NTSC Decoding Basics (Part 1)
What? There's More?
Now, the color encoder uses each of the difference signals (R-Y, B-Y) to modulate the 3.58 MHz reference carrier. The difference signals modulate the amplitude and phase of the 3.58 MHz subcarrier. The amplitude modulation represents the intensity, or saturation, of a color and the phase modulation represents the hue, or specific color intended. Prior to this step, the subcarrier reference signal is separated into two reference channels within the encoder. One channel is the in-phase component (called "I") and the second channel is phase-shifted 90 degrees (called "Q"). Information contained in the R-Y channel controls the 'I' component of the subcarrier. The B-Y channel controls the 90-degree phase shifted 'Q' component of the subcarrier. This outcome is shown in Figure 1 as "I" and "Q" signals. The 3.58 MHz subcarrier signal with its modulated 'I' and 'Q' signals is added to the Y signal and composite NTSC is the result. During all this processing, which takes time, the Y channel must be delayed to account for the time difference. In order to keep the TV receiver in synchronism with the transmitted color signal, a sample of the subcarrier, called "color burst", is added to each horizontal line during the picture blanking interval. Burst lasts for about 8 - 9 cycles of the subcarrier and is placed inside the video signal horizontal blanking interval. See Figure 4 for a representation of the color bar test pattern as composite NTSC.
Although I and Q signals are used for evaluation of the TV signal, they are not used in standard consumer television receivers. In actuality, the bandwidth of the I channel is reduced to about that of the Q channel and each is phase-shifted an additional 33 degrees. This facilitates lower cost decoding circuitry in the receiver.
Now that we see how the NTSC composite signal is assembled, in the next installment, we'll begin to see just how challenging it is to disassemble. And, if you can call or email our editor and tell us if this series is accomplishing Dave's request. Don't worry, we'll get through it before you need to buy your digital TV set.