1080i is the shorthand name for a category of video modes. The i stands for interlaced, the 1080 for a vertical resolution of 1080 lines, usually with a horizontal resolution of 1920 pixels and an aspect ratio of 16:9; that is high-definition television. If not implied by context, the field rate (not the frame rate) in hertz is given after the letter. The two field rates in common use are 50 and 60 Hz. 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 former NTSC countries (North America, Japan). Both variants can be transported by both major digital television formats, ATSC and DVB.
There are progressive-scan versions of the 1080-line resolution, but due to bandwidth limitations of broadcast frequencies, it is only practical to use them at 24, 25, and 30 frames per second (1080p24, 1080p25, 1080p30). Progressively-scanned material at the higher framerates of 50 and 60 hertz can only be sent over higher-bandwidth channels, and is not part of the broadcast standards, but is defined for internal use.
It is relatively easy to convert between 1080i60 and 1080p24 with a telecine system when the source material is 24 frames per second, such as in most films. In fact, it is more sensible to broadcast a movie at 1080p24 than 1080i60, since about 20% less data needs to be compressed to carry the images. Displaying a p24 broadcast on a 50 Hz system would be problematic, thus traditionally temporal compression is used, i.e. the movie is sped up to 25 frames per second.
Due to a curiosity of the NTSC format, the field rate of actual broadcasts is usually 0.1% slower than the numbers listed here. For example, a "60 Hz" transmission actually puts out 59.94 fields each second. Both the straight 24/30/60 and 23.976/29.97/59.94 frequencies are supported by current standards.
