o the monochrome signal. The NTSC system, and any other system that combines luminance and chrominance to represent images, is called composite video. In the case of NTSC, the composite signal is achieved not only by frequency interleaving but also through three different modulation techniques (phase, amplitude, and frequency). Each of the three conveys different elements of the signal. Audio is frequency-modulated, as is the luminance information contained in the signal. Color television is encoded through a process of color differencing. Subtracting luminance information, referred to as Y, from each of the primary color signals, R, G and B, forms signals. Only two of the three color-difference signals are needed; the third is redundant. NTSC uses (R-Y) and (B-Y) that are combined to create two new signals, I and Q. The formula for deriving I and Q is: 0.74(R-Y) - 0.27(B-Y) 0.48(R-Y) + 0.41(B-Y) The I and Q signals are used to modulate two color subcarriers at the same frequency but 90 degrees out of phase-an approach that is also referred to as quadrature modulation. This technique exploits the fact that the polarity of an alternating current is constantly reversing. Phase is measured in degrees. The full cycle of a sine wave describes a 360-degree arc. The sine wave crosses the base line at zero volts, achieves its highest positive voltage at 90 degrees, dips down to touch the base line again at 180 degrees, achieves its lowest negative voltage at 270 degrees and finally returns to the zero volt base line at 360 degrees. Phase-modulation electrically alters this pattern of positive and negative undulation to convey information-in this case, the color of a pixel. Phase is hard to determine without a phase reference, SO a short eight-cycle burst of the 3.58 MHz color subcarrier frequency serves as a phase reference. This color burst, which is transmitted during horizontal blanking as a reference for establishing the picture color, allows synchronization. The luminance and the modulated color signals are added to