Composite artifact colors

Lore & Background

Composite artifact colors were most common on the IBM PC (with CGA graphics), TRS-80 Color Computer, Apple II, and Atari 8-bit computers, and used by the Ultima role-playing video games. Software titles such as King's Quest for the IBM PC usually provided an option to select between 'RGB mode' and 'Color Composite mode'. On PAL displays the effect is also present but generates more limited colors; depending on the exact PAL system (PAL-M or PAL-N), color artifacts similar to NTSC might be possible. Although related, artifact colors are not the same as horizontal blurring, which is a general effect of composite connection that creates new colors due to a mix of adjacent horizontal pixel values.

Reader's Guide

The technique was significant because it allowed programmers to overcome the limited hardware color palettes of early computers, producing new colors by exploiting the NTSC color subcarrier's phase relationships. On the IBM PC CGA, both the standard 320×200 four-color and 640×200 color-on-black graphics modes could be used, with early efforts yielding a usable resolution of 160×200 with 16 colors. Later demonstrations increased the maximum number of colors the CGA could display simultaneously to 1024 using a text mode tweak. On the TRS-80 Color Computer, the 256×192 two-color mode became a 128×192 four-color mode with black, orange, blue, and white. The technique was generally only used with NTSC, as PAL's line-to-line phase changes made the tricks infeasible, and SECAM's frequency modulation required far more precise timing. Artifact colors should not be confused with horizontal blurring, which is dependent on original pixel values and more pronounced at higher resolutions with saturated colors.

Did You Know?

• The Apple II and CGA used a pixel clock of 14.318 MHz, four times the NTSC subcarrier frequency.
• On the TRS-80 Color Computer, the bit patterns representing orange and blue were not predictable, so games often asked users to press reset until colors were correct.
• Artifact colors are theoretically possible in any analog television system because color information resides within the same bandwidth as luminance information.
• The CGA's 80×100 effective resolution with 1024 colors was achieved using ASCII characters such as U and ‼ in a text mode tweak.

The Subcarrier Trick

The technique exploited a specific relationship between pixel timing and the NTSC color subcarrier. In broadcast NTSC, each line contains 227.5 subcarrier cycles, causing the apparent phase to reverse every line. Early computers like the Apple II and IBM's CGA card stretched each line to a full 228 cycles, which stayed within display tolerances but produced straight vertical stripes instead of checkerboard patterns. Because each horizontal pixel position maintained a constant phase relationship to the subcarrier, lighting up a pixel at a specific index always produced the same color effect. Both machines used a 14.318 MHz pixel clock—exactly four times the subcarrier frequency—meaning each line was 912 pixel cycles long. With only 560 addressable pixels on the Apple II or 640 on CGA, each pixel could occupy one of four predefined phase positions relative to the color burst, letting programmers construct a fake subcarrier that the display interpreted as color.

Games, Machines, and the Mode Switch

This technique found its primary home in video games rather than text display, since the pixel manipulation inherently reduced effective horizontal resolution. The most common platforms were the IBM PC with its CGA graphics card, the TRS-80 Color Computer, the Apple II, and Atari 8-bit machines. Notable software titles like the Ultima role-playing series and King's Quest for the IBM PC leveraged the expanded palette. Because the effect depended on the viewer's display, these programs typically offered a menu choice between RGB mode and Color Composite mode, letting users select whichever suited their monitor. The CGA card's native color capability added another layer: by forming patterns from its built-in colors, the hardware's real subcarrier would interfere with the pixel-pattern-generated fake one, producing entirely new colors the display interpreted as unique hues beyond the card's standard palette.

Not Blurring, Not Dithering

A common misconception conflates artifact colors with horizontal blurring, but the two are fundamentally different phenomena. Blurring is a general side effect of any composite video connection, where limited bandwidth in both luminance and chrominance causes adjacent pixel values to average together, creating new intermediate colors. The exact result depends entirely on the saturation and specific hues of the neighboring pixels. This effect grows more pronounced at higher resolutions and with saturated colors, particularly blue and red. Game artists on machines with high resolution but limited palettes—such as the ZX Spectrum, Mega Drive/Genesis, NES/Famicom, and Amiga—exploited blurring deliberately through dithering patterns to generate intermediate palette colors. Artifact colors, by contrast, are arbitrary and position-dependent, not derived from neighboring pixel values.

Why NTSC Won the Day

Although color information technically resides within the same bandwidth as luminance in every analog television system, making artifact colors theoretically possible in all of them, the practical reality favored NTSC exclusively. In the PAL system, the subcarrier phase is interpreted differently from line to line, and the color burst phase must strictly alternate on successive lines, rendering the pixel-position tricks described for NTSC infeasible. SECAM takes a different approach entirely, using frequency modulation for color, which would demand timing far more precise than simply synchronizing a pixel clock to a subcarrier frequency. The one partial exception within PAL was that PAL-M and PAL-N variants could sometimes produce color artifacts resembling NTSC results. For these reasons, the technique was generally confined to NTSC displays, with PAL screens offering only a more limited set of artifact colors when the effect appeared at all.

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