Analog Video
Analog video contains a rapidly changing signal (analog) that represents cthe luminance and color information of a video picture. Sending a video picture involves the creation and transfer of a sequence of individual still pictures called frames. Each frame is divided into horizontal and vertical lines. To create a single frame picture on a television set, the frame is drawn line by line. The process of drawing these lines on the screen is called scanning. The frames are drawn to the screen in two separate scans. The first scan draws half of the picture and the second scan draws between the lines of the first scan. This scanning method is called interlacing. Each line is divided into pixels that are the smallest possible parts of the picture. The number of pixels that can be displayed determines the resolution (quality) of the video signal. The video signal breaks down the television picture into three parts: the picture brightness (luminance), the color (chrominance), and the audio.
There are three primary systems used for analog television broadcasting: NTSC, PAL, and SECAM. The National Television System Committee (NTSC) is used for the Americas, while PAL and SECAM are primarily used in the UK and other countries. The major difference between the analog television systems is the number of lines of resolution and the methods used for color transmission.
There have been enhancements made to analog video systems over the past 50 years. These include color video, stereo audio, separate audio programming channels, slow data rate digital transfer (for closed captioning), and ghost canceling. The next major change to television technology will be its conversion to HDTV.
Figure 1 demonstrates the operation of the basic NTSC analog television system. The video source is broken into 30 frames per second and converted into multiple lines per frame. Each video line transmission begins with a burst pulse (called a sync pulse) that is followed by a signal that represents color and intensity. The time relative to the starting sync is the position on the line from left to right. Each line is sent until a frame is complete and the next frame can begin. The television receiver decodes the video signal to position and control the intensity of an electronic beam that scans the phosphorus tube (“picture tube”) to recreate the display.
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