Video data
Initially, video contained only gray-scale (also called black-and-white) information. While color broadcasts were being developed, attempts were made to transmit color video using analog RGB (red, green, blue) data. However, this technique occupied three times more bandwidth than the gray-scale solution, so alternate methods were developed that led to using Y, R–Y, and G–Y data to represent color information, where Y represents the luma (black-and-white part), and R–Y and G–Y represent color difference signals made by subtracting the Y from the red and blue components. A technique was then developed to transmit this Y, R–Y, and G–Y information using one signal, instead of three separate signals, and in the same bandwidth as the original gray-scale video signal.
Today, even though there are many ways of representing color video (R′G′B′, YIQ, YCbCr, YPbPr, YUV, and others are covered in Chapter 2), they are still all related mathematically to RGB.
S-Video was developed for connecting consumer equipment together (it is not used for broadcast purposes). It is a set of two analog signals, one gray-scale (Y) and one that carries the analog R–Y and B–Y color information in a specific format (also called C or chroma). Once available only for S-VHS, it is now supported on most consumer video products.
Although always used by the professional video market, analog RGB video data has made a temporary comeback for connecting high-end consumer equipment together. Like S-Video, it is not used for broadcast purposes.
Insider Info
A variation of the Y, R–Y, and G–Y video signals, called YPbPr, is now commonly used for connecting consumer video products together. Its primary advantage is the ability to transfer high-definition video between consumer products. Some manufacturers incorrectly label the YPbPr connectors YUV, YCbCr, or Y(B-Y) (R-Y).
Digital Video
The most common digital signals used are RGB and YCbCr. RGB is simply the digitized version of the analog RGB video signals. YCbCr is basically the digitized version of the analog YPbPr video signals, and is the format used by DVD and digital television.
Technology trade-offs
There is always the question of “What is the best connection method for equipment?” For DVD players and digital cable/satellite/terrestrial set-top boxes, the typical order of decreasing video quality is:
Some will disagree about the order. However, most consumer products do digital video processing in the YCbCr color space. Therefore, using YCbCr as the interconnect for equipment reduces the number of color space conversions required. Color space conversion of digital signals is still preferable to D/A (digital-to-analog) conversion followed by A/D (analog-to-digital) conversion, hence the positioning of HDMI RGB above analog YPbPr. The computer industry has standardized on analog and digital RGB for connecting to the computer monitor.