it in very simple way.
It covers many of the fundamentals of analog video. It is divided into four sections: "Picture Basics" covers how a video picture is generated; "Resolution: Visual versus Format" discusses the different resolution formats and how resolution is specified and measured; "Formats and Interfaces" includes different types of video signals, waveforms, and interfaces; and the glossary at the end defines terms specific to video.
Picture Basics
A picture is "drawn" on a television or computer display screen by sweeping an electrical signal horizontally across the display one line at a time. The amplitude of this signal versus time represents the instantaneous brightness at that physical point on the display. Figure 1shows the signal amplitude relationship to the brightness on the display.
Figure 1. Horizontal scan versus display brightness.
At the end of each line, there is a portion of the waveform (horizontal blanking interval) that tells the scanning circuit in the display to retrace to the left edge of the display and then start scanning the next line. Starting at the top, all of the lines on the display are scanned in this way. One complete set of lines makes a picture. This is called a frame. Once the first complete picture is scanned, there is another portion of the waveform (vertical blanking interval, not shown) that tells the scanning circuit to retrace to the top of the display and start scanning the next frame, or picture. This sequence is repeated at a fast enough rate so that the displayed images are perceived to have continuous motion. This is the same principle as that behind the "flip books" that you rapidly flip through to see a moving picture or cartoons that are drawn and rapidly displayed one picture at a time.
Interlaced versus Progressive Scans
These are two different types of scanning systems. They differ in the technique used to "paint" the picture on the screen. Television signals and compatible displays are typically interlaced, and computer signals and compatible displays are typically progressive (non-interlaced). These two formats are incompatible with each other; one would need to be converted to the other before any common processing could be done. Interlaced scanning is where each picture, referred to as a frame, is divided into two separate sub-pictures, referred to as fields. Two fields make up a frame. An interlaced picture is painted on the screen in two passes, by first scanning the horizontal lines of the first field and then retracing to the top of the screen and then scanning the horizontal lines for the second field in-between the first set. Field 1 consists of lines 1 through 262 1/2, and field 2 consists of lines 262 1/2 through 525. The interlaced principle is illustrated in Figure 2. Only a few lines at the top and the bottom of each field are shown.
Figure 2. Interlaced scanning system.
A progressive, or non-interlaced, picture is painted on the screen by scanning all of the horizontal lines of the picture in one pass from the top to the bottom. This is illustrated in Figure 3.
Figure 3. Progressive (non-interlaced) scanning system.
Resolution:Visual versus Format
The visual resolution of a video signal or display is the amount of detail that can be seen. This is different from the resolution format of a signal or display. For example, in a computer application, a XGA signal has a format resolution of 1024 horizontal pixels and 768 vertical pixels (lines), and is the implied visual resolution. However, if the signal or display has any limitations that can degrade the performance, it may not be possible to actually view all of this detail.Visual Resolution in Television Systems
Visual resolution in television systems is accurately specified in terms of a parameter called "TV lines." This parameter is typically used to indicate horizontal resolution, but the same technique can be used for vertical resolution. TV lines are determined by viewing a test pattern consisting of alternating black and white lines that are placed closer and closer together. The pair of lines with the closest spacing that can be distinguished as separate lines determines the resolution. The lines that can be extrapolated across the screen to a width equal to one picture height are the TV lines of resolution. Figure 4 shows a representative picture for determining resolution.
Figure 4. Representative visual resolution test pattern.
Visual Resolution in Computer Systems
Computer resolution formats are typically specified by the visible number of pixels in the horizontal and vertical dimensions. For example, a VGA format signal has 640 visible pixels in the horizontal direction and 480 visible pixels in the vertical direction. An XGA format signal has 1024 visible pixels in the horizontal direction and 768 visible pixels in the vertical direction. In a well-designed computer system that is specified to reach a given maximum format resolution, all of the signal processing would be designed such that the visual resolution would be at least as good as the format resolution. If any circuit in the chain does not have the required performance, the visual resolution will be less than the format resolution.Formats and Interfaces
There are many different kinds of video signals, which can be divided into either television or computer types. The format of television signals varies from country to country. In the United States and Japan, the NTSC format is used. NTSC stands for National Television Systems Committee, which is the name of the organization that developed the standard. In Europe, the PAL format is common. PAL (phase alternating line), developed after NTSC, is an improvement over NTSC. SECAM is used in France and stands for sequential coleur avec memoire (with memory). It should be noted that there is a total of about 15 different sub-formats contained within these three general formats. Each of the formats is generally not compatible with the others. Although they all utilize the same basic scanning system and represent color with a type of phase modulation, they differ in specific scanning frequencies, number of scan lines, and color modulation techniques, among others. The various computer formats (such as VGA, XGA, and UXGA) also differ substantially, with the primary difference in the scan frequencies. These differences do not cause as much concern, because most computer equipment is now designed to handle variable scan rates. This compatibility is a major advantage for computer formats in that media, and content can be interchanged on a global basis.Table 1. Typical Frequencies for Common TV and Computer Video Formats
| Description | |||||
| Vertical Resolution Format (visible lines per frame) | |||||
| Horizontal Resolution Format (visible pixels per line) | |||||
| Horizontal Rate (kHz) | |||||
| Vertical Frame Rate (Hz) | |||||
| Highest Frequency (MHz) |
There are three basic levels of baseband signal interfaces. In order of increasing quality, they are composite (or CVBS), which uses one wire pair; Y/C (or S-video), which uses two wire pairs; and component, which uses three wire pairs. Each wire pair consists of a signal and a ground. These three interfaces differ in their level of information combination (or encoding). More encoding typically degrades the quality but allows the signal to be carried on fewer wires. Component has the least amount of encoding, and composite the most.
