1080i is a 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 a high-definition television (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 frame/s (in countries that use 50 Hz alternating current) and 30 frame/s (in 60 Hz countries), with the former (1080i25) generally being used in traditional PAL and SECAM countries (Europe, Australia, much of Asia, Africa), and the latter (1080i30) being used in traditional NTSC countries (e.g. United States, Canada and Japan). Both variants can be transmitted by both major digital television formats, ATSC and DVB. The European Broadcasting Union (EBU), prefers to use the resolution and field rate separated by a slash, as in 1080i/30 and 1080i/25, likewise 480i/30 and 576i/25.
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At the least, HDTV has twice the linear resolution of standard-definition television (SDTV), thus showing greater detail than either analog television or regular DVD. The technical standards for broadcasting HDTV also handle the 16:9 aspect ratio images without using letterboxing or anamorphic stretching, thus increasing the effective image resolution.
The optimum format for a broadcast depends upon the type of videographic recording medium used and the image's characteristics. The field and frame rate should match the source and the resolution. A very high resolution source may require more bandwidth than available in order to be transmitted without loss of fidelity. The lossy compression that is used in all digital HDTV storage and transmission systems will distort the received picture, when compared to the uncompressed source.
1080i is directly compatible with 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. Of note, a very popular panel size used in mid-range HDTVs is 1366 x 768 and are often advertised as 1080i "compatible" - however these TVs, while accepting a 1080i signal will scale it down to the panel size of 1366x768 as they are physically incapable of displaying 1920x1080 resolutions. [1]
Due to 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. In European countries where PAL has been used, it is common to code 1080i in 25 frame/s which is equal to 50 fields/s.
The interlaced image format 1080i/25 requires about 20% more bit-rate than the progressive image format 720p/50 to obtain the same subjective image quality[2].
Progressive format 1080p (in 50 Hz or 60 Hz) is becomming common in production and is foreseen as the future broadcasting standard[3]. Today EBU recommends that 720p at 50Hz is used for distribution[4].
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