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=Crominanza e Luminanza: i colori nel video=
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When color TV became commercially possible, a number of black and white TV sets were already in people´s restrooms and it would make no sense to throw them away. Also, the idea of transmitting color TV with different signals for the three basic colors (Red, Green and Blue, the famous RGB) was impossible in practice, because of the large bandwidth required for analog transmission. It would be too much information to fit in one standard television channel, 6MHz wide.
Engineers got a brilliant solution, one more time benefiting from a limitation of the human eye. They created a color system based on '''subtraction''' which became known as YCC, as opposite to the addition system used for RGB.
The main part of the analog color TV signal is made of '''luminance''', which defines only the brightness, ranging from full black to full white. This is exactly what the black and white TV needed, so those old TV sets could remain useful just by ignoring the color information. Added to it, but taking much less bandwidth, the '''chrominance''' signals were added for the colors. These signals are processed locally by the receiver with a little colors logic. If the blend of all colors in full bright result in pure white --- which is also the maximum value possible for the luminance signal ---, you could get color values from subtracting those chrominance values from the luminance signal.
To be exact, the blend for making white is 30% red, 11% blue and 60% green. So, to keep the required bandwidth as low as possible, the two chrominance signals blended to the luminance are those for red (C<sub>r</sub>) and blue (C<sub>b</sub>). Getting the amount of green in a pixel is as simple as G = Y - C<sub>r</sub> - C<sub>b</sub> . Blue is the result of Y + (C<sub>b</sub> - Y), and red comes from Y + (C<sub>r</sub> - Y).
Then, a coloured image in TV is made half by the luminance signal, and the rest for chrominance (a proportion of 4:2:2). The color information itself can use less bandwidth because the human eye is much more sensitive to the bright and contours of an image than the color differences. You can prove that by looking at drawings made only from colors without black or darker contours.
Although this solution was made for analog television, and digital video allows complete color information in a narrower bandwidth, the YCC scheme was preserved for allowing better compression. The DV25 standard for digital video uses even less bandwidth for the color information, in a luminance/chrominance proportion of 4:1:1 --- which can lead to visual artifacts in operations such as compositing an image with other of a person shot against a blue background (the famous ''chromakey'' composition). The more professional-driven DV50 standard uses 4:2:2 color sampling by default.
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Quando si iniziò a progettare la commercializzazione di televisori a colori, un grande numero di persone possedeva già un apparecchio in bianco e nero, e sarebbe stato impensabile costringerle a buttarlo. Inoltre
l'ipotesi di trasmettere programmi televisivi a colori utilizzando tre distinti segnali per i tre colori fondamentali (rosso, verde e blu, in inglese Red, Green e Blue, da cui la sigla RGB) era irrealizzabile a causa dell'ampiezza di banda richiesta dalla trasmissione di tipo analogico. Sarebbe stato necessario trasmettere troppe informazioni per la capacita di un canale televisivo standard, dell'ampiezza di 6 MHz.
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