What is the difference between 720p and 1080i?

720p and 1080i are both high definition television formats and both use the same data rate. The two different resolutions were adopted into the HD specifications and any HD television can display either of them.

1080i has 1080 lines in each picture which are refreshed 25 or 30 times each second. It displays the full image in two passes called fields. The first field displays all the odd numbered lines and the next displays all the even numbered lines. The field rate is of course twice the frame rate so there are 50 or 60 fields each second. The alternating field method is known as interlacing, hence the "i" in the format.

720p has, as the number suggests, 720 lines in the picture. The "p" stands for progressive scanning and the full picture is refreshed from top to bottom 50 or 60 times each second. There are no alternating fields with progressive scanning. Although the spatial resolution is lower, the picture is updated twice as fast. Therefore, the same amount of data is used for both formats. For those who are interested, HD raw data uses 1.483 Gigabits per second, compared to SD which uses 270 Megabits per second.

720p was included to allow broadcasters the option of using higher refresh rates for content such as sports. The fast movement typically seen in sports material is better captured with the faster refresh rate as the expense of a slightly lower resolution.

All HD televisions and recorders will handle either format without any user intervention. For viewers, it is a little academic as the television will simply display the format that the broadcaster delivers.

1080p is mentioned frequently. At present, broadcasters are not offering 1080p and it is not being used for live production. Because it is a progressive scan, it uses twice the data rate of 1080i and 720p. Current live production equipment rarely supports the format and transmission channels are not given the bandwidth too properly carry the format. It is currently limited to local sources such as games consoles, Bluray players and computer displays.