1080i
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1080i is the 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, and a field resolution of 1920 × 1080 / 2 (because it's interlaced) or about 1.04 million pixels. The field rate (not the frame rate) in hertz can be either implied by the context or specified after the letter i. The two field rates in common use are 50 and 60 Hz, with the former (1080i50) generally being used in traditional PAL and SECAM countries (Europe, Australia, much of Asia, Africa, and South America), the latter (1080i60) in traditional NTSC countries (North America, 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 frame rate instead of the field rate and separate it with a solidus from the resolution as in 1080i/30 and 1080i/25, likewise 480i/30 and 576i/25.
1080i is directly compatible with numerous CRT-based HDTV sets. 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 curiosity of the NTSC format, the field rate of actual 1080i broadcasts is usually 0.1% slower than is implied. For example, a 1080i60 or "60 Hz" transmission actually displays about 59.94 fields each second. Both the straight 24/30/60 and 23.976/29.97/59.94 frequencies are supported by current standards.
For a comparison between 1080i and 720p, see the 720p article.
1080i vs 1080p
The progressive-scan versions of the 1080-line resolution is known as 1080p. Current digital television broadcast systems and standards are not equipped for 1080p50/60 transmission. It is less bandwidth-intensive to broadcast a movie at 1080p24 than 1080i60, 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 1080i60 format using a 3:2 pulldown process (see telecine). On the other hand, most consumer televisions in the world are currently not equipped to receive or decode a 1080p signal at any frequency. 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).
Therefore a hybrid is often used for movies and the like: 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 1080i50/60 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.
It would be inefficient to transmit any signal with a frame rate significantly higher than its source’s, which is 24 Hz for movies.
External links
| Designation | Usage examples | Definition (lines) | Rate (Hz) | |
|---|---|---|---|---|
| Interlaced (fields) | Progressive (frames) | |||
| MP@LL | LDTV, VCD | SIF) | 24, 30; 25 | |
| Standard; MP@ML | SDTV, SVCD, DVD, DV | 480 (NTSC, PAL-M); | 60; | 24, 30; |
| 576 (PAL, SECAM) | 50 | 25 | ||
| Enhanced | EDTV | 480; 576 | 60; 50 | |
| High; MP@HL | HDTV, HD DVD, BD, HDV | 720 | 24, 25, 30, 50, 60 | |
| 1080 | 50, 60 | 24, 25, 30 | ||
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| This table illustrates total horizontal and vertical detail via box size. It does not accurately reflect the screen shape (aspect ratio) of these formats, which is always stretched or squeezed to 4:3 or 16:9. The table assumes an average vertical detail loss of .75x due to interlace. The actual loss is variable due to content, motion, opinion on acceptable levels of flicker, and possible success of deinterlacing. 1920 × 1080i is not included because all common use of 1080i is filtered to 1440 or less. | ||||
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