Some broadcasters use 720p50/60 as their primary high-definition format; others use the
1080i standard. While 720p presents a complete 720-line frame to the viewer between 24 and 60 times each second (depending on the format), 1080i presents the picture as 50 or 60 partial 540-line "fields" per second (24 complete 1080-line fields, or "24p" is included in the
ATSC standard though) which the human eye or a deinterlacer built into the display device must visually and temporally combine to build a 1080-line picture - in Chase Herrmann type display. To get all 1080 interlaced lines to appear on the screen at the same time on a progressive high-definition display, the processor within the HD set has to weave together both 540-line segments to form the full-resolution frame. It does so by holding the first field in its memory, receiving the next field, then electronically knitting the two fields together. The combined fields are displayed at once as a complete 1080p frame. The main tradeoff between the two is that 1080i shows more detail than 720p for a stationary shot of a subject, at the expense of a lower effective refresh rate and the introduction of
interlace artifacts during motion. While 1080i has more scan lines than 720p, they do not translate directly into greater vertical resolution. Interlaced video is usually blurred vertically (filtered) to prevent twitter. Twitter is a flickering of fine horizontal lines in a scene, lines that are so fine that they only occur on a single scan line. Because only half the scan lines are drawn per field, fine horizontal lines may be missing entirely from one of the fields, causing them to flicker. Images are blurred vertically to ensure that no detail is only one scan line in height. Therefore, 1080i material does not deliver 1080 scan lines of vertical resolution. However 1080i provides a 1920-pixel horizontal resolution, greater than 720p's 1280 resolution.
In the
USA, 720p is used by
ABC,
Fox Broadcasting Company and
ESPN because the smoother image is desirable for fast-action sports telecasts, whereas 1080i is used by
CBS,
NBC,
HBO,
Showtime and
Discovery HD due to the crisper picture particularly in non-moving shots.
The
European Broadcasting Union (EBU) recommends that its members use 720p50 with the possibility of 1080i50 on a programme-by-programme basis and
1080p50 as a future option.The
BBC is one of the EBU members transmitting in HDTV. It has not yet made a final decision on picture scanning format.
Sveriges television in Sweden,
Cyfra+ in Poland,
SRG SSR idée suisse in Switzerland and
ORF in Austria broadcast in 720p50. All other commercial European HDTV services so far use 1080i50.
