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Video signals are separated into several channels for recording and transmission. There are different methods of color channel separation, depending on the video format and its historical origins. For example, broadcast video devices were originally designed for black-and-white video, and color was added later. This is still evident in today’s video formats that break image information into separate black-and-white and color information. On the other hand, video and image processing on computers is more flexible and developed later, so a three-color RGB model was adopted instead of a luma-chroma model.
The luma (black-and-white channel) and chroma (color channels) information can be recorded and transmitted several different ways in a video signal.
The luma signal is derived by combining R, G, and B signals in proportions similar to the way human vision perceives those three colors. Therefore, the luma signal approximates the way the human eye sees brightness in color images. Humans are most sensitive to the green portion of the visible spectrum, and therefore the luma channel mostly consists of the green channel. The color difference channels are so named because they are derived from RGB by subtracting signals from the luma channel for each of the color channels (for example, R-Y or B-Y).
Black-and-white televisions are unaware of the color subcarrier, and so only the luma (Y′) channel is shown. Color televisions reverse the composite process, re-creating the Y′CBCR component signal and then the RGB signal for display. Because the chroma and luma channels are superimposed, they do not separate perfectly, causing artifacts in the resulting image.