CGA

CGA Color Palettes

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The infamous default 4-color palette with pink, cyan, black and white is probably the first thing that comes to mind when somebody starts to talk about CGA. I previously wrote a post about 16-color modes available for composite monitors but it’s good to add also something about the palettes for RGBI TTL monitors. These started to be used heavily when people stopped using TVs with their IBM PC compatibles.

The CGA palettes were designed for good viewing on NTSC TVs. That’s the reason behind the strange color combinations. The default one can, however, be modified using a video chip register – it replaces pink with red but also disables color burst on the composite output. Such trick was used in many games, but it did nothing on newer cards (EGA/VGA).

CGA also supports changing the color 0 (usually black) to any other color. Several games used this with the default palette to get blue, cyan, pink and white which allowed for better color transitions. Anyway, the easiest way to get more visually appealing games was to use the second palette – red, green, yellow and black.

Flight Simulator 3.0 Running on IBM PC with CGA Graphics

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Users with composite monitors (or TV sets) could use the 16-color video mode that was based on the same trick as was used in Apple II. On the other side, it was not possible to use more than four colors at the same time in the game when the system was connected to a standard RGBI (TTL) monitor. This was typical for many early PC games.

Double CGA Display in Olivetti Quadero

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Although 640×400 (“double CGA”) LCD screens usually could display only “black” and “white” (so the 320×200 4-color mode was handled using multiple 2×2 1-bit patterns) this was an exception. The screen on Olivetti Quaderno can display four shades of grey with a good contrast. It reminds me playing games on old Game Boy handhelds…

Old CGA Laptops and Monochrome TV Output

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When I tried a composite video output on my Bondwell Model 8 for the first time I was disappointed that there were no colors in the picture. I searched over the internet and old reviews and found that the output is “monochrome only”.

Bondwell used the V6355 chip sold under Yamaha brand. This chip was quite popular in early CGA laptops (and MSX computers) and according to a datasheet it can handle multiple output modes: digital monochrome LCD, TTL RGB, analog RGB (for SCART connection) and color/mono composite. The problem is that the chroma pin on the chip is shared with signals required for LCD and wrong voltages/clocks on the pin could damage the LCD screen.

It looks like engineers wanted to have color composite output as there are missing parts on the logic board around these signal traces. However there was probably no business justification for having it in the laptop. Mobile users used the composite output mostly on the road when stayed at hotel (any hotel TV was better than the first generation of laptop LCDs).

I have found that Toshiba used monochrome TV outputs on their LCD CGA laptops as well (and IBM probably too). Since adding a color burst logic to the laptop would need heavy hardware modifications and some disassembling of BIOS I have to stay without a mobile device that could handle special multi-color (>4) CGA modes.