider acting if appropriate to recompense data providers according to the amount of use made of their respective parts of the data on the disc. Such a system can be used, e.g. to assemble a large quantity of periodical published material on one disc, the respective publishers being eventually paid in direct proportion to the amount read. A known such system has been commercialised under the name ROI by Personal Library Software of Rockland, Maryland, USA.

The present invention aims to provide data retrieval systems which combine great flexibility of operation with flexible realtime billing of data users.

According to one aspect of the present invention, there is provided a data retrieval system comprising a computer and at least one data carrier, the data carrier(s) including in stored form an accessible catalogue of first data elements and also corresponding second data elements in a form only accessible on presentation of an electronic key, and a computer program for accessing the catalogue and enabling search for, selection of and optionally processing of required data elements selected from the set of first data elements, means for requesting an electronic key from a remote computer and means for receiving such a key and using the same to locate and download selected second data elements from the data carrier(s) to the computer in processible form.

Advantageously, the database is stored on a CD-ROM disk Such disks are very portable and can store very large amounts of data, e.g. 500 Mbytes.

The remote computer which provides the electronic key to the second data elements is preferably connected to the computer by a modem and telephone line. The system thus preferably includes a modem and software capable of dialling the remote computer and interrogating the remote computer for the electronic key. Once the key is obtained, the second data elements may be accessed by the user, and if desired processed, for example simply viewed, or recorded for example on a floppy disk or a hard disk in the user's computer.

For a better understanding of the invention, reference will now be made, by way of example only, to the accompanying drawing which is a block diagram of a data retrieval system according to the invention.

The system shown comprises a database 1 consisting of data recorded on a removable CD-ROM disk which is accessed via a computer 2. The database 1 consists of a large number of elements which a user may wish to use from time to time. The CD-ROM disk contains two types of data: a catalogue of the elements available to the user, which catalogue can be accessed at any time, and the available elements themselves, but e.g. recorded in encrypted form with access only following a selection from the catalogue.

A program on the disk enables the user to browse and search for elements of interest in the catalogue using the computer 2. Once the desired elements have been selected, the user tells the computer the selection he has made and the program then dials a service centre computer 3 automatically via a modem 4 connected to a telephone line. The service centre computer can then, if appropriate after suitable interaction with the user, e.g. the request for and giving of a password, provide the user's computer with the information needed to find the selected elements on

the disk and to access and/or decrypt them. Once accessed and decrypted, the selected elements may if allowed be transferred to a "read/write" medium 5, such" as a floppy or hard disk in or associated with the computer 2. The selected elements can then be used at the user's convenience. Alternatively, they may be used at once.

The invention may be applied in a wide variety of areas, and the detailed operation may vary widely. For example, a journal may provide its publications at low cost for the contents and abstract pages, with full text articles available at additional cost. Alternatively, a single CD-ROM may include a plurality of journals on it, e.g. every journal in a given field for that month, with the contents pages of each accessible and the full text only available on payment. The host computer would, of course, have no difficulty in rewarding each of the contributor journals according to its usage.

However, these are examples of applications where the database is a text database. The present invention is of very substantial value when applied to a graphical database, e.g. a photo-library or typeface library, where low resolution images may be freely accessed and searched e.g. via keywords, and high resolution data then obtained, on payment, only if chosen. The major advantage in this connection is that of time: while current techniques can send text down telephone lines fast, high definition graphics can take a very long time, with risk of corruption by transmission errors adding to the difficulties.

In one specific example, that of a typeface library, the CD-ROM may contain, in addition to the dedicated software to enable the invention to be worked, an accessible catalogue of the various typeface elements. The program

may be arranged to give a screen display at the same size as that element is stored in the catalogue, and to enable such a displayed element to be scaled, cropped or positioned as desired on the screen.

The jumbled and encrypted elements correspond to catalogue elements but at a higher resolution. When the higher resolution element is accessed, the program may be arranged to effect that the high resolution element replaces the low resolution element with any scaling, cropping etc applied to that element applied to the higher resolution element automatically.

The CD-ROM disk does not contain all the information needed to access the selected elements at high-resolution and this information is obtained from the service centre computer. Once this information is obtained via the modem, the selected elements are downloaded into the computer and onto a disk, and the user is invoiced or his account charged according to the data released to him.

The distinction between the low and higher resolution elements occurs because printers and typesetting machines are capable of producing a much higher quality image than a computer screen but use more storage, (for example, up to 2500 dots per inch for a printer as opposed to 72 dots per inch for a screen) or a different approach to storing the data, e g a set of mathematical equations. A five inch square image needs 16 to 130 kilobytes at screen resolution but as much as 1 megabyte at print quality resolutions. Similarly for typefaces, a high resolution description is required to produce a high quality printout. The limited screen bitmap can be used to illustrate the typeface but print out of such a bitmap is inadequate for most uses.

Most high quality data elements are too big to be practically downloaded using a modem and telephone line. 100 kilobytes of data would take approximately 15 minutes to download (at 1200 baud). However, if the data is stored on a CD-ROM disk having a storage capacity of over 500 megabytes, it could be downloaded in a few seconds, and securing the electronic key takes only a short while, most of which is taken up by the automatic dialling of the remote computer.