Me and Him

The chroma upsampling* error, or "chroma bug", occurs because a DVD (and also the current U.S. digital TV standard) only provides color for every other scan line. In other words, for film source material, scan lines 1 and 2 share the same color (it should be the average of the two scan lines), scan lines 3 and 4 share the same color, and so on, although light/dark detail may be unique for each line.

In technical terms the color resolution is half the luminance resolution. This compromise was made to make the entire program fit on a 5 inch disk or be transmittable in the broadcast channel bandwidth allowed. The human eye is not as sensitive to minute errors in coloration compared with fine light and dark detail.

But DVD players always decode the disk data as interlaced. The color for scan lines 1 and 2 is brought out for the odd scan lines and gets applied to scan lines 1 and 3. The color for scan lines 3 and 4 comes out for the even field and is applied to scan lines 2 and 4. So these minute color errors occur. Sometimes these errors are conspicuous, such as where a dark blue patch meets a bright red patch. The light/dark relationships remain correct and unaltered, and the interchanged coloration in this example produces some bright blue where bright red should be as well as the less obtrusive dark red where dark blue should be.

For progressive scan output the player has the opportunity to assemble the entire frame and then apply the color correctly. Unfortunately many players' circuitry still applies the color to the scan lines first (color for scan lines 1 and 2 applied to scan lines 1 and 3) before assembling a complete frame.