NTSC Decoding Basics (Part 2)
by Steve Somers, Vice President of Engineering
Here we are again. Figure 1 illustrates the basis of the NTSC decoder system. Now, we have the dubious task of wading through the various methodologies for its disassembly. The most critical portion of the system is the first box in that figure… the Y/C separator. Decoder quality totally depends on the approach taken in this important signal processing function. Some have devoted their entire career to the process of Y/C separation…you know who you are. It is the function within this one box in which this entire series of articles is concerned.
There are fundamentally two methods for separating the luma (Y) information from the chroma (C ) information. Both involve filters that are designed to discern one portion of the signal from the other. One is called a simple "notch/bandpass filter" and the second is called a "comb filter". The comb filter variants are numerous and comprise the successive parts of this series. In this article, the notch/bandpass approach is presented.
We learned about the creation of NTSC in Part 1. Remember that the visual image is made up of two components… brightness and detail information (the higher bandwidth monochrome portion) and the color information portion (low bandwidth portion). The brightness and detail portion is the Y channel representing the luma information, which comprises most of the signal bandwidth. The color information, or chroma portion, is the C channel comprised of less bandwidth. In the composite signal, the chroma is superimposed onto the luma channel. This is accomplished through careful interleaving of one component over the other by selection of the chroma subcarrier frequency…i.e. the 3.58 MHz (the frequency is really 3.579545…MHz).
Figure 1 — Basic NTSC decoding system