Most TVs use composite cables that send single transmissions that include the color and luminescence information in one burst. While this single transmission can be displayed on a TV screen, it causes errors in interpreting the transmission, such as dot-crawl, in which the dots from shapes and figures crawl outward like a low-resolution image. A digital comb filter takes these two pieces of information — color and luminescence — and separates them; this causes the transmission to enter the TV at a slightly slower rate, but it dramatically increases picture quality. Several different digital comb filters exist, with newer ones able to process information with better accuracy and in less time. Such a filter is unneeded, and will not do anything, if the TV is receiving a super-video (S-video) transmission.
Normally, when a TV receives a transmission, it gets an analog transmission in which both the luminescence and color information, known as chrominance, are merged. This type of transmission offers low-quality output and was used for the first TV units. Common errors that viewers see include too bright or too dim pictures, dot-crawl, incorrect colors and blurry images that can be visually indecipherable.
To correct this error, a digital comb filter is commonly employed. This device first separates the luminescence and chrominance into two different transmissions, making the analog stream a digital stream. This slows down the overall processing time of the TV, but only by a fraction of a second. Both streams are separate, and therefore easier to understand, so the TV is able to display the image with higher quality. Lines are typically sharpened, making the quality even better.
As of June 2011, only four digital comb filters are on the market. The least effective type is the glass, or analog, comb filter, which is found in smaller TVs and does not handle the stream interpretation very well. The two-line digital comb filter is able to consecutively interpret two horizontal lines at once. The three-line version is the same, only it works on three lines at once as opposed to two. The 3D digital comb filters handle multiple scan fields and three lines of information, and they work even better than the other types at removing issues that affect quality.
If an S-video TV is being used, then there is no reason to use a digital comb filter. In an S-video TV, the analog stream is automatically converted to digital. The filter's job is already done, so having a comb filter would be a waste.