Composite video
Before dedicated RGB monitors became common in the late 1980s, many home PC users connected their computers to TVs or composite video monitors. The NTSC composite signal introduces interference patterns that produce unexpected colors - a phenomenon known as artifact colors - expanding CGA's usual 4-color palette to as many as 16. It sounds like a bug, but developers deliberately exploited it.
CGA's two graphics modes - 320×200 medium resolution and 640×200 high resolution - both produce artifact colors on a composite display, though the higher-resolution mode yields a broader and more distinct palette, and was the more popular choice for games built around composite output. Counterintuitively, 640×200 is a monochrome mode - it outputs only black and white pixels - but it's precisely that high-frequency alternating pattern that the composite signal smears into color.
Composite emulation is available for the CGA, IBM PCjr, and Tandy 1000 machine types.
Composite support in games
Sierra On-Line was particularly enthusiastic adopters: games like King's Quest and Space Quest would prompt you at startup to choose between RGB and composite modes, with the composite graphics being the more colorful and detailed of the two. On RGB without that mode selected, they appear as garish 4-color affairs. Earlier titles like Microsoft Decathlon and Ultima II : Revenge of the Enchantress used composite color from the outset, without bothering to offer an RGB fallback at all.
dosbox-automation automatically enables composite emulation when a game switches
to the 640×200 composite mode. For the more common 320×200 modes,
you'll need to enable the composite setting manually.
Notable games with composite video support
See the linked Google Spreadsheet in this blog post for a complete list.
- Ancient Art of War, The (1984)
- Bard's Tale II: The Destiny Knight, The (1986)
- Below the Root (1984)
- Black Cauldron, The (1986)
- Bruce Lee (1985)
- California Games (1987)
- Demon's Forge (1987)
- Donald Duck's Playground (1986)
- Dragon Wars (1990)
- Gold Rush! (1988)
- Jumpman (1984)
- King's Quest (1984)
- King's Quest II (1985)
- Leisure Suit Larry (1987)
- Maniac Mansion (1987) (CGA version)
- Microsoft Decathlon (1982)
- Mixed-Up Mother Goose (1987)
- Pitstop II (1984)
- Seven Cities of Gold, The (1985)
- Space Quest (1986)
- Transylvania (1986)
- Ultima II : Revenge of the Enchantress (1983)
- Ultima III: Exodus (1985)
- Where in the U.S.A. is Carmen Sandiego? (1986)
- Zak McKracken and the Alien Mindbenders (1988) (CGA version)
Tuning the composite colors
Artifact colors are sensitive to the phase of the composite signal, which
means the hue can drift between different cards and setups. Real NTSC monitors
had a physical tint dial for exactly this reason, and IBM even included a
"COLOR ADJUST" trimpot on the PC motherboard to compensate. The hue setting
lets you replicate that adjustment - if colors look off, this is the first
thing to tweak. Adjust the hue setting for each game until the colors look
as expected (e.g., blue skies, green grass).
Early and late CGA card revisions produce slightly different artifact
patterns, and some games were designed with one revision in mind. The
era setting lets you choose which to emulate.
Configuration settings
You can set the CGA composite video parameters in the [composite]
configuration section. CGA composite monitor emulation is only available for
cga, pcjr, and tandy machine types.
composite
Enable CGA composite monitor emulation. This allows the emulation of NTSC artifact colors from the raw CGA RGBI image data, just like on a real NTSC CGA composite monitor.
Possible values:
auto(default) -- Automatically enable composite emulation for the 640x400 composite mode if the game uses it. You need to enable composite mode manually for the 320x200 mode.on-- Enable composite emulation in all video modes.off-- Disable composite emulation.
Note
Fine-tune the composite emulation settings (e.g.,
hue) using the composite hotkeys, then
copy the new settings from the logs into your config.
era
Era of CGA composite monitor to emulate.
Possible values:
auto(default) -- PCjr usesnew, and CGA/Tandy useold.old-- Emulate an early NTSC IBM CGA composite monitor model.new-- Emulate a late NTSC IBM CGA composite monitor model.
hue
Set the hue of the CGA composite colors (0 by default). Valid range
is -360 to 360. For example, adjust until the sky appears blue and the
grass green in the game. This emulates the tint knob of CGA composite
monitors which often had to be adjusted for each game.
convergence
Set the sharpness of the CGA composite image (0 by default). Valid
range is -50 to 50.