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.
RGB (Red, Green, Blue): This is the native format for most computer graphics and video files. This signal is also used inside traditional color CRTs, video cameras, flat-panel displays, and video projectors. Red, green, and blue signals can be combined to make any color, as well as grayscale images ranging from black (no signal on any channel) to white (full signal on every channel). RGB signals do not have a separate luma channel, because black-and-white signals can be represented by equal amounts of R, G, and B signals.
Component YUV: This three-channel Y′CBCR signal has a luma (Y′) signal and two color difference channels (CB and CR). (The two color difference channels have different names depending on the particular format but serve a similar function in all formats. Some common names for different color channels include CB, CR; R-Y, B-Y; and U, V.) Component video was invented in the 1950s as a way of making color television signals compatible with existing black-and-white televisions. Black-and-white televisions could use the luma signal, and color televisions could convert Y′, CB, and CR back to RGB for display.
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).
S-Video (Y/C): An S-Video signal is also considered a component video signal because the luma and chroma signals are separate. However, the C signal is derived by combining the CB and CR component signals, which reduces the quality of the color channel compared to Y′CBCR.
Composite: The luma (Y′) and chroma (C) signals are combined into a single composite video signal for broadcast. The chroma signal is placed on a color subcarrier frequency related to the main luma frequency. This method of superimposing color information on top of the black-and-white information indicates that this format originated in the early days of color television, when black-and-white TV compatibility was critical for widespread adoption.
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.