SDTV stands for “standard-definition television,” referring to the overall clarity of the image on the screen. The term only arose when high-definition television entered the scene; before that, all televisions were standard definition and the distinction was unnecessary. Standard-definition televisions provide less clarity than high definition, and normally constitute older tube box TVs rather than newer plasma screen and LCD screens.
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For many decades, television stations broadcast in analogue, which means that the signal was analogous to the source. Analogue takes up a great deal of space on the bandwidth, which limited the parameters of the screen definition. Because they couldn’t fit any more information into the signal, most resolutions were basically the same. In 2009, all US stations switched to digital broadcasts, which send the signals out as a series of 1s and 0s. Because they take up much less space on the bandwidth, digital signals can hold much more information, allowing stations to broadcast in varying definitions and necessitating the need to create SDTV.
Standard-definition television screens contain 640 vertical lines of pixels and 480 horizontal lines of pixels. Televisions with a higher definition than SDTV have more horizontal pixel lines in their screen--720 or 1080--which provides more detail than 480 lines and thus a sharper and clearer image.
Progressive and Interlaced
Standard-definition signals were originally interlaced, which means that only half the pixel lines appeared at any time. The image flickered back and forth between one half and the others faster than the human eye could perceive, making the image appear whole. It also creates the flicker effect which appears in older TVs. Progressive scans, on the other hand, show every line of pixels instead of flickering back and forth the way interlaced screens do. SDTV currently encompasses bother interlaced and progressive signals, though they don’t display more than 480 lines of pixels.
Almost all older cathode ray tube TVs--the bigger, boxier TVs with a screen ratio of 4:3--are SDTV. Newer televisions list their screen resolution on their packaging materials, with a number indicating the horizontal pixel lines and an “i” or a “p” to indicate interlaced or progressive (720p, for example, or 1080i).
Free TV--including the five national networks and numerous local stations--continues to broadcast in standard definition, ensuring that anyone with a TV can receive the signals (though older TVs may require a digital converter in order to accept the digital signal). While some network shows are broadcast in high definition for free, cable and satellite companies charge an additional fee to send HD signals. Standard definition worked well for many decades, and still provides a clear image, while costing less than high definition to boot.
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