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On early color computers, it was possible to add a feature by which you could turn off the color burst output, restricting the display to black and white but making it significantly sharper and clearer; this was useful for text. I don't know how much it cost to provide this feature, but I am inclined to think it would be useful for any machine intended for serious use rather than just games; its historical rarity suggests either the cost was nontrivial or there is another issue I'm not taking into account.

The Apple II had it from early on (including a useful mode where you could have most of a screen of color graphics, with four lines of crisp black and white text at the bottom).

The Tandy CoCo tried to provide it, but the first version allowed the color burst to leak through, and a revised chipset that would have fixed this, was rejected in testing because it broke games that had been written to exploit the artifact colors in what was supposed to be black and white mode.

Did any other early color computers (that output RF or composite video) allow the color burst to be turned off?

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    Not a real answer, but in principle a demo coder could achieve it in effect on an Atari 2600 because vertical sync is programmatic and is combined with horizontal sync via XOR. So with a very, very delicately-timed loop you could turn off the real horizontal sync and put your own somewhere else. Which wouldn't remove the colour burst, but would move it so that it wasn't where it should be, so was no longer a colour burst. I would dare imagine other platforms might offer similar demo-coder exploits even where it's not an intentional feature?
    – Tommy
    Aug 29, 2018 at 19:53
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    One of the benefits of separated video (luminance+chromiance) on early machines like the C64 and Atari 800 is the luminance signal can be used by itself to drive a B&W monitor with no color interference. This is because the luminance signal includes the H-sync and V-sync as well.
    – Brian H
    Aug 29, 2018 at 21:27
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    Some monitors allow switching between green and color. Maybe someone can answer, do they do a good enough job of removing the color burst signal, or is it significantly better to not send the signal to the monitor in the first place?
    – snips-n-snails
    Aug 29, 2018 at 21:50
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    The Atari 2600 doesn't allow enough control over vertical timing to produce a usable signal without colorburst, but there wouldn't be much point since it can directly produce 8-level grayscale, and its resolution is low enough that a grayscale picture won't tend to pick up unwanted color fringes.
    – supercat
    Aug 31, 2018 at 19:09
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    Does IBM PC with CGA adapter count as early? I can add that as an answer, but it might be pointless to list each computer as a separate aswer as all answers are equally correct.
    – Justme
    Mar 9 at 10:32

5 Answers 5

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The V9938 video chip in the MSX2 home computer can be programmed to disable the color burst but I don't know if any utilities or applications provided such an option to the user. That would have certainly been beneficial for the 80 column text mode and 2-color high resolution bitmap mode.

Some MSX2 variants had a V9958 instead which has no composite video output and no color burst control function so it depends on the particular manufacturer.

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The BBC Microcomputer did the exact opposite with its composite video output.

The Beeb has three different video outputs, RF (for use with a television), composite, and RGB. As shipped, the composite output was black and white only. However, by soldering a capacitor between two transistors (on early issue boards) or fitting a wire link (on later ones), the colour burst of the RF output could be connected to the composite video signal.

A similar modification could be done to the Model B+, Master series, and the Electron, with the same effect of "turning on" the colour burst.

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An Apple II did not turn off color burst for the 4 lines of black and white text at the bottom. Instead, it output wide enough (low-res) pixels for an NTSC television or monitor to interpret the IQ phase (with respect to color burst phase) as equivalent to white or black.

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    I don't think this is right. The Apple II definitely turned off the color burst in text mode (no color fringes on the text), and I don't see why it shouldn't do that in mixed mode when essentially switching to text mode at the bottom. Yes, wide pixels would be white or black, but they still had the color fringes in hi-res mode. In doubt, one could look at the schematics.
    – dirkt
    Aug 30, 2018 at 5:35
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    The effect of turning color burst on and off mid-frame is apt to depend on enough factors as to be essentially unpredictable. If one were to turn on colorburst for some area in the middle of the display (possible with a well-timed switch to text mode 40% of the way down each frame, followed by a switch to graphics mode 60% of the way down) most monitors would continue to behave as though the colorburst were present during most or all of the portion of the screen where it was off, but the color phase would drift). Depending upon how much the phase drifted, they may then...
    – supercat
    Aug 31, 2018 at 19:06
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    ...behave as though the colorburst were absent during some of the scan lines where it is present. It would have been helpful if enabling the hires soft switch without the lores one would have produced a non-colorburst hires mode, while enabling both would yield color hires, but the Apple ][ didn't do that.
    – supercat
    Aug 31, 2018 at 19:07
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    I suspect a physical switch was used because the effect of taking what's supposed to be a monochrome image, decoding NTSC color, and re-encoding PAL color, would be far worse than the effect of feeding such a signal to an NTSC monitor. If the Apple ][ had a soft-switch combination that would yield hires without colorburst (I'm not sure if that would have required any extra circuitry) then PAL systems could have used that to control the decode/re-encode process, but no such feature exists in the NTSC machines.
    – supercat
    Sep 1, 2018 at 18:34
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    @PeterI: Thinking about it, perhaps what would have been most useful might have been a potentiometer to control the blend between transcoded chroma and delayed (accurately to be in sync with chroma) luma, since different programs might benefit from different trade-offs between luma clarity versus colorfulness.
    – supercat
    Sep 1, 2018 at 22:43
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On the Tandy Color Computers 1 & 2 you need to build a video adapter to take the Y signal at pin 28 of the video generator (MC6847) - this is Luminance and Sync, essentially a B&W signal, invert it and buffer it as an output of pure B&W composite video. There is no software approach (POKE a register or something akin to it) to kill the color burst, it has to be a hardware solution - could be made switchable with the addition of a crafty switching scheme.

On the CoCo 3 there is no VDG, the GIME chip can manipulate BASIC's PALETTE registers to simulate a B&W output. I used this technique while a starving student in college on my CoCo 3 setup (used 99% for word processing) and the 12" B&W TV I could muster for monitor purposes: changing the background color on the 40 & even 80 column screen with a white foreground provided for a poor man's "composite" like B&W image, just had to deal with the over scan using the horizontal hold control. Where there is a will...

There is a very detailed article on Hot CoCo's July 1983 issue, written by Marty Goodman, on the monochrome composite video adapter for a CoCo 1 and 2:

https://colorcomputerarchive.com/repo/Documents/Magazines/Hot%20Coco/Hot%20Coco%20Vol.%201%20No.%202%20-%20July%201983.pdf

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The amiga has a color burst bit

http://amiga-dev.wikidot.com/hardware:bplcon0

09 COLOR Enables color burst output signal

Most games make sure that it's turned on.

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