Let us focus on the active video part of the NTSC signal.The technique employed for sending color information to the TV is analog QAM. The signal is given by
Given R, G, B , we can determine Y,I,Q and vice versa. Representing the chroma signal in terms of one carrier signal, we have,
Let us now consider a square wave and represent that using the fourier series.
If 1/T = f = 3.579545 MHz is the fundamental, higher harmonics are odd multiples of the fundamental frequency. These will be 10.73Mhz, 17.89Mhz and so on. All of these are removed by the chroma band pass filter in a TV. All that remains after filtering will be the fundamental.
So, as far as the TV color detecting circuit is concerned, a color carrier square wave looks identical to a sine color carrier.
Let us now see some numerical examples of the way QAM works.
Suppose phase angle 0 is to be produced, then I = 0 and Q = 1. ( This is as per our earlier definition and NOT on NTSC signal definition ).
Suppose phase angle 45 is to be produced, I = 1 and Q = 1.
Similarly, if I = 2 and Q = 1, phase angle is 63.44 degrees.
By mixing Sine and Cosine waves of different amplitudes, hue can be spanned.
Using the previous result, all hue can be generated by mixing square waves of phases 0 , 90 degress with their amplitude scaled appropriately.
1. SX PONG by Rickard Gunee
2. AVR PAL Color Generator by Jakub Trznadel
3. NTSC Tutorials