An electronic circuit that breaks down the composite NTSC video signal into its components. It’s the receiver side of the encode, decode process used to compress three channels of color information into one, then expand it back to the three channels required for viewing. There are two basic types of decoders, Comb Filter and Notch Filter. There are a number of subdivision in the category of Comb Filter, including one that uses the Notch Filter.
There are several steps in this decoding process and a number of ways of accomplishing each step.
In order to meet the guide lines set out for compatibility with black and white TV's, our NTSC composite color signal contains a black and white signal plus the two additional channels of color information needed to derive red, green, and blue. Each of the two channels of color information ride along with the black and white on their own 3.58 MHz subcarrier. The black and white information exists in frequency space all the way through and past the color information and its sidebands.
The first stage of decoding is to separate the color 3.58 MHz carriers and their information from the black and white. This is required not only in TV sets but in consumer VCR's.
In the early days of decoding NTSC, we couldn't build inexpensive circuits that would allow us to preserve the black and white information occupying the area of the color signal. We used a simple circuit called a Notch Filter to break these two signals apart. Everything around 3.58 MHz was treated as color information and all else was treated as black and white. Since that essentially means that there was no high frequency detail in the black and white, the Sharpness control was added to your set to eke out every last bit of detail left after the notch filter had done its damage.
The Notch Filter decoder is still alive and well today. It can be found in most every inexpensive TV sets and most standard VHS machines. The presence of a Notch Filter in a TV is easy to detect. Display the SMPTE Resolution Chart Video Essentials VE T17 C13 at the composite video input to the monitor and look for the horizontal resolution wedge, the vertical lines on either side of the center circle, to show a lot of color from about 240 lines on up. A Notch Filter can also be incorporated in a Comb Filter decoder.
As technology progressed, versions of the Comb Filter came along. They allow the color information, which is interleaved with the black and white signal, to be combed out, preserving much more detail in the black and white. If a Comb Filter is incorporated in a TV set, the horizontal resolution wedge will remain in black and white over its entire visible range. There are exceptions. Sometimes you'll see a little bit of color in the wedge. This is usually an indication that the Comb Filter hasn't done a good job of separating the two.
The first generations of Comb Filter decoders were analog. They incorporated a delay line so that adjacent TV lines could be added or subtracted, yielding black and white at one point and color at the other. Once the color carrier and its information is separated from the black and white, it's necessary to pull the two pieces of color information away from their 3.58 MHz carriers. The absence or presence of Dot Crawl in the picture is an indication of how well the 3.58 MHz carrier has been removed. In less expensive implementations of the Comb Filter, the subcarrier is notched out, taking some black and white information with it. Most consumer sets with a Notch on or off option in the menu system are applying the notch to the color signal rather than using it to separate the color from the black and white.

This separated black and white and color signal forms what is known as an "S-Video" signal. It's two channels of information, the black and white on one and the color sub-carriers and their information on the other.
As we mentioned, delay lines are required for comb filters. If the exact amounts of delay can be tightly controlled, a better job of decoding can take place. Normally we just compare adjacent lines in the same field, 2D decoding. If we can get enough delay, we can compare lines from the next field, 3D decoding. It turns out to be less expensive to delay signals in the digital domain than in the analog domain. This is where the Digital Comb Filter fits in.
Each type of decoding has advantages and disadvantages in picture quality, depending on the picture content. An Adaptive decoder can switch among the various modes, at any given time, to pick the best type of decoding. An adaptive filter is limited by what's built in. It could be 2D adaptive or a 3D adaptive comb filter.
The first step in decoding a color video signal is to separate the high frequency color carrier from the black and white. The second step is to separate the two color signal from their carriers. By the way, this takes a finite amount of time. The black and white signal is delayed while this process is taking place. Once the two color difference signals arrive out of the color processor, without their carriers and timed with the black and white, we have a three channel system. This is a stage of Component Video. This is the three channel form that will be recorded on DVD, is now available on DSS, and will be available in our Advanced TV system.
The black and white plus two color difference signals are then converted to another component video format, called red, green, and blue, the format needed to drive the display.
What's important in picture quality? There isn't any clear cut answer. Some of the best video processing equipment on the market uses a 2D adaptive comb filter. While 3D adaptive filter have the potential of more picture resolution, it comes at a cost of possible picture artifacts.
The problems associated with decoded look of composite video signals are properly solved by not encoding the signal in the first place. That is the direction of digital video. It is a component video system.