What's teletext?


Teletext is free, simple and up-to-date (well, mostly). It consists of a variety of numbered frames that you can access via a suitably equipped television or computer.

These frames contain up-to-date news, TV guides, chat, advertising, and other things that the broadcaster thinks may be of use to you. Possibly the most useful things provided by teletext are:

You cannot ordinarily receive Program Delivery Control (PDC) with your computer. This is a special 'packet' of data in the teletext signal. Its purpose is to inform your video recorder about what is currently being broadcast. Using PDC, you never need to worry about programmes being broadcast late, or interrupted for some reason. PDC will tell your video recorder when to start and stop in order to correctly record your desired programme.

On the other hand, though, the clock signal (an extended packet, not the clock at the upper-right of the teletext page) is useless for any television (although some TVs display the channel identification that is also present). It is used, instead, by a number of video recorders to set the internal clock correctly. Also, my !Teletext software uses it to accurately update your computer's internal clock. Given a suitable broadcaster, it will even change to and from "daylight savings" time for you.


Teletext is broadcast in the "Vertical Blanking Interval" (VBI) lines of the television signal. This is an area unused by the actual television - it has been left to give the television's electron beam time to return to the top of the screen, these days it is more historical than anything else.
This picture shows you what I mean...

VBI example; GIF 8K

The electron bean scans your television screen from left to right, then flies back to scan the next line. It actually scans every other line, doing this some 300 times for each 'field'. Then the beam needs to return to the top left to begin scanning the missing lines, or the next frame (this is known as 'interlacing'; there are 625 lines on a PAL/SECAM television (of which around 576 are 'visible'), and 525 lines on an NTSC television; there are 25 complete frames (30 for NTSC) every second, and each complete frame is built up from two fields, every odd line and every even line).
This return-to-top takes slightly more time than a simple fly-back, and this is why the VBI exists. The VBI is larger then you might expect, as it also includes unmistakable synchronisation data so each and every television knows exactly where in the broadcast signal each frame begins, and can lock on to that, knowing also when each line begins.
So, then, we have this 'blank' bit of the television signal, right? Wrong - just because the television doesn't display this part of the signal doesn't mean it is mostly junk. You can hide all sorts of stuff in there (don't get paranoid, it's mostly just boring 'test' information). One thing tucked away in the VBI is teletext. If you were to 'play' with the Vertical Hold on your television, until the picture started to roll slowly, you would see a few lines of blinking and flashing dots of all sorts of colours, towards the bottom of the VBI and above the normal picture.
Those coloured dots are the teletext signal, and look something like this:
VBI example; JPEG 13K

(this example is slightly blurry as it is from a video recording, and most videos do not preserve enough of the VBI to make it possible to 'record' teletext)

HDTV is new, and different, and offers two resolutions (720p or 1080i) which have little in common with the older broadcast formats described here.


If you are interested in the actual format of the television frame, or about how teletext actually works, or even "what's that black hammer-shaped thing at the top of the VBI for?", a myriad of books how been written on it. Simply visit your local library (or on-line bookshop, if you prefer) and look for books with interesting names such as "Television Receiver Servicing". If you are handy with a soldering iron and the type that buys old video recorders (or if poking around inside an old VCR sounds 'cool' to you), then look for the "Video Recorder Servicing Guide". It is packed with all sorts of useful information. I have borrowed it may times from my local library (when I lived in England), and used both it and a wodge of blu-tack to get numerous old Betamax VCRs up and running again...


One question often asked is "Why is teletext so bloody SLOW!!!?". The answer to this is two-fold. Firstly, teletext has a very limited bandwidth. I cannot remember the exact figures, but you get fifty 'fields' (25 frames) each second (60/30 for NTSC) and it takes a couple of frames to make a complete teletext page - thus meaning your data rate is about 12 to 20 pages per second, though I am open to correction on this number.
There are numerous possible teletext pages - usually up to '99' in a magazine, with up to seven magazines. While the pages are actually numbered in hex (base 16), you'll rarely see pages with numbers like 14F because television users have no way to tap in the 'F'.
Also, some things, such as the main index page, are broadcast more frequently than other pages (so people just turning on teletext don't see a blank screen for ages). Also in the teletext signal is the TSDP (where the accurate clock information is taken from), and subtitles (if provided).
So, you see, there are a few extra things... Well, let's assume these extra things take a quarter of a page worth of data each, that's one less page every second - so it does factor into the time taken, even if only a little bit.

