It’s DOOM, But In Teletext

We’ve seen the 1993 id Software classic DOOM running on so many pieces of unexpected hardware, as “Will it run DOOM?” has become something of a test for any new device. But will it run in the circuitry of a 1970s or 1980s TV set? Not quite, but as [lukneu] has demonstrated, it is possible to render the game using the set’s inbuilt Teletext decoder.

Teletext is a technology past its zenith and which is no longer broadcast in many countries, but for those unfamiliar it’s an information service broadcast in the unseen lines hidden in the frame blanking period of an analogue TV transmission. Its serial data packets can contain both pages of text and rudimentary block graphics, and we’re surprised to learn, can include continuous streams to a single page. It’s this feature that he’s used, piping the game’s graphics as a teletext stream which is decoded by the CRT TV and displayed as a playable if blocky game.

Delving further, we find that DOOM is running on a Linux machine on which the teletext stream is created, and the stream is then piped to a Raspberry Pi which does the encoding on to its composite video output. More powerful versions of the Pi can run both processes on the same machine. The result can be seen in the video below, and we can definitely say it would have been mind-blowing, back when DOOM was king. There are plans for further refinement, of which we’d say that color would be the most welcome.

21 thoughts on “It’s DOOM, But In Teletext

    1. Well it certainly beats playing Bamboozle.
      Who knew it was capable of updating that fast.
      Making use of the unused Teletext chip could certainly be something that could make it’s way into other projects.

      1. When you’re sending the data over tcp, like in the demo, then you can update as fast as you want. Injecting it into a RF stream and then having the TV decode the teletext itself might not work as well….

  1. “we’re surprised to learn, can include continuous streams to a single page”

    Why is that surprising? Half the point of Teletext was to be able to provide closed captioning on live TV, and continuous streaming is the key to making that work.

    Am I just old?

    1. Hm, not sure. Where are you from, if I may ask ? I’m from Germany and I can’t remember ever using subtitles via Videotext (that old fashion type as received via analogue terrestrial TV directly or from analogue satellite TV receiver – via RF modulator or Composite). I rather remember using Videotext to read news, admiring those graphical weather charts and so on. The subtitle feature is something that I don’t remember using. 🤷‍♂️

      1. Some networks had special pages with the closed captioning included and many times the reason was to provide captioning for the hearing impaired. YMMV depending on where you lived at the time though.

        I should add that some networks also used the teletext as a data-channel to transmit data, like scheduling information for example or remote control of equipment.

        Some networks, like MTV back in the day, had a special page where all the scheduling information was available listing everything down to the second although it was a bit of hassle to access it since they inverted the data-signal specifically for that page and you needed to modify your TV-set with some extra logic so it could decode the signal correctly. Quite handy when you wanted to know when the next ad-break was coming up.

      2. Here in Sweden we often used page 199 for closed captioning. Often at the start of the program it would show a small text showing that closed captioning is availible for the program. I was always impressed by the animations on some of the pages as a kid.

    2. Teletext was a completely different technology than that which was used for subtitles. The BBC introduced it first with CeeFax in 1974, well before subtitling. It spread across Europe and even had a brief stint in the US in a couple markets using a different technology(but the same blanking interval concept), NABTS. Teletext technology was so much more than displaying a line or two of static text in a small box. There were interactive explorable pages(that inspired the BBSes of early telephony data transmission) that were not only full-screen, but could be updated in real-time. There were classified ads, dating included, chat rooms, real-time sports updates, stock tickets, and so many other clever uses of the text. Most of the US never received this, and the regions that did get it didn’t get the investment in programming that made it such a hit in the UK and rest of the world. Parallel to the UKs teletext, the US Public Broadcasting Service had the first subtitled for the hearing impaired broadcast in 1973. As this gained traction the FCC dedicated line 21 of every broadcast frequency to Closed Captioning and set top boxes(which cost as much as or more than a new TV) were on the market for people who had impaired hearing. The US was also heavily investing in the Internet and telephonic data communications which would render Teletext service moot for the US market.

      All that to say, due to the very small amount of text transmitted through CC, it can update relatively fast but expanding that to a whole screen of moving graphics rendered at up to 29.97 fps(NTSC) or 25(PAL) is another feat entirely.

    3. Continuous streaming for subtitles is not really continuous – it’s just updated every time the text changes. Like most pages are updated every 30 seconds(?) or so, to cycle through multiple subpages on the same topic. But for this Doom teletext encoder, since there are (I assume) no other pages being transmitted it’s possible to repeatedly refresh the same page at full speed.

  2. The streaming thing reminded me of something that Teletext could do back in ye olden dayes in the UK. You could buy a small optical reader that sat in front of the TV screen (bottom right IIRC). The other end connected to your BBC Micro. The broadcaster could stream data to the screen and the reader dumped the it into the Micro. Et voilà, you had just downloaded a programme for your computer to run.

    The “data area” of the screen was about one square inch so you could still watch TV too.

    Yes, it’s a very hacky way to download data but in the 1980s it was indistinguishable from magic for many people.

    1. I don’t remember that, but the BBC produced a teletext adapter that plugged into your aerial and there were pages you could then download BBC micro software from. If you looked at those pages on a TV then you could make out strings but it was hard to read tokenised basic and assembly language that was just dumped out to the screen (some of the codes would change color, height, move the cursor etc as well)

    2. Reminds me of those bar code readers of the 1980s.
      Some computer magazines used them to save the readers of typing down listings.
      Unfortunately, it didn’t catch on. It was more convenient to just include a 5,25″ floppy.

      Then, there was Channel Videodat here in Germany.
      It used the invisible parts of a video image to story data and needed no back channel.
      Lots of freeware, shareware and multimedia files were transmitted that way.
      In a way, it was like receiving the newest shareware (‘shovelware’) CDs with hundreds of MBs for free.
      An access software for DOS and Windows 3.x (?) was available in the early 90s, I believe.
      Unfortunately, it didn’t co-exit with Videotext technology.
      So the not-so successful Videodat had to go once the broadcaster wanted to jump on the Videotext wagon.

      Which in retrospect was sad a bit, maybe. Bildschirmtext (BTX), a Videotex service, was superior to Videotext anyway and better looking, if it only wasn’t so pricey. Videotext and Videotex both are based on CEPT glyphs, essentially.
      And both technologies were superior descendents of the old Ceefax and Prestel, as well. And Minitel, too. ;)

      In some way or another, the experience was akin to Digital Radio Mondiale (DRM) on shortwave.
      Both had some sort of passive Mosaic-style browser with images and hyper links.
      Just have a look at DREAM software for DRM.

      Pictures (Dream):

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