NTSC Decoding Basics (Part 2)

Hey, we only opened one box


Well, we've only ventured through one box in the decoder diagram (Figure 1). Unfortunately, there isn't space to cover the whole process here [amen. - ed.]. Each of the remaining boxes deserves its own article (let the editor, or me know if you are interested). But, let's just wrap it up in general.

That first box in the chain is the most critical. After Y and C are obtained, the Y information must be delayed to account for the longer processing time required to synchronously detect the phase and level differences in the color subcarrier in order to derive R-Y and B-Y, the color difference signals. The color burst sample that is sent on the back porch of the video signal is used to key the local 3.58 MHz oscillator so as to synchronize or "genlock" it to the incoming signal. When this local reference oscillator frequency is compared to the incoming C-channel information within the color difference decoder, the result is the R-Y and B-Y signals.

Once the color difference signals are derived, they are input along with the Y channel to the matrix decoder. Remember that we can derive G-Y algebraically if we have two of the three components. The matrix decoder derives the G-Y signal from the two difference signals. In addition, it algebraically adds each of the difference signals to the Y channel. The result is the red, green, and blue video signals used to drive the display device.