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WHAT IS VIEWDATA?
Conventional terminals, such as those used to access BT GOLD, operate using just raw ASCII characters, they have no facilities for colour, graphics or other features such as reveal A system was developed where sequences of ASCII characters could be recognised to change colours, introduce flashing and so on. This system is paged based and is called Viewdata. The PRESTEL service, run by British Telecom is one example of Viewdata in action
ASCII terminals operate by scrolling a screen of displayed text upwards when the bottom line is reached. Viewdata on the other hand organises information into frames, each frame occupying the whole of the screen. A frame consists of 24 lines of 40 characters, a total of 960 screen locations.
A page of Viewdata information may use one or more of up to 26 frames, each page can be accessed at any time with its unique page number The frames within the selected page are identified with a lower case letter in the range a to z. displayed after the paae number
Tne graphics of viewdata are made up using a system of alphamosiacs. Each of the 960 screen positions is actually made up of six smaller blocks, any combination of which may be displayed. These mosiacs allow simple block graphics to be implemented, as the screen pictures show.
There are eight colours: black, white, red. green, cyan, magenta. yellow and blue that can be displayed. Also, colours may be flashing, and characters can be single or double height. Any combination of characters may be concealed from view, for instance the answer to a quiz, and revealed by pressing a button on a keyboard.
Until recently most Viewdata pages were built up character by character, line by line, but now new techniques have been developed that allow any part of a displayed page to be changed dynamically, that is at any time, rather than line by line. This has been used to great effect in creating moving pictures. The Micronet Christmas Card demonstration is a good example.
The PRESTEL sen/ice is British Telecoms Viewdata service. Within PRESTEL are thousands of pages covering all aspects of home and business applica tions PRESTEL. like many other Viewdata systems, is two-way, that is information may be sent to PRESTEL as well as being received from it. An example of this is the mailbox facility which allows you to send a message to any other user providing that you know their mailbox number. The next time the recepient logs on. they will be informed that there is a new message for them which they may read at any time.
The procedure for logging onto PRESTEL is quite straightforward. Firstly, check that your interface and MODEM are connected up and selected for 1200/ 75 operation and then dial up the PRESTEL service. Almost all regions have a local number for PRESTEL which can be obtained either from the operator, or when you are on- line. If you cannot obtain a local number then the Enterprise computer in London can always be accessed on 01- 618 1111.
The first page that is displayed informs you which computer you have been connected to and then asks you for your customer identification code. If you have not yet subscribed to PRESTEL, then a demonstration number can be used to log-on to give you a sample of the service that can expect; the ID code for this demonstration number is AAAAAAAAAA (that's a big ten-four, little buddy).
When a valid ID code has been entered, the next screen is displayed which asks for the password for your ID code. This is a four digit number, and for the demonstration service is 4444. Having entered the password, you are finally logged into the PRESTEL service.
The next page that is encountered is the main index page for the PRESTEL service. From this page, a directory of what is available and where you can find it can be accessed.
Most of the time, you will find that you will be guided through the frames by being prompted for a keypress at the end of each one. However, you can move to a particular page at any time by keying in its number. The sequence required is to first enter a star (*) character, followed by the page number and terminated by a hash (#) Character.
It is also possible to step back through the frames that you have called until you reach the last 'a' frame that you accessed by typing a star followed by a hash A couple of good pages to try are '13888# and '60161# The first is a demonstration of graphics and includes several 'galleries'containing some very striking pictures. The second is a demonstration of dynamic frames, which also serves as a very good test for your software - if it can handle these OK then the chances are that the rest will be displayed without problem. When you have finished using PRESTEL you should logoff by going to page 90 using *90# , whicn will talce to the exit page and then log you off from the PRESTEL computer
The pages on PRESTEL are provided From a wide range of sources, these sources are officially termed Information Providers or IPs for short. The information ranges from financial information provided from the City to the latest sports news provided by news agencies. Not all the information on the PRESTEL service is available to all of its users. It is possible for an IP to permit only certain customer IDs to access their pages, and more often than not these customers are charged every time they use these pages. This system is known as a Closed User Group (CUG). An example of a CUG is MICRONET 800 which provides information about home computers and peripherals as well as carrying software reviews and a 'Swap Shop' service to get rid of your old system when you reckon its time to 'upgrade'.
Viewfax 258. one of the IPs on PRESTEL, have been developing a system to download software over the telephone to an Amstrad computer. The system is due to go on line from October. By inserting special character sequences and encoding programs into pages from PRESTEL, it is possible to send data in a form that can be converted back to a program at the receiving computer.
Needless to say, any commercial software that you download you will be charged for on your 'phone bill although there is some software that is provided completely free.