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gstanton

Maria CTRL Register

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I've been programming/playing around with the 7800 and I wanted to know more about the CTRL register, specifically what the Color Kill (CK) bit does? According to the documentation it shuts off color burst to prevent arifacting? What does that mean exactly? Does the screen turn to black and white, or does this just happen for the current scanline? I know some of the commercial games (Desert Falcon, etc) turn the CK on and off during gameplay along with the border control bit.

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It lets you turn off the colorburst so that text-only lines will show up better without chroma artifacts. It's an on/off thing, so you have to keep turning it on and off... unless you're doing a text-only display, of course. (All I need is a 7800 keyboard and code for the Apple II or C64 version of the interpreter, and I could put Zork on a cartridge.)

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According to the documentation it shuts off color burst to prevent arifacting?  What does that mean exactly?

 

When a high-resolution image is displayed on a color TV, small dots will never be exactly the right color because closely spaced brightness changes interfere with the integrity of the 3.579MHz color signal (to make matters worse, on all NTSC Atari 8-bit systems the pixel clock is a multiple of the NTSC color clock).

 

Anyway, this interference causes artifacting, which is a discolored display. White text on a back background will not be pure white, but will rather have some coloring visible in it. This is simply a limitation of NTSC and the way in which the Atari generates video.

 

To get pure white text, the 7800 allows you to turn off the color burst signal that is present in the off-screen part of every scan line. The TV uses this short signal to align its color level and phase to the source signal. If the signal is not present on a scanline, the TV's color decoding circuit will not operate, and the line will be in black & white.

 

-Bry

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Just my C$0.025

 

When the CK bit is set, MARIA doesn't generate a colorburst as part of the horizontal sync portion of each scanline. This results in TVs decoding the video as black & white. This was intended to allow the 7800 to produce high resolution (320x240) greyscale for displaying 40 column text. I've also used it for SpaceWar! 7800 to avoid artifacting.

 

Color artifacting occurs when the luma (B&W) portion of a composite video signal is at or near the colorburst frequency (3.58Mhz). This is because the color portion of a composite signal is modulated at the colorburst frequency. Unfortunately, the NTSC 7800's high resolution modes (320 pixels per line) pixel clock is the same as the one used for colorburst; which means any color game which uses the high resolution mode has to deal with color artifacting. (Notably Tower Toppler, which uses it to its advantage.)

 

It certainly is possible to turn the CK bit on and off on a per-line basis. Of course, there may be some modern hyper-intelligent TVs out there which might get wigged out by it.

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Do you all use the Cuttle Cart to program the 7800? What kind of hardware or software do you use? I've been facinated by the 7800 for sometime now and any advice is helpful. I would like to make games for the 7800 system.

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Do you all use the Cuttle Cart to program the 7800?  What kind of hardware or software do you use?  I've been facinated by the 7800 for sometime now and any advice is helpful.  I would like to make games for the 7800 system.

 

The Cuttle Cart 2 seems to be the popular choice though there are other options available.

 

Mitch

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