On the other hand, though, teletext is completely free. You don't pay any extra (over your normal licence fee) to receive it, and you don't need to connect to any sort of service to request pages. Instead of being 'request-driven' like the internet, teletext just sends each frame to you in sequence, over and over. If you had fast enough hardware (and I mean really fast), you could probably grab a complete snapshot of all the pages broadcast in about twenty or thirty seconds. I've not seen such a thing. Instead, we typically fetch a page, then fetch another page, and so on.
A method I used to use is to make a note of the pages I wanted to read, and as soon as one page appeared, I'd tap in the number of the next. How well this works depends on the speed of your broadcaster's teletext (some are quite fast, some are tediously slow), and how quickly you can read.

If you are viewing teletext with a television, then it isn't all gloomy. Many televisions tend to try to speed up teletext by...

Obviously this system can be tricked if you fetch, what appears to the television to be, apparently random frames. For example, on CNN, I might fetch: It is unlikely that auto-fetch would help much. However many users read things in sequence, like news pages 102 to 111 in order. Auto-fetching would make things faster for them.

If you are viewing teletext with a computer, then how you can benefit depends a lot upon the capabilities of your teletext hardware. The Ground Control unit, in common with many, can fetch one frame at a time (although the unit has the unrealised potential of fetching four pages independently). Some, more expensive, receivers can fetch two or more pages at once.
Most of these problems can be solved in software, if you don't mind an initial wait.
For example, the Teletext+ software advises you to give it about fifteen-twenty minutes of 'warm up' time. In this time, it'll work it's way through the pages it knows you like to read (each page is 'scored'), so when you come to read the teletext information, it appears instantly.
My own Teletext (RISC OS version only) works differently. There is an auto-fetch system, where it will just work through each possible frame in turn, but it isn't very effective. Instead, it is recommended that users write a 'script'. This script will direct Teletext to fetch certain pages from certain channels. Then, you can either view the pages that you fetched, or - the coolest feature - you can get my Teletext to turn the pages fetched into a document of some description. All I need to do to get my regular digest is to load !Teletext, and click on 'OK' when it asks if I want to run the '!startup' script. Then I just forget about it...
...I can write letters, do some programming, whatever I choose...
...and, say, ten minutes later up will pop an Ovation Pro document, all nicely formatted and automatically opened. I don't actually need to look at the teletext viewer to read anything; though it is there if I choose to. Where my system wins over Teletext+ (in my opinion) is that I can customise the scripts - like show me only the current affairs stuff. In addition the script language has date awareness so it can actually look for different things depending on what day of the week it is.
With my !Teletext, and quite possibly with Teletext+, it is possible to do all sorts of other things with the script language. From CNN I can receive up-to-date currency information. Well, sorry, but it doesn't really help to look at the Pound-Euro rate, and see a number like 1.6235. So I wrote a simple script to read all these boring numbers, and show me, instead, a nicely formatted table giving me Euro amounts from 5eur to 100eur (in 5eur increments), and showing what each amount is worth in GB pounds, US dollars, etc...


So, to conclude: Teletext might be old (I think it was initially devised and put into service in the late '70s), and it might be slow, but it is useful. Sure, if you have internet, then go look up stuff on-line. Many broadcasters have impressive and feature-packed pages (the Radio Four website allows you to search the schedules, backwards and forwards, and you can even listen to recently-broadcast programmes and interviews!); but if you don't have internet (or don't want to connect), all is not lost...
I can tell you who is on Jay Leno or Larry King this week, I can tell you the latest hole the British Labour Party have dug for themselves, I can tell you the top twenty in England, and I can even tell you the birthday of that girl presenting that programme on MTV right now (her name is Nora Tschirner (don't ask me to pronounce that!), and she was born in Berlin 1981/06/12 - a veritable baby compared to a crusty old git like me (1973/12/16!))... simply by looking on teletext.


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Copyright © 2003-2006 Richard Murray