Definition

The Vidéotex is a telecommunication service allowing the sending of pages made up of texts and simple graphics a user in answer to a request of this last (interactivity). These pages are intended to be visualized on a cathode screen, for example on a television or any other screen with the format of television. The Minitel French is the terminal adapted to this service. The name of the terminal ended up being used, in the language running, like generic name of the service and thus in French became synonymous with Videotex. The service uses a standard of communication based on a syntax of description of the pages. The service Télétel (or Minitel) uses the standard Antiope (Option 2 of the T.100 international standard)

The service is generally rendered by a comprising system of the terminals of the keyboard/screen type connected by the phone network commutated to a specific access point ensuring connection to a waiter through a network of data transmission (in France the network TRANSPAC of commutation of data per packages) worms of the waiters, even worms of the even terminals. Only the service Teletel (also called Minitel, of the name of the terminal), based on an option of consumption to the request permitted by the use of Access points Videotex integrated like automatic exchanges in the commutated phone network, knew a one significant duration commercial exploitation. The British services (Prestel), Germans (Bildschirmtext), Canadian (Télidon) and Japanese (Captain), all based on a system of subscription, were not business success and were stopped shortly after the end of the experimental phases. Nevertheless, the system Viewdata (whose Prestel was the trade mark) continuous to be used by the travel agencies in the United Kingdom.

History of the Standard

The first world technical standard of the Videotex is the T.100 Recommendation of CCITT, ancestor of UIT-T. This standard gathers the characteristics of the four systems exploited in 1980, date of adoption of this standard:

• System Viewdata, used in the United Kingdom for the service Prestel
• System ANTIOPE (Numerical Acquisition and Télévisualisation d' Images Organized on Pages of Writing) used in France for the services Teletel and Electronic directory (the name of the terminal, the Minitel ended up becoming the common noun to these services).
• system NAPLPS (North-American Presentation Level Protocol Syntax), used in Canada for the service Télidon
• system CAPTAIN (Characters and Pattern Telephones Access Network Information), used in Japan for the service éponyme of NTT

In the same moment, the international standard Recommandation F.300 of the CCITT defines the rules of exploitation and the characteristics of the service him even. It was into force until 2004, France being the only one has to still maintain the service.

One second international standard, adopted by the CCITT in 1984 defines a certain number of functions of ordering of the network corresponding roughly to the functions of the Access point Vidéotex.

First European standard (T/CD 06-01), adopted in May 1981 by CEPT (European Conference of the administrations of the Stations and Telecommunications) gathers the two European versions of T.100 and adds technical functions to it coming from German proposals (Bilschirmtext) and Swedish. It was updated regularly until 1988 and is currently maintained and published like European standard of telecommunications by ETSI under the title " ETS 300.072 Syntax of data of the layer of presentation of Vidéotex". Its last versions include the photographic videotex (use of the standard JPEG), and the transmission of forms of characters (DRCS or redefinable character sets). The successive European standards were:

• T/TE 06-02 (ETS 300.073) in May 1988: Geometrical mode of the syntax of data of the presentation layer of the Videotex
• T/TE 06-03 (ETS 300.074) in May 1988: Syntax for the transparent transfer of data (intended for the remote loading of software and other forms of data)
• T/TE 06-04 (ETS 300.075): Data retraitables for the Videotex (which one can compare to information of active scripts in the terminal)

and finally

• T/TE 06-05 (ETS 300.076): Identifier of the capacities of the terminal of Videotex (TFI)

Features of the standard of presentation of Videotex (CCITT T.100)

Characteristics common to options 1 and 2: alphamosaic mode

The alphamosaic mode is the base of the services of European Videotexes and they have, fortunately, a whole of common characteristics. They are based on a model of alphanumeric terminal traditional of the teleinformatics of the Seventies: a memory of maintenance of page (RAM) contains the codes of the characters to be visualized and a memory (ROMANIAN standard dead) contains the forms of the characters. A base of time organizes the periodic reading of the RAM in order to papillottement post the page with a frequency (50Hz or more) avoiding.

The originality of the alphamosaic system, imagined in the United Kingdom about 1971 by the engineers of BBC within the framework of the project Ceefax, consists of several characteristics:

• the capacity of the screen is limited to 24 or 25 lines of 40 characters, is a total of approximately 1000 characters. This makes it possible to use memories of 1kbit which, in the Seventies constitute the standard of the market but also to visualize the page on the screen of a domestic television set.
• N the other hand of this limitation, the screen gains the possibility of visualizing the pages colors. Eight colors are used corresponding to the 8 combinations of the three primary colors. This possibility constitutes the introduction of the concept of attributes of the characters .
• In addition to the traditional data-processing alphabet (international alphabet N°5, ISO CEI 646 also called ASCII in the United States), an alphabet says graphic or mosaic. The mosaic characters are composed of a matrix of 2 points of width on 3 points height, representative thus 64 different characters. They can be regarded as the pieces of an image which will be stuck the ones against the others to constitute it. In addition to the attributes of color, this mosaic alphabet has the attribute of joint (the character is jointed or separate). A photograph of the gallery gives an example of use of this attribute: this page with a few hundreds of others is one of the first pages of the parallel mode presented publicly in September 1976 within the framework of a presentation of the system Antiope to exposure СПОРТ 76 to Moscow.

