1080i is an abbreviation referring to a combination of frame resolution and scan type, in the
domains of high-definition television and high-definition video. The term assumes a
widescreen aspect ratio of 16:9, a spatial resolution of 1920 pixels × 1080 lines, and
a temporal resolution of 50 or 60 interlaced fields per second. This format is used in
the SMPTE 292M standard. The choice of 1080 lines originates with Charles
Poynton, who in the early 1990s pushed for "square pixels" to be used in HD video formats.
Broadcast standard
Within the designation "1080i", the i stands for interlaced scan. A frame of 1080i video
consists of two sequential fields of 1920 horizontal and 1080 vertical pixels. The first
field consists of all odd-numbered TV lines and the second all even numbered lines. Consequently
the horizontal lines of pixels in each field are captured and displayed with a one-line
vertical gap between them, so the lines of the next field can be interlaced between them,
resulting in 1080 total lines. 1080i differs from 1080p, where the p stands for progressive
scan, where all lines in a frame are captured at the same time. In native or pure 1080i,
the two fields of a frame correspond to different instants, so motion portrayal is good. This
is true for interlaced video in general and can be easily observed in still images taken
from fast motion scenes, as shown in the figure on the right. However when 1080p material
is captured at 25 or 30 frames/second it is converted to 1080i at 50 or 60 fields/second,
respectively, for processing or broadcasting. In this situation both fields in a frame do
correspond to the same instant. The field-to-instant relation is somewhat more complex for the
case of 1080p at 24 frames/second converted to 1080i at 60 fields/second, as explained
in the telecine article. The field rate of 1080i is typically 60 Hz
for countries that use or used System M as analog broadcast television system, or 50 Hz
for regions that traditionally used television systems with 50 fields/sec rate. Both field
rates can be carried by major digital television broadcast formats such as ATSC, DVB, and ISDB-T
International. 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. The European Broadcasting Union prefers
to use the resolution and frame rate separated by a slash, as in 1080i/30 and 1080i/25, likewise
480i/30 and 576i/25. Resolutions of 1080i60 or 1080i50 often refers to 1080i/30 or 1080i/25
in EBU notation. 1080i is directly compatible with some CRT
HDTVs on which it can be displayed natively in interlaced form, but for display on progressive-scan—e.g.,
most new LCD and plasma TVs, it must be deinterlaced. 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. Preferably video material
with 50 or 60 motion phases/second is to be converted to 50p or 60p before display.
Worldwide, most HD channels on satellite and cable broadcast in 1080i. In the United States,
1080i is the preferred format for most broadcasters, with Discovery Communications, Viacom, Time
Warner, Comcast owned networks broadcasting in the format; along with most smaller broadcasters.
Only 21st Century Fox-owned television networks and Disney-owned television networks, along
with MLB Network and a few other cable networks use 720p as the preferred format for their
networks; A+E Networks converted from 720p to 1080i sometime in 2013 due to acquired
networks already transmitting in the 1080i format. Many ABC affiliates owned by Hearst
Television and former Belo Corporation stations owned by Gannett Company, along with some
individual affiliates of those three networks, air their signals in 1080i and upscale network
programming for master control and transmission purposes, as most syndicated programming and
advertising is produced and distributed in 1080i and must be additionally downscaled
to 720p. This also allows local newscasts on these ABC affiliates to be produced in
the higher resolution to better compete with local newscasts from the 1080i networks.
Some cameras and broadcast systems that use 1080 vertical lines per frame do not actually
use the full 1920 pixels of a nominal 1080i picture for image capture and encoding. Common
subsampling ratios include 3/4 and 1/2. Where used, the lower horizontal resolution is scaled
to capture and/or display a full-sized picture. Using half horizontal resolution and only
one field of each frame results in the format known as qHD, which has frame resolution 960x540
and 30 or 25 frames per second. Due to the chosen 16x16 pixel size for a compressed
video packet known as a macroblock as used in ITU H.261 to H.264 video standards, a 1080
line video must be encoded as 1088 lines and cropped to 1080 by the de-compressor. The
720 line video format divides perfectly by 16 and therefore doesn't need the eight wasted
lines. See also
List of common resolutions 4320p, 2160p, 1080p, 720p, 576p, 576i, 480p,
480i, 360p, 240p Interlaced video
Field Sony HDVS Wideband analog video
References
External links High Definition Image Formats for Television
Production 720p vs. 1080p HDTV: The final word - CNET
Review - December 5, 2007