Apple MA261Z/A User Manual - Page 1029

XIII The luma (black-and-white channel) and chroma (color channels) information can be recorded and transmitted several different ways in a video signal. Â 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 and flat-panel displays. Red, green, and blue signals can be combined to make any color, as well as grayscale images 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. Â Component YUV or Y´CBCR: This three-channel signal has a luma (Y') signal and two color difference channels (CB and CR)1. 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 while 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 similar proportions to the way human vision is sensitive to those three colors. Therefore, the luma signal approximates the same detail we see for various colors in the spectrum, which happens to be most sensitive to detail in 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). Â 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. Â 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, recreating the Y´CBCR component signal and then the RGB signal for display. Because the chroma and luma channels are combined, they do not separate perfectly, causing artifacts in the resulting image. 1. The pair of color difference channels has different names depending on the particular format, but they all serve a similar function. Some common names for color difference channels include: CB, CR; R-Y, B-Y; or U,V. Appendix A Video Formats 1029