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As we know that RGB monitors requires separates signals for red, green, and blue components of the image but television monitors uses single composite signals. For this composite signal use YIQ color model.
My question is why we can say YIQ is composite signal and RGB is 3 different signal?
N. B. - I am computer science student, I want to understand just intuition rather than details because much using technical term it may creates unnecessary confusion. So please explain easily.
Not so sure what that question is about, as global terms like this can have different meanings, depending on context. Also it seems as if it mixes physical definitions of signal transmission with colour models. Most basic:
About Signals:
About Colour Models:
Colour models do not tell anything about composite or not. For example, NTSC is YIQ over a single signal, while S-Video is the same, but using two lines, one for lumina (Y) and one for colour (IQ). Although some models are more easy to be turned into composite signals then others - in case of YIQ, it's already made as relative encoding, so combination is way less effort than would be needed for a composite RGB signal.
*1 - Making luminosity a dedicated component is what made it colour TV compatible with B&W TV, as now the basic B&W signal could be encoded like before, while adding the new colour component in a way old devices would not detect.
YIQ and related luminance (Y) plus chrominance (some other two channels) relate to the history of broadcast television — originally monochrome, with colour added in a backwards-compatible fashion — but that stems from the nature of human vision.
Eyes have a high density of rods, which are comparatively quite sensitive and detect luminance only, and a much lower and less even density of cones, which act to detect frequency and therefore colour.
As a result humans see a high resolution of brightness and overlay that with a low resolution of colour. Which means that usually video is encoded with a high resolution of luminance and a low resolution of chrominance — usually half or a quarter as mich information.
Video engineers and televisions therefore think most natively in terms of 1d luminance plus 2d chrominance, and that is partly expressed in the connectivity standards, including original colour composite, S-Video and modern component.
So: the reason is partly continuous evolution of the standards, partly keeping things in terms of what most closely correlates with perception.
As television signal was originally a single brightness channel, there was a need to add colour in a way that is compatible with the older standards. This way it is possible to watch colour transmissions on a B&W TV, and B&W transmissions on a colour TV.
Three components are required for transmitting colour, but the brigthness signal is needed for compatibility. So RGB is translated to three other signals, one that is equal to the B&W brightness Y, and two other signals that are called the colour difference signals, U and V.
In NTSC colour transmission, the two U and V are slightly processed (scaled and rotated by 33 degrees) to end up with two similar signals called I and Q.
Basically U&V, or I&Q, is a sine (or cosine) wave at the color carrier frequency (3.579545 MHz) which encodes colour in the amplitude of the sine wave (indicating saturation, how much colour there is) and in the phase of the sine wave (indicating hue, which colour there is). The phase reference, the colourburst, is sent on each line before video.
So in short, two colour signals are quadrature amplitude modulated at the colour carrier frequency and added to the brightness signal with reference to end up with the composite signal.
Thus the single composite signal does not contain the three YIQ/YUV signals directly, but in encoded form. So composite must be decoded into the three YIQ/YUV signals before converting back to RGB for display.
A lot of detail is in the other answers. I try to focus on the basic concept. For a color image to be displayed on a color CRT, you need three components. They can be red, green, blue (RGB) or a general brightness (Y) combined with information about the cyanness (or orangeness, which is just negative cyanness) an well as magentaness (or greenness, which is just negative magentaness).
Now we need to talk about how the three components get transmitted to the monitor. On the one hand, look at cable radio: You have one cable, but by tuning your receiver, you can receive different programs from the same cable. On the other hand, look at an analog PBX telephony system: If you have three phones in one office, you have three (pairs of) wires running from the PBX to the office, each one only carrying one call. The composite signal is like cable radio (single cable, three programs), whereas RGB is like the phone lines (three cables, three programs).
There is nothing inherent to the YIQ (or the related YUV) color models preventing them to be tranmitted on three different cables. Essentially, that's exactly what "component video" used for analog HD transmission is doing.
