What Is an HDTV 3D Comb Filter?

By David Lipscomb

High-definition televisions provide stunning image quality in millions of homes. Most people simply turn the set on and marvel at the picture without giving the underlying processes a second thought. One HDTV technology is the comb filter or, on some TV sets, the 3D comb filter. The filter processes the incoming image, ensuring that what you see is color-accurate and devoid of noisy artifacts.

Essential Comb Filter Functions

Comb filters separate luminance and chrominance information. These two parameters reference the level of brightness intensity and colors, which helps reduce the common TV artifact problem of "dot crawl" -- small dots that cluster along the edges of objects, especially during fast motion. Other noise manifests itself as moire patterns, also seen when objects on the screen move from a still position.

3D Comb Filters

The 3D comb filter examines the incoming signal at a rate of three horizontal scan lines per pass. This improvement over conventional two-line and notch filter versions of the process dramatically reduces the appearance of hanging dots and moire. Color bleeding with a 3D comb filter is also virtually eliminated, ensuring clean transitions between intense colors. The filters improve vertical resolution during still or slow-moving frames.

3D Y/C Comb Filters

3D comb filters in an HDTV examine video image frames before the viewer sees them. Advanced 3D Y/C (luma/chroma) filters perform all the functions of a 3D filter. The 3D Y/C comb filter enhances standard 3D versions by evaluating the information in the image and comparing it to the succeeding frame. This capability is necessary with the high data rates and sheer quantity of video bandwidth in high definition content.

Help With Motion

As good as 3D and 3D Y/C comb filters are, sometimes intense quick-moving objects require the video processor to "downshift" into 2D comb filter mode. Switching between 3D and 2D happens seamlessly, allowing both still images and fast-moving ones to appear as clean and artifact-free as the incoming signal allows. Although 2D filters do a marginal job at reducing picture artifacts, these tend to be obscured during periods of fast motion. These filters are inherently limited by how much resolution they can handle, which explains why older HDTVs "lose" vertical resolution during periods of fast motion on-screen.