720p and 1080i are both high definition video formats, but that is where the similarity ends. There are significant differences between the two that may affect the TV you buy and your TV viewing experience.
720p is 1,280 pixels across a screen horizontally and 720 pixels down the screen vertically. This arrangement yields 720 horizontal lines or pixel rows that are sent to a TV or other display device progressively, or each line sent following another (that is where the "p" comes from). In other words, the entire image (720 lines or pixel rows) is sent every 60th of a second (or twice every 30th of a second).
1080i represents 1,920 pixels arranged horizontally and 1,080 pixels arranged vertically. This yields 1,080 horizontal lines (pixel rows), sent to a TV alternately (referred to as an interlaced signal). In other words, all the odd lines or pixel rows are sent to the TV, followed by all the even lines or pixel rows.
This means that 1080i, since it is interlaced, only sends 540 lines (or half the detail) every 60th of a second, with all the detail sent every 30th of a second. On the surface, 1080i produces more detail than 720p, but since the increased detail is only sent every 1/30th of a second, rather than 1/60 of a second, fast moving objects, will exhibit slight interlacing artifacts - which can appear to look like jagged edges or a very slight blurred effect.
720p, 1080i, and Your TV
HDTV broadcasts from your local TV station, cable, or satellite service are either 1080i (such as CBS, NBC, WB) or 720p (such as FOX, ABC, ESPN).
However, although 720p and 1080i are the two main standards for broadcasting HDTV signals, that does not mean you are seeing those resolutions on your HDTV screen.
It is important to note that 1080p (1920 x 1080 lines or pixel rows progressive scanned) is not used in television broadcasting, but is used by some cable/satellite providers, internet content streaming services, and, of course, 1080p is a part of the Blu-ray Disc format standard.
Also, it must be pointed out that most TVs that are labeled as 720p TVs actually have a native pixel resolution of 1366x768, which is technically 768p. However, they are usually advertised as 720p TVs. Don't let this through you off, these sets will all accept 720p and 1080i signals. What the TV has to do is process (scale) any incoming resolution to its native 1366x768 pixel display resolution.
In addition, since LCD, Plasma, and DLP TVs can only display progressively scanned images, they cannot display a native 1080i signal. If a 1080i signal is detected the TV has to scale the 1080i image to either 720p or 768p (if it is a 720p or 768p TV) or 1080p (if it is a 1080p TV). As a result, the quality of the image you see on the screen depends on the how well the TV's video processor works - some TVs do better than others. If the TV's processor does a good job, the image will display smooth edges and have no noticeable artifacts for both 720p and 1080i input sources.
However, the most telltale sign that a processor is not doing a good job is to look for any jagged edges on objects in the image. This will be more noticeable on incoming 1080i signals as the TVs processor only has to scale the resolution up to 1080p or down to 720p (or 768p), but also has to perform a task called "deinterlacing". Deinterlacing requires that the TV's processor combine the odd and even lines or pixel rows of the incoming interlaced 1080i image into a single progressive image to be displayed at least every 60th of a second. Some processors do this very well, and some don't.
What all these numbers and processes mean to the consumer is that there is really no such thing as a 1080i LCD, Plasma, or DLP TV. If an LCD, Plasma, LCD TV is advertised as being a "1080i" TV, it really means that while it can input a 1080i signal - it has to scale the 1080i image to 720p for screen display. 1080p TVs, on the other hand, are simply advertised as 1080p TVs and any incoming 720p or 1080i signals are scaled to 1080p for screen display.
In the final analysis, when inputting a 1080i signal on either 720p or 1080p TV, what you see on the screen is the result of many factors, including resolution, motion response, color processing, contrast, brightness, and background video, and video scaling and processing.