The two alphamosaic systems are different by the management style from their attributes qualified respectively from series (or serial) for the British system of origin and parallels for the system of French origin.

Option 1: alphamosaic mode series

It is the original mode of the Viewdata system. It is built so as to ensure a visual compatibility with the system Télétexte (Ceefax) and thus to use the same technology of terminal. Syntax of it is however different in order to allow the transfer on a transparent data network.

Model of terminal

The principle of construction of the terminal is derived from that of Ceefax in which the codes received on line are put directly in memory. Like the transmission used by Ceefax is done by binary words of 7 characters, the memory thus comprises 7 kilobits. On the 128 combinations, 32 of them are codes of attributes and the 96 others represent characters. When the code is a code of caracère, it is transmitted to the generator of characters, if it is a code of attribute, it is put in memory in a latch (latch) and maintained until a new code of attribute arises. The contents of the latch order the operation of the generator of effect. the effects used are the color of the character (one among 8), the fill color, the flickering, the typestyle (alphabetical or mosaic).

The principal consequence of this architecture is that a code of function occupies on the screen the place of a character which is generally represented then like a space.

In figure Ci below, word VIEWDATA is written with a space between each letter because of the change of color between each letter.

Tables of coding

The two images below represent the same page, photographed on a console of creation of pages functioning according to the series mode. On the second version, one activated the function of visualization of the codes of attributes in the form of characters visible. This version makes it possible to include/understand the constraints of the series mode by showing the number of necessary spaces in particular to manage the attributes. One can see that the characters blast through , not requiring no space to be visualized when one is in " mode; graphique" (as one calls also the mosaic mode). One can also see the sequence of 3 or 4 necessary codes to begin a page when there is a bottom: Color of the character again followed bottom fix the fill color. If one remains about it there, the characters will be registered in the same color as the bottom, which would make them not very visible. It is thus necessary again to fix the color of the character which will follow. If that Ci is alphabetical, one remains about it there. If not, one prescribes also a " maintenance of the graphique" who allows to replace the space required by the codes of attributes to come on the same line by a repetition from the preceding graphic character: one sees what that gives on the first line for the code " mosaic vert" on the level of Pas-de-Calais.

Option 2: parallel alphamosaic mode

It consists of a syntax of description of pages independent of the layer of transfer of the data. Thus, the data can they use a system of telecommunications (including the Phone network commutated) but also use a system of broadcasting of data provided that it respects independence between the layer of transfer and the syntax of data.

The pages are made up of 25 lines of 40 columns of symbols, which can be either of the alphanumerics (correspondent with a wide repertory required by the 39 European languages using a form of Latin alphabet, or of the mosaic characters allowing the composition of elementary graphics. The first line is never used directly, it plays the part of line of state, posting the price of the communication for example. Each character has a certain number of attributes, which are the color of writing and basic (8 colors are available on a Minitel), the visual attributes the such video inversion, the flickering, and for the mode text underlining, and posting cuts it (normal, double width, double height or both joined together). The alphabetical attributes of each character as mosaic can be définis' freely and independently of those of the others caractères' .

Possibilities of other natures (mosaic smoothed, characters dynamically redefinable, characters nonLatin such as Cyrillic, Arabic or Greeks, even nonalphabetical for Chinese or Japanese were gradually introduced.

The functions of size and the independence of the choice of the colors are not available in Option 1 derived from the Viewdata system used by Prestel, because of the party taken of visual compatibility with the British diffused teletext in which there is not independence between the syntax of data and the protocol of transport. In the same way, the alphanumeric repertory is limited to the not accentuated characters of the English language.

Model of terminal

This type of approach supposes that in memory, the codes of characters and those of the attributes occupy each one part of the memory of maintenance of page and thus this type of terminal is more expensive in memory capacity. Up to three times more taking into account the number of standardized attributes:

• Color of the character (presence or absence of red, green, blue either 3 elements)
• Size of the character (simple or double in width and height or 2 elements)
• Flickering: 1 element,
• Fill color: 3 elements,
• Nature of the character (alphanumeric or mosaic) is 1 element which can be multiplied if the terminal accepts for example nonLatin characters.
• reversed Fund: 1 element,
• Underlined: 1 element,
• Masked: 1 element,

One adds moreover an attribute which is especially used in the mode broadcast Télétexte to allow subtitling (encrusted or not in an external image) what leads to 14 binary characters of memory for the attributes, that is to say 22 in very if a repertory of 256 characters is counted.

Information coming from the line is decoded before being put in memory. The decoder is not illustrated on drawing Ci below.

