HiDef Image Formats

The term High Definition or HD, has become a standard in our lives, and I would like to explain the different types of HD formats that are out there.
In High Definition, there are basically 3 types of formats: 720p, 1080i, and 1080p. These formats are described as follows:

720p - 1280x720 pixels @ 50/60Hz Progressive
1080i - 1920x1080 pixels @ 50/60Hz Interlaced
1080p - 1920x1080 pixels @ 24/25/50/60Hz Progressive

That's alot of numbers to read, I know. But let me explain. The differences between these formats are the resolutions- measured in pixels, and their scanning mode- progressive or interlaced. Interlaced and progressive scan are the different ways we capture, send, or display the video content. If you look at the diagram below, you can see the differences in the formats.

Interlaced Scanning
In order for us to understand the different formats, we need to understand the history of it all. Interlaced scanning was created way back in the early stages of television to provide a good picture quality ( ie..many picture lines) without using too much bandwidth (ie..broadcasting capacity). An interlaced video frame is made from two scans, the first displays every other picture line, and the second fills in the gaps left by the first. When displayed on a cathode ray tube or CRT, the system relies on the afterglow of the phosphor inside the tube and human's persistence of vision to hide the fact that only half the image is being updated at a time. By using this method, a sufficiently high scan rate can be used to avoid an image flickering, while using half the bandwidth a sequentially or (progressively) scanned image at the same scan rate would have needed.

The Progressive Scan system doesn't split the video frame in to two separate fields as interlacing does. Instead, the image in drawn sequentially from top to bottom, in order, in a single scan.
This system is ideal for use with modern displays like LCDs and plasmas which are, of course, progressive devices, drawing their pictures by updating sequential lines from top to bottom of the screen.

HDTV Formats
HDTV broadcast formats come as 720p and 1080i. Interlacing can cause visible flickering or jagged images when displaying motion, resulting in a reduction in perceived vertical resolution as well as image quality. For fast moving content such as a sport, a progressively scanned image is better, so it is likely most sports content will be broadcast in 720p, which provides 50/60 frames a second (Hz) of 1280x720 pixel resolution.
Less fast-moving content can be safely interlaced, and the bandwidth can be used to deliver a higher resolution - 1080 line image in the case of 1080i, making it suitable for other types of programming. The 1080i format delivers 50/60 higher resolution (1920x1080px) fields which combine to produce half the frame rate at 25 or 30 frames per second. Since display devices like LCDs and plasmas are inherently progressive devices, this makes them natively unsuitable for displaying 1080i or any other interlaced content.

The third of the mainstream high-definition formats, is 1080p, also known as 'full-HD'. This format, as its name suggests, has the same 1080 line resolution (1920x1080px) of the 1080i format but is delivered progressively, hence 1080p.
1080p content is availablein other sources, including the next-generation DVD formats; HD-DVD and Blu-ray, and also from next-generation games consoles such as the Xbox and Playstation 3. The quality of 1080p over the other high-definition formats will be much smaller than the upgrade from standard to high-definition, but it's advertised as much more, and in my opinion, the main reason for the major hype over HDTV.