Analogue Teletext (Ceefax)

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• published: 29 Jun 2012
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Just before the analogue switched off (from bluebell hill) I was surprised to find that teletext on BBC 1 was still being transmitted. Teletext is a very weird information service invented in the 70s. It is transmitted though the TV picture. The analogue signal is designed to be fed directly into a CRT (with no processing required by the TV). In-between TV frames there is some time while no picture is transmitted. This is where the teletext is transmitted. (there can be up to 25 lines of teletext transmitted on each frame). Some TVs display the teletext lines by mistake, When this happens you can see a flickery effect at the top of the TV picture. Teletext works by each pixel on the line being either white or black (White is 1 and black is 0). There are 360 pixels per teletext line. (analogue doesn't actually have pixels because it is a waveform. There is normally 720 pixels across, but teletext only has 360 pixels, so the waveform is less dense and will have less error) The 360 pixels (which act as bits) are thought off as 45 bytes. (each byte is 8 bits) The first 3 bytes (or 24 bits) are a set pattern to tell the TV that the line is a teletext line (and also tells the TV the pixels positions in the waveform creating the line) The first 3 bytes (or 24 bits) are 101010101010101011100100 This leaves 42 bytes for teletext. The teletext on one TV channel is divided into 8 different sections called magazines, each work independently. (pages 100-199 is magazine 1, pages 200-299 is magazine2, and so on...), The lines of teletext are continently transmitted on the TV picture, and the lines for the magazines are transmitted all intertwined. The TV will look for the lines for the magazine your in, and filter out the rest. Now lets look at the teletext data, looking at the lines of just one magazine (ignoring the others)... The teletext pages are continuously looping around. Each teletext line line is called a packet. For each page that is transmitted, lets look at the transmitted packets. Each packet has 45 bytes first 3 identify it as a teletext line. The 4th byte on ALL packets says what magazine the packet is for. The 5th byte on ALL packets says the packet number. For each page the first packet is number 0 (so byte 5 = 0) bytes 6 - 13 contain information about this new page (e.g. if its a subtitle page, if its a boxed page with transparency ect.) Also this is where the subpage number is. (The subpage number is used to identify the different pages, when one page keeps changing every-time it come around like with the shares pages) and the rest of the bytes in this packet will be the page header which is displayed at the top. The following packets will literately be the text of each line in the page. (packets 1 to 24) (bytes 1 - 5 are always the same as above, and the remaining 40 bytes are each charactor on that line of the page). As for the colour in the pages, this is done using a mark up language (which means curtain charactors in the page are hidden and cause the following text to change color or background color or hidden text where you press the reveal button) Packet 25 is optional and is a replacement page header Packet 26 is only used on certain listing pages Packet 27 contains 6 links to other pages, the first 4 links are for the colored buttons. the 5th link doesn't do anything!, and the 6th link is for the homepage button on some remotes (this is normally page 100) Packet 28 (optional) is what charactor set to use There may also be some more packets which say other infomation. After this the next packet is number 0 for the next page You may have noticed some of this is not the best use of data. (things like the magazine number uses an entire byte, when it only is ever 1 - 8. (lazy programming?) And the higher packet number get more and more odd, where the existing specification is added to. Also teletext has a few strange features... -some text can be hidden and you have to press the reveal button (rarely used) -page numbers can contain letters so you can't type it in and only access it though the colored links (this was once used on a teletext quiz game) -The number of the subpages on each page can be 4 characters long! (no way would there be this many sub-pages) The reason for this was an idea for a subpage alarm clock system (which never took off) -one time in the past you could call a phone number which let you controll your own dedicated teletext page. -In the middle of the night on BBC 2, they show a TV program that shows pages from Ceefax, In the 70s this was originally for people who's TVs couldn't receive teletext. It is still shown today for some unknown reason (probably to fill time in the night) http://beno.org.uk