Analog NTSC Television

NTSC Color Scan Frequencies

You may have wondered why NTSC color television uses 29.97 frames per second, not 30 fps; why the odd number. The answer is that the video is synchronized to the 4.5MHz sound subcarrier, which is 3/4 of the 6MHz channel spacing. Likewise, the horizontal scan frequency was also adjusted so that it could be synchronized with the sound  subcarrier, ie 15,750 became 15,734. The nearest integer sub-multiple of 4.5MHz to the original 15,750 is 4.5MHz/286, ie Fh = 15734.26573426573426... Hz  Then, divide that by 525 lines per frame, and you have  Fv = 29.9700299700299700... frames per second. Synchronizing the picture scan frequencies to the sound subcarrier prevents any crosstalk from the sound, from creating noise that craws across the screen. Instead, any distortion due to crosstalk will be motionless, and therefore not nearly as noticeable.  It's very likely that these frequency tweaks were unofficially used by savvy TV station technicians before NTSC color television institutionalized them. It's interesting that 29.97Hz can not be synchronized with 60Hz North American AC power, the reason 30Hz was originally used. But if you do a little math, you see that the beat frequency is 0.03Hz, or about every 33 seconds. A slight wiggle in your picture that slow is hard to detect, and a good DC filter in the TV power supply will eliminate it. BTW, as you probably know, the vertical scan frequency is not 29.97Hz, but twice that (59.94Hz) producing an interleaved scan, and a reason that 525 scan lines has to be an odd number.

Comb Filters

I'm sure you've seen comb filters mentioned in the specs for any good color TV.  A comb filter is a delay line and two summing amplifiers. Summing the current video signal with video that has been delayed one scan line, almost completely eliminates the colors sub carrier because the subcarrier in adjacent scan lines is phased 180 degrees. Taking the difference leaves you with just the color sub carrier and almost none of the luminance signal. This works because the color subcarrier is synchronized to the horizontal scan frequency. It is 227 and one half times the horizontal scan frequency. (3579545.4545... Hz) The one half part of 227.5 means that the phase of the subcarrier alternates 180deg every scan line, for the same horizontal position. 

Notice that this does not require bandpass filters. In practice, some bandpass filtering is applied, but the use of comb filters allows the color and luminance (black and white) signals to share bandwidth. It is assumed that adjacent scan lines look very much alike, but to improve the plan some TVs offer "three line comb filters". That means that two delays are used so that the current scan line is compared to an average of the previous line and the one following, ie a more accurate estimate of the current scan line.

It's called a comb filter because when you add a signal to a delayed copy of itself, you get a frequency response that is a picket fence pattern. The harmonic frequencies of the two video signals are clumped alternately every 15,734Hz. Frequencies that are a multiple of 15,734Hz are luminance information frequencies, and, chroma information is made up of frequencies that are  N+1/2 times 15,734Hz.

 Color Signals

The chroma signal is actually two signals. Two 90 degree phases of the chroma subcarrier are modulated by a ~Red (R-Y) signal and a ~Blue (B-Y) signal. This means that the total chroma subcarrier is both amplitude and phase modulated. A phase reference burst is sent at the beginning of each scan line, and your TV tint control is an adjustment of this reference phase as used by the R-Y and B-Y phase detectors. Because the luminance (black and white signal) is the sum of Red, Green and Blue,  Green can be extracted by subtracting Red and Blue. The color sums are weighted to optimize for human vision.

Sound Subcarrier

NTSC 4.5MHz sound subcarrier is FM modulated and therefore fairly immune to static and crosstalk. Like FM radio, the baseband signal is the sum of left and right channels in order to be backwards compatible with mono sound. Stereo, SAP, and some video scrambling systems use subcarriers above the baseband audio.