PC CGA Composite Color - Four Devices Compared
19In this video I am running a program that displays the 16 direct colors an IBM CGA or compatible clone can display in the 40-column text mode and then the 15 artifact colors seen in the high resolution 640x200 graphics mode using white as the foreground color.
Direct color is the CGA's ability to generate hues directly with its onboard circuitry. Through a series of flip flops and delay lines, CGA can display pixels that correspond to hue phases on the NTSC color wheel. Direct color is typically seen with text modes, but text modes have the ability to turn the NTSC color burst off in the IBM PC, XT and many clones, leaving only the monochrome representation of the pixels. Direct color will usually be seen with the 320x200 graphics mode and the standard cyan/magenta/white/background color and green/red/brown/background color palettes, especially for large areas of solid color. Direct composite color is very similar to the colors produced on an RGBI monitor with the notable exception of brown. Direct color 08 is intended to be brown on an RGBI monitor and is accomplished by reducing the green component of the color when that particular color is being displayed. Composite monitors have no such "brown-aware" circuitry and are likely to display the color as dark yellow.
Artifact color is the CGA's ability, in conjunction with an NTSC color composite monitor or TV, to show colors that should not be present and to alter the expected color to another color. NTSC combined a higher frequency luminance signal (4.2MHz) with a lower frequency color signal (3.58MHz), but when the luminance signal is significantly higher and an even multiple of the color signal, which is the case in the CGA's high resolution graphics mode (14.318MHz), the luminance signal will be decoded as a color. In this case, as the luminance signal is 4x the color signal, four high resolution pixels will chose the color for one "low" resolution pixel.
I have presented in this video four CGA-compatible devices which display composite color. All were tweaked to show correct black (0,0,0) and white (255,255,255) RGB levels from the capture card (a Dazzle DVC-100). Saturation was adjusted to get "good" colors from every adapter. The names of the direct colors are taken from IBM's Technical Reference manuals. As IBM never gave names to artifact colors, and these colors were inspired by Apple II artifact colors, their names are taken from the Apple II's Color Demosoft program. For all adapters, the gray artifact colors 5 and 10 are identical but direct color grays must be distinguishable from themselves as well as black and white.
1. IBM PC/XT with an Old-style IBM CGA
00:00 - Direct Color
00:11 - Artifact Color
IBM's original CGA was designed for displays with a greater tolerance for a strong video signal and shows artifact colors identical to an Apple II Plus's and later models Graphics Modes. The Old-style IBM CGA's signal is so strong that neither of my capture devices and handle the signal correctly without reducing the card's composite output voltage significantly. This is the only card which requires a voltage reduction to show all direct colors (light gray will look identical to white without reducing the voltage).
Old CGA is by modern standards not a signal with strong color saturation at normal levels of saturation. In order to show properly pleasing colors, I set my capture card's saturation to the maximum value.
2. IBM PC/XT with a New-Style IBM CGA
00:21 - Direct Color
00:31 - Artifact Color
In 1984 IBM designed the composite video output of the CGA card to show 16-more or less distinct levels of brightness (versus 6 with Old CGA) with a monochrome display by feeding the luminance values of each color back into the output. This change coincided with the release of the IBM PC Portable, which had a built-in amber screen. This change also caused some slight color differences to the old CGA card. Direct colors remain more-or-less the same accounting for changes in brightness and saturation, but artifact colors have changed slightly. The most notable change is that magenta becomes red. The artifact colors can be adjusted from old CGA to new or from new to old in high resolution graphics mode with a white gray foreground color with a minor adjustment to the tint control of the monitor.
Description continued in first comment due to length/