What Are 3D Comb Filter LCD TVs?

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What Are 3D Comb Filter LCD TVs?
Comb filters in LCD televisions process the incoming picture information. (Jupiterimages, Brand X Pictures/Brand X Pictures/Getty Images)

Liquid crystal display televisions have devices called comb filters, deigned to handle the incoming video signal. Depending on the quality of the filter, picture artefacts are reduced or remain unaffected. The essential function of any 3-D comb filter in an LCD television is to separate luminance and chrominance, also known as colour and brightness/colour intensity information. Better 3-D comb filters make television images appear smoother and more natural. To understand 3-D and 3DYC comb filters in modern LCD sets, you have to examine the evolution of the comb filter and its reasons for existence.

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What is a Comb Filter?

All picture information contains colour and brightness/intensity information. The essential task of any comb filter is to separate these elements of the image, reducing picture anomalies known as "dot crawl" and "hanging dots." These appear on screen as small dot-like artefacts surrounding the borders of on-screen images. You may also see a rainbow effect along on-screen diagonal lines. Comb filters also help make abrupt transitions between colours, especially during fast motion, appear more natural. Early or cheap filters such as notch or bandpass versions offer very little in terms of picture enhancement and severely reduced detail as the screen resolution increases.

2-D Comb Filters

Earlier iterations of the comb filter were designed to address and properly handle analogue, NTSC video. Typically, this video was transmitted along one cable into the television, either from a composite or coaxial video feed. As such, the set had to separate the information that the transmission system did not. Simple comb filters, known as 2-D filters, measure the vertical and horizontal lines drawn on the screen. Taking into account both horizontal and vertical picture information helps the filter better detect hanging dots and other artefacts, often at the expense of increased overall graininess or fine noise in the image. When on-screen action is fast moving, however, you do lose some resolution as the filter attempts to keep colour and noise normalised.

Three-Line Filters

These filters almost completely eliminate dot-based artefacts from the image, increasing smoothness and actively decreasing noise to a large degree. Interestingly, there are points where the on-screen motion is so rapid that the filter must "downshift" into 2-D mode. These periods are brief, however, and often undetectable in normal viewing conditions. Three-line filters analyse the horizontal scan lines, as opposed to two on 2-D versions. In most cases, this significantly improves picture quality, especially when viewing non-HD content.

3D Y/C Comb Filters

The evolution of the 3-D filter adds another significant processing element. These filters, used in modern high-definition sets and better analogue televisions, are known as 3D Y/C comb filters. These exhibit near-perfection when it comes to still or slow-moving objects. These filters are common in modern high-definition LCD televisions, necessary with the high resolutions and data density involved with the broadcast medium. Their success lies in the fact that these filters examine frames in the video content in advance, pre-processing them to ensure consistently smooth images. This feature makes the 3D Y/C comb filter vastly better than conventional 2-D or 3-D versions.

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