Within the monitor, the four signals are interpreted to drive the red, green and blue color guns. When using a direct-drive monitor, the four color bits are output directly to the DE-9 connector at the back of the card. The fourth "intensifier" bit, when set, increases the brightness of all three color components (red, green, and blue).The lower three bits represent red, green, and blue color components.When four bits are used (for low-resolution mode, or for programming color registers) they are arranged according to the RGBI color model: In the medium- and high-resolution modes, colors are stored at a lower bit depth and selected by fixed palette indexes, not direct selection from the full 16-color palette. Note the use of dithering to simulate gray tones and non-square pixel ratio that deforms the fonts.ĬGA uses a 16-color gamut, but not all colors are available at all times, depending on which graphics mode is being used. The 40×25 text and 320×200 graphics modes are usable with a television, and the 80×25 text and 640×200 graphics modes are intended for a monitor. IBM intended that CGA be compatible with a home television set. 80×25 with 8×8 pixel font (effective resolution of 640×200).40×25 with 8×8 pixel font (effective resolution of 320×200).Some software achieved greater color depth by utilizing artifact color when connected to a composite monitor. 640×200 in 2 colors, one black, one chosen from a 16-color palette.320×200 in 4 colors, chosen from 3 fixed palettes, with high- and low-intensity variants, with color 1 chosen from a 16-color palette.160×100 in 16 colors, chosen from a 16-color palette, utilizing a specific configuration of the 80x25 text mode.Some third-party displays lacked the intensity input, reducing the number of available colors to eight, and many also lacked IBM's unique circuitry which rendered the dark-yellow color as brown, so any software which used brown would be displayed incorrectly. Īlthough IBM's own color display was not available, customers could either use the composite output (with an RF modulator if needed), or the direct-drive output with available third-party monitors that supported the RGBI format and scan rate. IBM produced the 5153 Personal Computer Color Display for use with the CGA, but this was not available at release and would not be released until March 1983. The RCA connector provided only baseband video, so to connect the CGA card to a television set without a composite video input required a separate RF modulator. The CGA card could be connected either to a direct-drive CRT monitor using a 4-bit digital ( TTL) RGBI interface, such as the IBM 5153 color display, or to an NTSC-compatible television or composite video monitor via an RCA connector. The highest display resolution of any mode was 640×200, and the highest color depth supported was 4-bit (16 colors). The original IBM CGA graphics card was built around the Motorola 6845 display controller, came with 16 kilobytes of video memory built in, and featured several graphics and text modes. 3.2 With a composite color monitor/television set.
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