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Digital Television (DTV) is a new type of broadcasting system that replaces traditional analog broadcasting. From the consumer’s point of view, DTV has two immediate advantages: it provides better resolution for a clearer picture and it offers a wider viewing screen for a theater-like experience. If there’s a downside to DTV, it might be the understandable confusion surrounding the technology with its many formats and overlapping acronyms. For example, DTV also refers to digital television sets, or sets designed to display digital TV signals.
DTV broadcasts can come in different flavors or resolutions. Resolution is determined by the number of pixels or dots that make up a single frame of video. DTV gives broadcasters choices and digital televisions are capable of displaying those choices, either directly, or by converting the signal to the highest resolution the set is capable of displaying.
To understand how resolution works, consider a video camera that captures footage one frame at a time at a rate of 30 frames per second (fps). Each frame is then rasterized, or processed into rows of tiny dots called pixels. Each pixel carries its own hue and brightness information so that when taken as a whole, the data reconstructs the frame. To ensure an analog TV will align the rows of pixels correctly, horizontal and vertical sync signals are combined with the rasterized video to make a composite video signal.
This data-intensive signal is broadcast using radio waves, with sound broadcast separately. An analog TV receives the audio and video broadcasts and reconstructs the composite video signal using 525 vertical lines of pixels, poor resolution by modern standards. (Your computer display set to its lowest possible resolution uses 640 vertical lines of pixels.) Broadcasting needed a digital facelift to improve resolution while consuming less bandwidth.
DTV can transmit video information in the digital language of ones and zeros. This data can then be compressed by an encoding scheme known as MPEG2, which allows broadcasters to choose how they would like to encode each program, or what resolution to use. The choices include standard (SDTV) or high-definition (HDTV) resolutions. Therefore, DTV is not always high-definition. SDTV is roughly equal to analog TV though SDTV provides a superior picture thanks to digital technology.
Due to the different possible formats in DTV, not all televisions that are digital televisions can display all DTV formats. Some digital televisions are not capable of displaying HDTV and must down-convert these broadcasts to a lower resolution. Conversely, an HDTV made to display the highest resolution must up-convert all programs that are broadcast in lower resolutions.
Digital resolutions are named after the number of vertical pixel lines the program has been encoded with. They are as follows: 480i/p (SDTV), 720i/p (HDTV), and 1080i (HDTV). The “i” stands for an interlaced scan, and the “p” for progressive scan. In an interlaced picture half the screen updates every 60th of a second, then the other half, so that the entire frame updates 30 times per second. In a progressive scan, the entire frame updates with each pass, making for a flicker-free picture.
Some high-end digital TVs offer a native resolution of 1080p, though nothing is broadcast in 1080p because it requires too much bandwidth. Instead, these TVs internally process 1080i broadcasts before displaying them, de-interlacing the frames to stream a 1080p picture to screen.
You might wonder why a network would choose to broadcast in a lower resolution. Broadcasting in 1080i consumes all available bandwidth for a particular channel, while lower resolutions leave room for multi-casting, or broadcasting on sub-channels within the same frequency band. This can be used for a second stream of data to provide interactive television, supplementary information like menu guides, or even more programming choices. Since not every type of show requires or is even best suited to a 1080i broadcast, DTV opens us to many new possibilities.
Programs that are well suited to 1080i include movies, many types of documentaries and travel logs. Sports are broadcast in 720p because progressive scan keeps fast movement and camera pans fluid. A network might choose to broadcast a newscast, game or talk show in SDTV to make room for multi-casting.
Finally, unlike analog TV with it's 4:3 aspect ratio, nearly square, DTV uses a 16:9 aspect ratio resembling a rectangular movie screen. For all of these reasons the switch from analog to DTV is occurring globally. In the United States, 12 June 2009 marks the end to analog broadcasting. Canada set a date of 31 August 2011, and in the European Union the switch is already complete in some regions with others following suit.
An analog TV can display DTV signals using a digital box converter. In the US, the government is providing coupons on its TV Converter Box Coupon Program Website to subsidize purchases for citizens who qualify.
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