NTSC Decoding Basics (Part 2)

Side effects…they make my chroma crawl


Visual image side effects are the result of luma and chroma components alternately making their way through the wrong decoding channel. You have seen the results. When chroma information is present in the luma channel, it creates "dot crawl" or "chroma crawl" on the vertical edges of color details. Probably the most noticeable time is on the vertical edges of red lettering or other similar color objects. This artifact is referred to as cross-luminance.

During moments when high frequency information, or horizontal detail, is present, some of this energy will find its way through the chroma channel. It presents itself as a "rainbow effect" in regions of the image having the high detail. Remember seeing this effect on people wearing finely textured suits or clothes with narrow stripes? This phenomenon is called cross-color. It can only occur within frequencies that "fit" within the bandpass of the chroma signal path…or, about 3.0 to 4.0 MHz.

These two common effects primarily characterize an analog notch/bandpass decoder scheme in a color display. And, of course, the other most noticeable effect is the loss of high frequency details in the image.