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PAL Composite Artifact Output
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1. Luma-to-Chroma Cross-Talk
A composite video cable sends brightness (luma) and color (chroma) down a single wire. The color data rides on a 4.43 MHz sine wave called the color subcarrier. When a computer outputs fine alternating black-and-white pixels, the sharp transitions mimic this 4.43 MHz wave — and the TV's decoder mistakes brightness changes for color information.
2. The PAL Phase Alternation Problem
PAL flips the color signal phase by 180° every other line, then averages adjacent lines to cancel errors. This normally destroys artifact colors — dither patterns cancel themselves into gray mush.
3. How the ZX Spectrum 128K Defies the Rule
The Spectrum 128K's clocks are phase-locked to the color subcarrier. Each scan line spans an exact integer multiple of the subcarrier wavelength. This means specific pixel patterns survive the PAL delay-line averaging, producing stable green, purple, and cyan fringes.
| Feature | Standard PAL | ZX Spectrum 128K |
|---|---|---|
| Clock Alignment | Unsynced; color phase drifts | Phase-locked to subcarrier |
| Delay Line Result | Averaging cancels artifacts → gray | Pixels survive averaging → color |
| Visual Stability | Shimmering "dot crawl" | Fixed, predictable fringes |