COMB FILTERS - WHY ARE THEY NECESSARY FOR CERTAIN DISPLAY DEVICES?
When shopping for a new TV you may have come across a feature listed on the television specification sheet called a comb filter. Somewhat confusingly, you may not need to worry about the comb filter at all depending on what type of signal you are using to carry information to your TV.
Certain types of signal will require a comb filter to improve the quality of the image, and there are different types and qualities of comb filter available to manufacturers of televisions which function in slightly different ways.
This Projector Source Canada article will explore what comb filters do to improve the quality of a TV''s displayed image. We will also consider why comb filters are necessary, and why some television signals will require a comb filter to improve the displayed image while others will not. The different types of comb filter will be explained so that you have the best information available when comparing television specifications.
Comb filters and video signals
Comb filters become an important consideration depending on the type of electronic signal used to carry information from the video source to the TV set.
Most widespread video signal, composite video, is also the video signal that outputs at the lowest picture quality. The reason behind this is that composite video is an analog format video signal, meaning that the signal''s broadcast lines have a very restricted bandwidth limit. Therefore, both the brightness signals (Y) and the colour signals (C) are purposefully designed to overlap into a single, lower bandwidth signal.
When televisions receive this signal they often have difficulty in fully dividing it back into its two separate brightness and colour parts. As a result the displayed image can sometimes display distracting fuzziness or image artefacts (unwanted speckles or noise).
What does a comb filter do?
To remedy the problems TVs have with converting a composite video signal back into two separate brightness and colour signals, comb filters were introduced to try to clear up the image and reduce any fuzziness or distortions. A comb filter functions by extracting the colour data (technically known as residual chrominance) from the brightness signal (technically known as luminance). For this reason, comb filters are also sometimes referred to as Y/C separators.
Depending on the type of comb filter used, the displayed image quality when using a composite video signal can appear noticeably better. Comb filtering increases the quality of fine details, enhances image outlines and removes the most unwanted colours from analog video signals. For example, the colour interference you sometimes see on a TV when someone is wearing a striped shirt can be removed with a good quality comb filter.
How do comb filters work?
Comb filters are called comb filters because the graphical representation of the video information they process looks like a comb -- i.e. a series of thin vertical lines. What a comb filter actually does is combine each scan line with the scan line before it, and then it takes the average of these two lines to draw the image on the screen. Since every other line transmitted has the colour information intentionally phase-reversed, when the average of the two scan lines is taken the colour information effectively cancels itself out leaving only the luminance details which can easily be drawn into the picture.
To retrieve the colour signal, the comb filter then subtracts one line from the line drawn before it, meaning that the luminance data is cancelled out and a pure, more accurate colour signal is obtained.
The four different types of comb filters
There are four different types of comb filter available for use in televisions. Usually, the larger and more expensive the television, the better quality comb filter it possesses. Each of the four types of comb filter is explained on more detail below, in descending order of quality.
3-D Digital Comb Filters
3-D digital is the most advanced form of comb filter currently on the market since it can analyze both previous and subsequent scan fields, while simultaneously analyzing three successive horizontal scan lines. The benefit of this technology, also known as 3D Y/C, is that it vastly improves the accuracy of the colour and the general image stability. Furthermore, it practically removes all dot crawl and colour bleeding issues.
3-Line Digital Comb Filters
As the name suggests, this type of comb filter processes three horizontal scan lines after separating the brightness and colour signals. The advantage of this kind of filtering is that both dot crawl and colour bleeding is greatly reduced.
2-Line Digital Comb Filters
2-line digital comb filters function by separating the brightness and colours signals while adjusting and processing two horizontal scan lines consecutively. This aims to decrease the effect of signal overlap and thus lessen the amount of cross colour interference. This type of basic comb filter is the sort described above in the "how comb filters work" section.
Analog/Glass Comb Filters
Sometimes referred to as just "analog" comb filters, analog/glass comb filters are usually only implemented in lower priced or small sized (below 24 inch) televisions. Consequently, these types of filters (which include charge coupled devices and glass) are extremely rare nowadays.
When is a comb filter not needed in a TV set?
Comb filters are not necessary when the signal source is being transmitted from the video source to the TV by component or s-video signals because these types of cables carry the luminance information separately from the colour information, meaning that the two signals do not need to be separated by a filter. Digital TV and DVDs are making comb filters less important but they are still needed for the best quality reproduction from standard broadcasts.
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