1080i is the shorthand name for a high-definition television (high-definition video) mode. The i means interlaced video; 1080i differs from 1080p, in which the p stands for progressive scan. The term 1080i assumes a widescreen aspect ratio of 16:9, implying a frame size of 1920×1080 pixels. Each frame of 1080i video consists of two fields of 1920×540 pixels each.
The field rate of 1080i is typically 60 Hz for countries that use or used System M as analog broadcast television system (such as United States, Canada, and Japan) or 50 Hz for regions that traditionally used television systems with 25 frames/s rate (such as in Europe, Australia, much of Asia, and Africa). Both variants can be carried by both major digital television transmission formats: ATSC and DVB.
The frame rate can be implied by the context, while the field rate is generally specified after the letter i, such as "1080i60". In this case 1080i60 refers to 60 fields per second or 30 frames per second. The European Broadcasting Union (EBU) prefers to use the resolution and frame rate (not field rate) separated by a slash, as in 1080i/30 and 1080i/25, likewise 480i/30 and 576i/25.[1] Resolutions of 1080i60 or 1080i50 often refers to 1080i/30 or 1080i/25 in EBU notation.
1080i is directly compatible with some CRT-based HDTV television sets on which it can be displayed natively in interlaced form, but for display on modern progressive-scan LCD and plasma TV sets it must be deinterlaced and often scaled. Depending on the television's video processing capabilities, the resulting video quality may vary, but may not necessarily suffer. For example, film material at 25fps may be deinterlaced from 1080i50 to restore a full 1080p resolution at the original frame rate without any loss.
Most UK HD channels on satellite, cable, and terrestrial platforms including BBC One HD and ITV1 HD broadcast in this format, while BBC HD broadcasts mostly in 1080p. In the United States, 1080i is the preferred format for CBS, NBC, and The CW, though some affiliates (especially those that broadcast two digital subchannels in HD) do broadcast in 720p. Fox and ABC/ESPN broadcast in 720p; many ABC affiliates owned by Hearst Television and Belo Corporation air their signals in 1080i and upscale ABC programming for master control and transmission purposes, as most syndicated programming is produced in 1080i and must be additionally downscaled to 720p.
Some cameras and broadcast systems that use 1080 vertical lines per frame do not actually use the full 1920 width of a nominal 1080i picture for image capture and encoding. Common subsampling ratios include 3/4 (resulting in 1440×1080i frame resolution) and 1/2 (resulting in 960×1080i frame resolution). Using half horizontal resolution and only one field of each frame (possibly with added anti-alias filtering or progressive capture) results in the format known as 540p, which has frame resolution 960×540 and 30 or 25 frames per second.
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