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480i is thevideo mode used forstandard-definitiondigital video[1] in most of theAmericas (with the exception ofArgentina,Paraguay, andUruguay),Japan,South Korea,Taiwan, thePhilippines, andMyanmar. The other common standard definition digital standard, used in the rest of the world, is576i.
It originated from the need for a standard to digitize analog525 line TV (defined inBT.601) and is now used for digital TV broadcasts and home appliances such as game consoles and DVD disc players.[1][2]
The480 identifies a vertical resolution of 480 lines, and thei identifies it as aninterlaced resolution. Thefield rate, which is 60 Hz (or 59.94 Hz when used with NTSC color), is sometimes included when identifying the video mode, i.e.480i60; another notation, endorsed by both theInternational Telecommunication Union inBT.601[3] and SMPTE inSMPTE 259M, includes theframe rate, as in480i/30.
Although related, it should not be confused with the analog "525 lines" resolution, mandated byCCIR Systems M and J and usually paired withNTSC color. This association explains why 480i is sometimes inaccurately called "NTSC", even though NTSC only exists in the analog domain.
For analog NTSC, there are a total of 525scanning lines per frame of which originally 483 lines were visible (241.5 visible lines per field + 21 lines ofvertical blanking per field = 483 + 42 = 525 lines per frame) and later 480 (240 complete lines per field). [For quad video recording systems, the math suggests 15 transverse head passes, each consisting of 16 lines of video, are required to complete one field.]
A full frame consists of two fields. One field contains the odd-numbered lines and the other contains the even ones. 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 equalizing pulses (or 2 and a half lines) earlier than on all other systems, includingSystems A (405-line) andE (819-line) even though they had no equalizing pulses, on the first equalizing 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.[4]
ForDV-NTSC only 480 lines are used. Thedigitally transmitted horizontal resolution is usually 720 samples (which includes 16 samples for the horizontal sync and horizontal blanking) or 704 visiblepixels with anaspect ratio of 4:3 (with vertically rectangular pixels) and therefore a display resolution of 640 × 480 (VGA); that isstandard-definition television (SDTV) with a 4:3 aspect ratio (with square pixels).[5][6][7]
Thefield rate (not theframe rate) is usually (60/1.001) = 59.94 hertz for colorTV and is often incorrectly rounded up to 60 Hz. There are several conventions for written shorthands for the combination of resolution and rate:480i60,480i/30 (EBU/SMPTE always use frame rate to specify interlaced formats) and480/60i. 480i is usually used in countries that conventionally use NTSC (most of the Americas andJapan), because the 525 transmitted lines at 60 hertz of analogue NTSC contain 480 visible ones.
In each case of the use of the ‘60’ terminology, it is merely shorthand for 59.94, to differentiate it from 30 (29.97) or 24 (23.976).
Color information is stored using theYCbCrcolor space (different from NTSC that usedYIQ) with4:2:2 sampling (also different from NTSC) and followingRec. 601 colorimetry.
480i can be transported by all majordigital television formats (ATSC,DVB andISDB) and onDVD.
