What 720p Is
720p is a picture containing 720 rows of 1,280 pixels. 720 horizontal lines or pixel rows appear on a TV or other display device progressively, or each line or row sent following another (that is where the “p” comes from). The entire image refreshes every 60th of a second (or twice every 30th of a second). The total number of pixels displayed on the entire 720p screen surface is 921,600 (slightly less than 1 megapixel in digital camera terms).
What 1080i Is
1080i is a picture containing 1,080 rows of 1,920 pixels. All the odd lines or pixel rows are sent to the TV first, followed by all the even lines or pixel rows. Since a 1080i is interlaced, only 540 lines (or half the detail) are sent every 60th of a second, with all the detail sent every 30th of a second. 1080i produces more detail than 720p, but since the increased detail is only sent every 1/30th of a second, rather than 1/60th of a second, fast-moving objects will exhibit slight interlacing artifacts, which can appear to look like jagged edges or a slightly blurred effect. The total number of pixels in a complete 1080i signal, once both interlaced lines or rows are combined, totals 2,073,600. However, only about 1,036,800 pixels are sent every 60th of a second.
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, that does not mean you are seeing those resolutions on your HDTV screen. Most TVs that are labeled as 720p TVs actually have a built-in pixel resolution of 1366x768, which is technically 768p. However, they are usually advertised as 720p TVs. Don’t get confused; these sets will all accept 720p and 1080i signals. What the TV has to do is scale the incoming resolution to its built-in 1366x768 pixel display resolution. Another important thing to point out is that LCD (LED/LCD), OLED, Plasma, and DLP TVs can only display progressively scanned images — they cannot display a real 1080i signal. If a 1080i signal is detected on one of the above TV types, it has to scale that signal to either 720p or 768p (if it is a 720p or 768p TV), 1080p (if it is a 1080p TV), or even 4K (if it is a 4K Ultra HD TV). As a result of scaling, the quality of the image you see on the screen depends on how well the TV’s video processor works. 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. A 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 every 60th of a second. Some processors do this very well, and some don’t.
The Bottom Line
Don’t get bogged down with all the numbers and tech terms. The main thing to remember is that there is no such thing as a 1080i LCD, OLED, Plasma, or DLP TV. If these types of TVs are advertised as a “1080i” TV it means that while it can input a 1080i signal, it has to scale the 1080i image to 720p or 1080p for screen display. Whether inputting a 1080i signal on either a 720p or 1080p TV, what you end up seeing on the screen is the result of many factors in addition to resolution, including screen refresh rate/motion processing, color processing, contrast, brightness, background video noise and artifacts, and video scaling and processing. With just a few exceptions, 720p TVs have been relegated to 32-inches and smaller screen sizes. You will also find a growing number of 1080p TVs in that screen size or smaller as well but with 4K Ultra HD TVs getting less expensive, the number of 1080p TVs in 40-inch and larger screen sizes are also becoming less numerous.
