Trick From 1903 Makes An Old Monochrome TV Spit Some Colours

Its safe to say that colour television is taken for granted nowadays. Consumed by the modern marketing jargon of colour dynamic range, colour space accuracy and depth, it is easy to overlook the humble beginnings of image reproduction when simply reconstructing an image with the slightest hint of colour required some serious ingenuity and earned you a well deserved pat on the back!

[anfractuosus] revisited an old gem of a technique, first patented in 1903 and used it to successful make an old monochrome TV produce a colour image. The idea in essence, is actually similar to what cheap image sensors and LCDs still use today. Rather than relying on true RGB colour generation by individually integrating colour sources as AMOLED does, we take an easier route: Produce a simpler monochrome image where each colour pixel is physically represented by four monochrome sub-pixels, one for each colour component. Now light up each of the sub-pixels according to the colour information of your image and rely on an external colour filter array to combine and spit out the correct colours.

He first used some image processing to convert a standard colour video into the aforementioned monochrome sub-pixel representation. Next, a Bayer colour filter array was printed on some acetate sheets using an inkjet printer (the original inventors used potato starch!), which when overlaid on top of the monochrome monitor, magically result in colour output.

There are some problems associated with this technique, mainly to do with the difficulty in measuring the size of the TV pixels and then producing and perfectly aligning a filter sheet for it. You should check out how [anfractuosus] went about solving those issues.

So now you know a bit more about colour image generation, but how about colour TV transmission? Check out an earlier piece to learn more.

26 thoughts on “Trick From 1903 Makes An Old Monochrome TV Spit Some Colours

  1. Yeah, why would you mention the cool hack and the ammount of work put into it, instead? Couldn’t add a shameless plug that way, right?

    Nice article and I didn’t even notice that horrible spelling mistake. As a non native English speaker, that is.

    Cheers, Jan

    1. As a well-established technical blog site, it’s expected that there would be a process to filter out most of these kinds of mistakes. In this case the mistake was fairly innocuous, but errors like this are not uncommon on Hackaday. At best they can simply be distracting, but at worst they could lead to the sentence having a different meaning.
      There’s a balance between pedantry and accountability.

      Cool hack though.

      1. What accountability? Hack a Day is a free website and you get what you pay for.

        If the quality doesn’t meet your expectations there’s always plenty of others you could go to instead. I wonder if you comment on Facebook posts complaining about typos. Facebook is a well-established blog site so it’s expected that there would be a process to filter out these kinds of mistakes, right?

  2. Most displays today use exactly the same way of color generation. Nearly all color LCDs work that way. Of course alignment is more difficult with an inherently analog solution like the picture tube – remember adjustment of color purity and convergence of CRTs? I do :-) Adjustment of an external filter is even more difficult as you not have the special adjusting magnet rings on a B/W CRT

  3. As I recall, I was amazed by a color commercial displayed on a B&W television many many tears ago. This was new years eve, and the commercial was (AIR) for hair color product for women. (colour in UK)
    No special filter required.
    As I was told, the colors were transmitted at different frequencies, and that was what caused the eye to view the black and white image as color.

      1. Or not.

        The frequency of a light wave causes our perception of the light we sense through our eyes and brain to appear as different colors ranging from red (low frequency) to violet (high frequency)

        1. I don’t think it’s doable. You would need to precisely modulate the luminance information for each “pixel” of the video. This signal, with some synchronization information would need to be modulated on a carrier wave of frequency of 45-214MHz. With 405 lines of image. And the carrier frequency must be at least equal to the signal frequency. And there are lenty of low-pass filters in the signal path between the demodulator and CRT. So yeah, it’s impossible with analog TV. Otherwise they wouldn’t invent the color TV in the form they did…

        1. You’re welcome, Jerry.

          However, this video only shows the effect, not really explaining it. It also doesn’t work for everyone (not for me, for example, but I’m partially blind) and it reacquires a strobe light or other form of flickering black and white pattern. Not very practical for daily use in TV…

        2. Not coming through very well on this laptop screen, better at full screen, can see a pale green cast to ZIP and pale blue in the flakes, then green also appears if I try rapid blinking, maybe a hint of red, not sure.

          1. On t’other machine… now ZIP is reddy browny orange while pouring, then seems to fade to green later. Think the top semicircle segment thing is pale blue this time too.

  4. The same technique, but in reverse is used for recording images and videos. Modern versions use 4 sub-pixels, two green, one red and one blue, That’s why we switched from blue screen to green screen for chroma keying. And we used blue because it’s the farthest color from any skin tone. Before CMOS and CCD sensors we used sensing tubes in two varieties. Older one was monochrome, so cameras had three of them, and some prisms and filters to split the image. Replace the tubes with CCD sensors and you get 3CCD system that was used for some time. The other type used striped filter in front of the single tube to get the color information.

    Field scanning was also used, especially for high-quality TV dislays used in broadcasting. There were even some attempts to make a hybrid color TV system by using B&W orthicon/vidicon to get the image, CRT to display it, and two synchronized rotating, transparent disks painted with different colors to add color to the image. This wouldn’t work for any reasonable TV, because the whole system would have to be the size of a building with disk rotating at extreemly high speed for its size…

      1. For chroma keying you can actually use any color you want. Digital cameras are, however, a bit more sensitive for green, as it’s used also for additional luminance processing, that’s why there are two green sub-pixels. The acutal chroma key color depends on the situation – if your actor wears green, you need to use blue screen. Or you could use rotoscoping, but that takes much more time…

    1. Eventually a rotating color filter system did make its way to our living rooms inside rear projection TVs and fairly small projectors using a single DLP device. Cheaper and more compact than using three DLP device, no convergence problems, just have to run the thing at 3x normal frame rate to synchronize the colors. Some of those had other tricks such as the DLP being half the horizontal resolution and rapidly deflecting left and right, sort of a vertical interlacing to go with the horizontal interlacing. When this wobulation works it looks decent. When it doesn’t, it drives you nuts with the jitter.

  5. Or you could just put a pair of nylon stockings or pantyhose over your B&W TV set to produce the same effect. Just make sure to choose at least an 80-100 gauge, at least 85% nylon blend!

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