Tuesday, June 26, 2007

Things figured out. Week 2. Teletext/Aertel

Many years ago, in an age before the internet, the bbc invented a thing called teletext. It was a revelation. Until now most people would say that magic invented Teletext. Most people should read the following.....

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...
The electron beam 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.
Phew!!!! Who on earth would of known that such a brilliant system worked so easily! It wasn't magic afterall just sheer genius.
Teletext was the english version, after a while RTE introduced Aertel to Ireland and went "on-line" 22nd of June 1987.
My personal favourite memory of Teletext was staying up all night waiting for Teletext to tell me the Fabrizio Ravenelli had signed for Tottenham..... my wait was in vain as eventually he signed for Middlesborough (what a cunt). Feel free to leave your favourite memories of Teletext/Aertel Below.

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