![]() ![]() One thing tucked away in the VBI is teletext. You can hideĪll sorts of stuff in there (don't get paranoid, it's mostly just boring 'test' information). Television doesn't display this part of the signal doesn't mean it is mostly junk. So, then, we have this 'blank' bit of the television signal, right? Wrong - just because the 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. This return-to-top takes slightly more time than a simple fly-back, and this is why the VBIĮxists. 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). ![]() ![]() It actually scans every other line, doing this some 300 times for each 'field'. The electron bean scans your television screen from left to right, then flies back to scan the The television's electron beam time to return to the top of the screen, these days it is more historical than anything else. This is an area unused by the actual television - it has been left to give Teletext is broadcast in the "Vertical Blanking Interval" (VBI) lines of the Given a suitable broadcaster, it will even change to and from "daylight savings" time for you. Also, my !Teletext software uses it to accurately update your computer's internal clock. It is used, instead, by a number of video recorders to set the internal clock correctly. 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). PDC will tell your video recorder when to start and stop in order to correctly record your desired programme. Using PDC, you never need to worry about programmes being broadcast late, or interrupted for some reason. Its purpose is to inform your video recorder about what is currently being broadcast. This is a special 'packet' of data in the teletext signal. You cannot ordinarily receive Program Delivery Control (PDC) with your computer. Possibly the most useful things provided by teletext These frames contain up-to-date news, TV guides, chat, advertising, and other things that theīroadcaster thinks may be of use to you. It consists of a variety of numberedįrames that you can access via a suitably equipped television or computer. Teletext is free, simple and up-to-date (well, mostly). Teletext - what's teletext? What's teletext? ![]()
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