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Understanding HDTV

Why three different digital TV formats?
Basically... the higher the screen resolution, the more bandwidth is needed for the signal. And the more bandwidth that’s used, the fewer the channels that can be transmitted during a given time slot.

Since TV stations and cable companies have limited bandwidth, this has presented those stations with a new business opportunity. A station broadcasting in HDTV will be transmitting at the highest possible quality, but it can only broadcast a single program since the HDTV signal uses all of the available bandwidth. Stations have realized that if they have programming that doesn’t require HDTV quality, they can broadcast in one of the other modes (EDTV and SDTV) and “simulcast” multiple programs.

Standard-Definition Television (SDTV)

This is your existing analog signal made digital. The horizontal frequency is the same as the current analog signal (a picture with 480 lines that is interlaced (480i). But since the signal is digital and suffers no loss in picture quality during transmission, you’ll enjoy television with greatly increased picture quality.

Enhanced Definition Television (EDTV)

This is a digital signal that is better than your existing analog signal but doesn’t have high enough resolution to be classified as HDTV.

High-Definition Television (HDTV)

This is a digital signal that has either two or three times the horizontal frequency of SDTV (480i). Thus, an HD signal may be either 1080 lines interlaced (1080i) or 720 lines progressive (720p). A 1080i signal has two times the horizontal frequency; a 720p signal has three times the horizontal frequency. The Consumer Electronics Association also stipulates that all televisions labeled High Definition” must be capable of displaying a 16:9 (widescreen) image.

Display Modes

In HDTV, the difference between 720p or 1080i scan line format is determined by how the television displays the signals on screen:

Progressive scanning (HDTV 720p, EDTV480P)doubles the number of visible picture lines per field by displaying all picture lines at once. Frames are drawn in order of all lines, 1,2,3,4, etc. which eliminates line flicker and provides a stable picture optimized for static or slow moving images. Interlaced scanning (HDTV 1080i, SDTV 480i) shows the picture by displaying the screen in two separate frames and drawing the odd lines first (1,3,5,7, etc.) followed by the even lines (2,4,6,8, etc.). There is flicker inherent in this scanning method. But by drawing more total lines per frame, this method is optimized for moving images.

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