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Video Basics

In order to fully understand the nature and implications of advanced video communications, it is necessary to comprehend the basics of video. The basic concepts are those of frame rate, scanning, resolution, aspect ratio, luminance, chrominance, and synchronization.

Frame rate
refers to the rate at which frames of still images are transmitted. Video is a series of still images that are transmitted in succession in order to create the perception of fluidity of motion. If transmitted in rapid succession, the perception is one of complete fluidity—24 frames per second (fps) is considered to be motion picture quality, while 30 fps is considered broadcast quality. If the frames are transmitted at a slow rate, the result is a herky-jerky video of poor quality. Particularly below 15 fps, quality suffers quite noticeably.
Scanning
refers to the process of refreshing the screen in horizontal lines. Odd lines are refreshed in one scan, and even lines in the next. Each set of odd and even lines refreshed constitutes a frame refreshed, with the scanning rate being a function of the power source of the receiver. For example, the American NTSC standard provides for 30 fps, involving 60 scans, which relates directly to the 60 Hz of the U.S. power source.
Resolution
refers to the definition, or sharpness, of the image. Resolution is determined by the number and density of the pixels (picture elements), which essentially are dots of picture. The greater the number and density of the pixels, the better the resolution. If the same number of pixels are spread over a greater area, the result is a grainier picture, as one can readily see by sitting close to a big-screen TV.
Aspect ratio
refers to the relationship between the width and the height of the image. The 4:3 (4 wide to 3 high) aspect ratio specified by the American NTSC standard is rooted in the early days of television, when round picture tubes made effective use of this approach.
Luminance
refers to intensity, or brightness, which can vary within an image. A video transmission varies the luminance by varying the power level, or amplitude, of the signal.
Chrominance
refers to color, with different standards allowing for varying levels of color depth. Clearly, the video image is more pleasing and life-like when the variation of color is as broad as possible.
Synchronization
includes vertical and horizontal synchronization, both of which are critical. Vertical synch is required to keep the picture from scrolling or flipping. Horizontal synch keeps the picture from being twisted.

Analog TV Standards

The initial television standards were set in the United States, where broadcast TV originated. The FCC set the initial VHF transmission standards in 1945 at 4.5 MHz. The National Television Standards Committee (NTSC) was formed in 1948 to standardize the characteristics of the broadcast signal. Ultimately, the Radio Corporation of America (RCA), which was owned by AT&T, lobbied the Electronics Institute of America (EIA) and set the initial black-and-white TV standards. Color TV was commercialized some years later. Among the first live color broadcasts were the Cotton Bowl and Rose Bowl games on January 1, 1954 [14-1]. As the cost of color TV sets was quite high, color didn’t really take off until the 1960s when sets became affordable for the masses. Major analog standards are NTSC and PAL, which are compared to digital HDTV in Table 14.1.

Table 14.1 Television standards compared: NTSC, PAL, and HDTV.

NTSC PAL HDTV

Analog/Digital Analog Analog Digital
Scanlines 525 625 1,920 or 1,280
Synchronization 40 49 N/A
Resolution (pixels per line) 640 640 1,080 or 720
Frame Rate (fps) 30 25 60, 40, or 24
Aspect Ratio 4:3 4:3 16:9

NTSC (National Television Standards Committee)
was established in the United States as the first standard (1953), setting the tone for broadcast TV. While other standards have since been developed, they all derived from the NTSC baseline. NTSC is characterized as analog in nature with 525 scanlines. There are 640 pixels per line, 485 of which are dedicated to the active picture. The frame rate is 30 fps and the aspect ratio is 4:3. An early analog standard that is viewed as being overly complex and ineffective in the contemporary digital world, NTSC is also said to mean “Never the Same Color” [14-6].
PAL (Phase Alternate Line)
was established in western Germany, The Netherlands, and the United Kingdom in 1967. PAL addresses the problems of uneven color reproduction that plagued NTSC due to phase errors associated with signal propogation. PAL inverts the color signal by 180º on alternate lines, hence the term Phase Alternate Line [14-6]. PAL is currently used in much of western Europe, Australia, and Africa. PAL is characterized as analog, with 625 scanlines. There are 640 pixels per line, with 576 dedicated to the active picture. The frame rate is 25 fps, and the aspect ratio is 4:3.
SECAM (Systeme Electronique Couleur Aven Memoire)
a variation of PAL, was developed in France. In addition to its use in France, it also is the standard in regions once under the influence of France, including areas of the Middle East.


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