Dot crawl
Also called: chroma dots, crawling dots, hanging dots, dot pattern
Dot crawl is the line of moving dots along colored edges in composite video. See what causes it and how to dial it in with PHANTOM CORRUPT in After Effects.
WHAT CAUSES IT
Dot crawl is a moving pattern of small dots along the boundaries between strongly colored areas, caused by leftover chroma subcarrier in the luma signal of composite video. NTSC puts the color subcarrier at 3.579545 MHz, which equals 227.5 cycles per line. Because of the half cycle, the subcarrier phase flips from one line to the next, and it also changes between frames. When a decoder fails to remove the subcarrier from the luma path, the residue shows as a fine checkerboard of light and dark dots at edges where color changes sharply. The checkerboard inverts every line and every frame, so the dots appear to crawl upward along the edge. A good adaptive comb filter removes most of it. A notch filter, which cuts the band around the subcarrier at the cost of horizontal sharpness, reduces it but does not remove it. S-Video and component skip the problem by never combining the signals.
WHAT IT LOOKS LIKE
A dotted fringe along vertical and diagonal color boundaries, such as the edge of a red logo against a blue background. The dots are a fine checker pattern, one or two pixels each, stacked along the edge, and they scroll slowly upward at a regular speed. They are most visible on saturated colors and on bright titles on colored backgrounds. In a still frame it appears as a zipper or a ladder along the edge. Moving the picture does not remove it; the crawl continues even on static graphics.
WHERE YOU SEE IT
NTSC composite video: broadcast television, early consoles such as the NES and Genesis over composite cables, LaserDisc, and VHS. It is conspicuous on game titles and network logos. Many viewers know it from retro console footage. In post it is a shorthand for the composite era, appearing in retro game compilations and throwback graphics. You can also catch it on classic television ID cards and on early digital capture of laserdisc menus.
RECREATE IT
PHANTOM CORRUPT
CONTROLS CORRUPT RAINBOW, PHANTOM NOISE, CORRUPT COMPOSITE
CORRUPT RAINBOW covers the composite decoding artifacts, so start with it low and see whether the edges pick up a fringe. PHANTOM NOISE adds fine structure that helps the fringe read as dots rather than smooth color. This is an approximation of the look, so judge it by eye on a saturated logo or title rather than by control values.
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HOW TO MAKE IT IN AFTER EFFECTS
1. Choose a shot with saturated colored edges, such as a title, a logo, or a bright costume against a contrasting background. 2. Apply PHANTOM CORRUPT to the layer. 3. Keep CORRUPT WARP and CORRUPT SHRED at their minimum. 4. Bring up CORRUPT RAINBOW until the edges show a colored fringe. 5. Add PHANTOM NOISE to give the fringe a fine texture, and check at 100 percent zoom, since dots disappear when the view is scaled down. 6. Use CORRUPT COMPOSITE to pull back the amount so only the edges show it. 7. For a retro game look, scale the footage down first so the dots are tied to a coarse pixel grid. 8. Keep the effect constant across the shot, because real dot crawl does not fade in and out. 9. Preview at full speed, since the crawl is a motion effect that you cannot judge on a still.
QUESTIONS
- Why do dots crawl up the edges of colored objects?
- Leftover subcarrier in the luma path forms a checkerboard that inverts every line and every frame. The inversion makes the pattern step upward as the frames advance, so it seems to climb the edge. Because the pattern depends on frame timing, a freeze frame of a crawl looks like a static zipper, and only the playing video shows the upward motion.
- How do I get rid of dot crawl?
- Use S-Video or component output where available, or a good comb filter decoder. After the fact, a temporal comb or specialized dot crawl filter can reduce it, but it cannot remove it entirely from an already combined signal. A notch filter also removes some of the real detail near the subcarrier, so the cleaner the connection to start with, the better the picture.
- What is the difference between dot crawl and rainbow banding?
- Both come from composite luma and chroma overlap. Dot crawl sits along color boundaries as a dotted fringe, while rainbow banding appears as false colors in fine patterns inside regions. Dot crawl is mainly an edge artifact visible on titles and logos, while rainbow banding needs fine repeating texture, so the same shot can show one without the other.
- Does PAL have dot crawl?
- PAL has a similar artifact, but its subcarrier phase alternates line to line and the pattern differs. It tends to look like a finer, less noticeable texture, though it exists. PAL's 4.43 MHz subcarrier and its alternating phase spread the pattern differently, which is why PAL footage rarely shows the obvious crawling dots seen on NTSC.