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Ceefax was a service that provided a series of pages of coloured text in a format similar to a 40-column display to a compatible television set. Each page was numbered, and you could address each page by its number. Typing in a number would bring up the relevant page with about the same latency as a dial-up modem.

This hasn't made sense to me since I knew how a television worked. The television can't transmit back to the transmission tower; even if it could, my aerial is much too small to send a signal that could reach the transmission tower without being drowned out by other signals from the transmission tower. Even if it was, there'd be interference from the other televisions' signals. And even if all of the signals managed to get to the transmission tower intact and there was the necessary infrastructure to process all of the signals, there'd be no way for the transmission tower to send an extremely directed signal to the specific aerial that transmitted to it... surely.

I am certain that there was not enough RAM in my old CRT to store all of the Ceefax pages whilst the television was in Standby mode (although it did seem to take a while for Ceefax to become available after power was given to the machine...), but I can't see any other ways that this could have worked.

How was Ceefax designed, implemented and transmitted?

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    Teletext is specified in ETSI 300 706, you can read about all the details. Basically it consists of 45 byte packets transmitted during vertical blanking on lines 6-22 and 318-335. That's fast enough to send a few 100 pages in a few seconds. – dirkt Feb 27 '17 at 19:02
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    @dirkt You should include that in an answer; that's enough technical details to create an implementation! :-) – wizzwizz4 Feb 27 '17 at 19:37
  • Too lazy to extract a summary. If you want to do so and add it as an answer, you are welcome. :-) – dirkt Feb 27 '17 at 20:21
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It's a one-way Teletext system. Basically, the pages are sent in conjunction with the broadcast signal. The TV receiver knows how to extract the page information from the analog video signal.

As you noted television broadcasts are one-way; all of the page content would be sent one page after the next. The receiver could cache this information if desired and capable; otherwise, there would be a delay until the requested "page" was sent again.

  • Was this really fast enough to send all of the pages? I suppose it must have been; text is much smaller than video / audio. – wizzwizz4 Feb 27 '17 at 18:29
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    @wizzwizz4: not all pages are transmitted in each frame. If you've ever used a TV with teletext, you'll see the currently received page number running through in the top line when selecting a new page. Without caching, you had to wait a few seconds. Some pages (e.g. the index page 100) are transmitted more often than other pages. BTW, Teletext is still in use e.g. in Germany and France, and broadcast digitally via DVB-T. – dirkt Feb 27 '17 at 18:55
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    @dirkt That's what that number was... I thought it was a glitchy frame counter. – wizzwizz4 Feb 27 '17 at 19:38
  • Didn't know about Teletext over DVB-T in France/Germany. Always thought there were good opportunities for a Teletext service on DVB, maybe even with images! Pity it was axed in the U.K. – jamesfmackenzie Mar 25 '17 at 8:31
  • @jamesfmackenzie It was less "axed" and more... deprecated. The underlying technology was dropped, so it just went. – wizzwizz4 Mar 7 '18 at 19:25
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Ceefax uses the Teletext system. This protocol encodes data, such as text, in the vertical blanking periods of the PAL video signal. Each packet is 45 bytes long, where each byte is normally sent with the LSB first.p17 Packets could be sent from lines 6 to 22 and 318 to 335 inclusive.p14 Each data packet occupies one TV line.p15 The first three bytes are used for identification and synchronisation, the next two are used to define the packet address, and the remaining bytes are used to carry data.p17 The areas of the protocol determining the actual data are complicated, but I hope to have a summary written out soon.

Specification: ETS 300 706 - Enhanced Teletext specification - ets_300706e01p.pdf
Thanks to dirkt for finding this document.

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    It would be good to work out the math here. (40 payload bytes / packet) * (1 packet / line) * (35 usable lines / frame) * (25 frames / sec) = 35kbyte/s or 280 kbit/s. That's enough to send a pretty substantial amount of text at a reasonable refresh interval. The latency, for the most part, would be the time waiting for the relevant bit of data to come around again. – hobbs Feb 27 '17 at 22:16
  • What's nice (also @hobbs) is that teletext was used to distribute source code for the BBC micro – Chris H Feb 28 '17 at 10:09
  • If your TV let you, you could play with the picture size / timing settings and see the black/white blobs in the first (and last?) few lines of the picture that aren't normally displayed. – TripeHound Feb 28 '17 at 10:43
  • @TripeHound If your TV let you, yes. It would probably be easier to do this on a modern TV with a software emulator, as CRTs need the blanking interval. – wizzwizz4 Mar 7 '18 at 19:23

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