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480i is the shorthand name for a video mode, namely the United States NTSC television system or digital television systems with the same characteristics. The i, which is sometimes uppercase, stands for interlaced, the 480 for a vertical frame resolution of 480 lines containing picture information; while NTSC has a total of 525 lines, only 480 of these are used to display the image for Digital video (DV)-NTSC.

Technical Detail

For analog NTSC there are a total of 525 scanning lines of which originally 486 lines were active (243 complete lines per field) and later 480 (240 complete lines per field). A full frame is split into even and odd fields where the even fields end with half a line and the odd fields begin with the other half of the line. By convention an NTSC video frame is considered to start with an even field followed by an odd field. The disparity of the line numbering compared to other systems is solved by defining the line numbering to start five equalising pulses (or 2 and a half lines) earlier than on all other systems (including Systems A and E even though they had no equalising pulses) on the first equalising pulse following an active line or half line. This has the effect of placing a half line of video at the end of the even (first) field and the beginning of the odd (second field). Thus the line numbers correspond to the real lines of the video frame. On all other systems, the field was considered to start with the falling edge of the first field pulse which gave the confusing position that the odd field (first) had a half a line of video occupying the latter half of a whole line and ended with a whole line of video but half a scanning line (and vice versa for the even field). The NTSC convention solved this confusion.<ref>Report 308-2 of the XIIth Planiary of the CCIR - Characteristics of TV systems.</ref>

For DV-NTSC only 480 lines are used. The digitally transmitted horizontal resolution is usually 720 samples (which includes 16 samples for the horizontal sync and horizontal blanking) or 704 visible pixels with an aspect ratio of 4:3 and therefore a display resolution of 640 × 480 (VGA); that is standard-definition television (SDTV) with a 4:3 aspect ratio.<ref>rpsoft2000 - US HDTV, 480i, 480p, 1080i</ref><ref>AfterDawn.com - 480i</ref><ref>CNET Networks (CNET) - Glossary - 480i</ref><ref>TheFreeDictionary.com - 480i</ref>

The field rate (not the frame rate) is usually (60/100.1% ≈) 59.94 hertz for color TV and can be rounded up to 60 Hz. There are several conventions for written shorthands for the combination of resolution and rate: 480i60, 480i/60 (European Broadcasting Union (EBU)/SMPTE) and 480/60i. 480i is usually used in countries that conventionally use NTSC (most of the Americas and Japan), because the 525 transmitted lines at 60 hertz of analogue NTSC contain 480 visible ones.

480i can be transported by all major digital television formats, ATSC Standards (ATSC), Digital Video Broadcasting (DVB) and ISDB.


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Original Source

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