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1080i

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1080i (pronounced "ten eighty eye") is shorthand name for a category of video modes. The number 1080 stands for 1080 lines of vertical resolution, while the letter i stands for interlaced or non-progressive scan. 1080i is considered to be an HDTV video mode. The term usually assumes a widescreen aspect ratio of 16:9, implying a horizontal resolution of 1920 pixels and a frame resolution of 1920 × 1080 or about 2.07 million pixels. The frame rate in hertz can be either implied by the context or specified after the letter i. The two frame rates in common use are 25 and 30 Hz, with the former (1080i25) generally being used in traditional PAL and SECAM countries (Europe, Australia, much of Asia, Africa), the later (1080i30) in traditional NTSC countries (e.g. United States, Canada and Japan). Both variants can be transported by both major digital television formats, ATSC and DVB.

Some people prefer to use the line number of fields, which is half that of frames, in their nomenclature and thus call this mode 540i, likewise 240i and 288i. Others, including the European Broadcasting Union (EBU), prefer to use the field rate separated with a solidus from the resolution as in 1080i/30 and 1080i/25, likewise 480i/30 and 576i/25.

1080i is directly compatible with CRT-based HDTV sets. CRT-based HDTV never entered the market in large volumes. 1080i is compatible with newer 720p- and 1080p-based televisions but must be deinterlaced first in order to be displayed on those sets.

Due to a revision of the NTSC format when color became available, the frame rate of actual 1080i broadcasts is usually 0.1% slower than is implied. For example, a 1080i30 or "30 Hz" transmission actually displays about 29.97 frames each second. Both the straight 24/30/60 and 23.976/29.97/59.94 frequencies are supported by current standards.

Progressive format 1080p50 or 60 is foreseen as the future broadcasting standard for production[1].

For a comparison between 1080i and 720p, see the 720p article.

1080i vs 1080p

To compare 1080i and 1080p it is important to compare framerates. Due to interlacing, 1080i has twice the frame-rate but half the resolution of a 1080p signal using the same bandwidth. This is especially useful in sports shows and other shows with fast-moving action. However, on some flat screens that don't support interlacing, this instead becomes smeared or jarred artifacts.

Current digital television broadcast systems and standards are not equipped for 1080p50/60 transmission. Also, the majority of consumer televisions offered for sale are currently not equipped to receive or decode a 1080p signal at any frequency. It is less bandwidth-intensive to broadcast a film at 1080p24 than 1080i30, since 20% less data would be transferred. In addition, when the source material is 24 frames per second, as are most films, it would be easy to convert a 1080p24 broadcast to an NTSC 1080i30 format using a 3:2 pulldown process (see telecine). Moreover, displaying a p24 broadcast on an i50 system (such as PAL) requires the speed of video and audio be increased by over 4% (to 25 frames per second).

For movies the frames (25 or 30 per second) are segmented into two interlaced fields with equal time index (psf, progressive in or with segmented frames). The deinterlacer has to perform a simple weave only. This ensures compatibility with 1080i25/30 with only little less coding efficiency than 1080p25/30 and half the bandwidth requirement of 1080p50/60, but the SDTV problems of PAL speed-up and Telecine judder remain.

See also

References

  1. ^ EBU Recommendation FUTURE HIGH DEFINITION TELEVISION SYSTEMS
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