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How Does It Work

In analog TV, a 6 MHz analog signal carries intensity and color information for each scan line of the picture. An analog TV signal in the U.S. has 525 scan lines for the image, and each image is refreshed every 30th of a second (half of the scan lines are painted every sixtieth of a second in what is called an interlaced display).

Digital television (DTV) is the transmission of pure digital television signals, along with the reception and display of those signals on a digital TV set. Signals can be broadcast over the air or transmitted by a cable or satellite system to your home, where a decoder receives the signal and uses it, in digital form, to directly drive your digital TV set.

HDTV is high-resolution digital television (DTV) combined with Dolby Digital surround sound (AC-3), HDTV is the highest DTV resolution in the new set of standards. (Can link to ATSC standards document) The higher resolution picture is the main selling point for HDTV. It offers 720 or 1080 lines of resolution compared to the 525 lines people are used to in the United States.

Of the 18 DTV formats, six are HDTV formats, on of which is based on interlaced scanning, and five of which are based on progressive scanning, which occurs at twice the speed of interlaced scanning. Of the remaining formats, eight are SDTV (four wide-screen formats with 16:9 aspect ratios, and four conventional formats with 4:3 aspect ratios), and the remaining four are video graphics array (VGA) formats. Stations are free to choose which formats to broadcast.

The formats used in HDTV are:

720p - 1280x720 pixels progressive

1080i - 1920x1080 pixels interlaced

1080p - 1920x1080 pixels progressive

"Interlaced" or "progressive" refers to the scanning system. In an interlaced format, the screen shows every odd line at one scan of the screen, and then follows that up with the even lines in a second scan. Since there are 30 frames shown per second, the screen shows one half of the frame every sixtieth of a second. For smaller screens, this is less noticeable. As screens get larger, the problem with interlacing is flicker.

Progressive scanning shows the whole picture, every line in one showing, every sixtieth of a second. This provides for a much smoother picture, but uses slightly more bandwidth.

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