NTSC Decoder (Cont.)

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.