Tables of coding

Option 3 or alphageometric mode

It is primarily based on the vectorial representation of the images and defines a syntax independent of the resolution (and other visual capacities) of the terminal which carries out the interpolation corresponding to its properties. The coordinates used in the instructions are expressed in fraction of screen unit (the screen is registered in a square unit) and the terminal is given the responsability to adapt the layout to the resolution of its display system. However, the use of the geometrical mode is optimum only if the model of terminal is based on the use of a memory of points (bit map) and not of a memory of characters. Canadian of the Télidon team showed the possibility of producing a geometrical decoder on an alphamosaic terminal in parallel mode.

Option 4 or alphaphotographic mode

It describes the pages in the form of an image coded by a process of compression. The original system, of Japanese origin (CAPTAIN) uses a code with variable length similar to that of the telefax. Its European version (described in standard CEPT) uses the photographic system of coding JPEG.

Network architecture of the Videotex

The network videotex generally supposes an access by modem on the commutated phone network.

In the services implemented in the Eighties, the modem of the terminal is an asynchronous modem of type CCITT V.23, the pages are generally sent to the terminal customer with 1200 bits/s. the answers of this one (mainly keys pressed by the user) are turned over to the access point to 75 bits/s. Of the Minitel rapids using a synchronous modulation to 4800 or 9600 b/s (the modem of the telefax) were sold in the Nineties. More recently, of the software of emulation of the Minitel on personal computers used accesses Internet to render the Vidéotex service.

Centralized systems (Viewdata, Bildschirmtext, Electronic directory)

In a centralized system, the terminal is connected directly to a single waiter. Generally, this single waiter is in fact a " cache" regional of the most asked pages, the whole of the pages being available in the National center. The service providers download their pages in this waiter and each page has a single national number giving arborescent access. The component number of figures this number is significant depth of the page in the tree structure. Thus, an information provider sees himself allotting a number for his banner page and all the numbers longer which start with this number. (Ci below, some photographs illustrate this matter). N the other hand of this heaviness, navigation, simple though long, requires only the telephone keyboard (digital and nonalphanumeric keyboard).

Decentralized systems (Teletel)

These are the decentralized systems whose architecture precedes more that of the Web on Internet network. The access point plays several parts:
1. With respect to the user

• Identification of the user by his phone line
• Echo of the characters emitted by the keyboard towards the screen of the terminal
• Accounting of the time of connection and transmission to the invoicing according to the mode and in " mode kiosk complet" according to the stage
• Preserves the anonymity of the user.

2. With respect to the network

• Put out of package of the characters emitted by the keyboard and sending for package full or by reception with the command code (Sending, continuation,…)
• Conversion of the address symbolic system into number TRANSPAC (similar to function DNS)
• Routing towards the waiter
• possible ISDN

3. With respect to the waiter

• Accounting on behalf of royalty to be transferred with the information provider

The French service comprises 3 modes:

• mode known as " 3613 " corresponds at a free public service or a service by subscription: the information provider pays the supplier of access and network the whole of the expenses of network. In the case of the subscription, the user remunerates the service directly. Nothing appears on its telephone bill.
• mode known as " 3614 " in which the user pays the share network but not the price of the contents: it is the mode which corresponds to the current majority use of the Internet. It does not prohibit a under-mode in which the price of the contents is directly regulated by the user with the supplier. It then belongs to the supplier to identify the user (by login and password). If not, the access point preserves the anonymity of the user compared to the service provider.
• the mode kiosk (and its various tariff stages) in which the telephone bill is used as means of payment, not only of the expenses of telecommunication, but also of the price of the information which is transferred with the information provider. This mode thus functions, from where its name, like a system of sale of the press: the newsstand ensures the collection of the payments of the customers and preserves his share by transferring theirs at the service of transport (NMPP) and at the editors (service providers).

Interactive commercial systems based on Videotex

• the Minitel, launched in 1982 in France, is the business service which was the most success, partly thanks to the variety of the services available and with its mode of invoicing, based on the use of access points integrated into the commutated phone network: no the subscription, time of connection accountancy in the traditional telephone bill. Teletel, the network of the Minitel, is always under operation nowadays. Derived systems were tested in the USA (Beautiful South), in China,… without giving place to commercial exploitations. DoCoMo uses today, in Japan, the architecture of the French system (access point and invoicing on the telephone bill) for its services emod
• Prestel, launched in 1979 with the the United Kingdom, based on option 1 of standard CCITT T.100, met much less success. The system was abandoned.
• Bildschirmtext (BTX) was launched in 1983 in Germany. The terminals, based on a syntax derived from the Antiope standard provided with a multitude with complementary options graphic, were expensive, which limited its adoption. The system was abandoned.
• Beltel was launched in South Africa in 1986.
• Telidon was launched to Canada in 1982. It is based on option 3 (alphageometric) of the standard CCITT T.100 which constitutes a prefiguration of the systems with vectorial graphics.

• AlexTel was launched to the Canada at the beginning of the Années 1990.

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