What Is SDTV?
The television industry has undergone a number of changes since high definition TV (HDTV) made its debut. Since then, broadcasters and TV manufacturers have been pushing to make the sharper, more detailed HD video more feasible and economical for both broadcasters and viewers. As this trend continues, it keeps the TV world moving further away from the lower-resolution standard definition television (SDTV).
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Definition
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Standard definition television is a term that relates to the specifications that comprise this specific type of TV signal as it is broadcast from its source and processed by the television. These specifications include the number of lines of resolution, frame rate and aspect ratio of the picture. These standards date back to analog television transmissions and continue today on non-HD channels.
History
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The specifications used with SDTV were first mandated by the National Television System Committee (NTSC), which created the standard first for black-and-white TV in 1941 and modified them for color TV in 1953. These standards were for the analog signals of the time, and later, as digital means of transmission were developed, the Advanced Television System Committee (ATSC) was formed to decide on those standards, both for SD and HD.
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Specifications
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A standard definition picture has a resolution of either 640 or 720 x 480 lines in each individual frame, as opposed to the 1,280 x 720 or 1,920 x 1,080 for HD, which creates the noticeable difference in picture quality between the two. SD has a frame rate (as with film, the video portion of a TV signal is composed of individual frames, which when shown in rapid succession gives the appearance of motion) of 30 frames per second (fps), while HD can have either 30 or 60 fps, depending on whether the signal is progressive or interlaced. SD also has a more square-like 4:3 aspect ratio, where HD has a wider 16:9. However, flat-panel HDTVs can stretch the SD picture to fit the wider screen.
Bandwidth
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Because of the much-higher amount of data used in the transmission of an HD signal, SDTV video signals require much less bandwidth, whether broadcast over the air or by cable or satellite. With the digital over-the-air signals, up to four individual SD channels can be broadcast in the same amount of bandwidth as that one HD signal uses. This practice is known as multicasting.
Channels
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As of 2010, the major broadcast networks and many cable-based networks broadcast their signals in both SD and HD, on separate channels. This is because the major cable and satellite providers currently do not have the bandwidth to deliver every channel in HD, though that will change as the technology continues to improve, and also millions of people have not yet upgraded to an HDTV, which keeps the need for standard definition channels relatively high.
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References
- Photo Credit old television image by inacio pires from Fotolia.com
