NTSC Decoding Basics (Part 2)

NTSC decoding…it's NOT easy

Focus one eye on Figure 1. Separating the luma component from the chroma component is the main task in that first box. The designers of the NTSC system knew theoretically that it would be possible to properly separate Y and C, but did not have a cost-effective way to do it in the early years. In fact, the more sophisticated methods of separation through "comb filters" did not arrive in the market until the late 70's, more than 20 years after the system was adopted.

So, early television receivers used the notch/bandpass filter system for Y/C separation because the method is low cost and easily implemented with reasonable results. In many situations, that approach is used today. In fact, most all of the digital decoders on the market automatically switch back-and-forth between notch/bandpass and combing as required. Watching a VHS tape? You'll most likely be operating in the notch/bandpass mode even if you have a comb filter in your display. Why is it called a notch/bandpass filter?

Figure 2

Figure 2 — Basic notch/bandpass filter system

Figure 2 illustrates the basic topology of this filter. The composite NTSC is input to a system having two analog signal pathways. One pathway substantially passes frequencies from the region of the 3.58 MHz subcarrier and lower. Here, a series-resonant type passive filter (capacitor and inductor) is employed whose resonant frequency is centered about 3.58 MHz. The operation of the filter is such that as energy approaches its resonant point, energy is dissipated in the filter; otherwise it is allowed to pass on. Therefore, little energy within the color subcarrier region passes by the filter. The result is a severe "notch" created in the Y channel bandpass centered at 3.58 MHz. This action substantially removes the chroma information from the incoming signal and we realize the luma or Y channel. But, we sacrifice high frequency information, or horizontal picture detail, above the color subcarrier frequency region.

Conversely, the second pathway passes frequencies just within the region of the 3.58 MHz subcarrier. Here, a parallel-resonant filter (capacitor and inductor) is employed whose resonant frequency is…can you guess? Yes, 3.58 MHz. A parallel resonant filter acts in an opposite manner in that it severely attenuates signals that are not near its resonant point. Signals near its resonant point are passed through as shown in Figure 2. The result is that substantially the chroma information makes it through the filter and we realize the C channel.

Now, mentally overlay the images of each filter characteristic shown in Figure 2 and you'll see that significant overlap occurs. Regions exist where some luma energy finds its way through the bandpass filter and some chroma energy finds its way through the notch filter. Hence, the notch/bandpass approach is far from ideal. Analog methods do not allow for extremely precise control of filter characteristics; or in other words, the sides of the these filters cannot be made very steep, so as to block unwanted information. These filter crossover regions are responsible for most decoding artifacts seen in color decoders using this methodology.