How to Avoid the HDTV Resolution Trap

by Kenny Hemphill

There's a great deal of nonsense talked about HDTV and much of the noise comes from manufacturers' and retailers' advertising which in some cases is extremely misleading. One of the areas of greatest confusion is resolution. Resolution is defined as the number of pixels - the elements which make up a digital image - in an image and is usually quoted as a x b, where a is the number of pixels across the screen (horizontal) and b is the number of pixels up and down (vertical, also referred to as the number of lines). In this article we'll explain everything you need to know about the number of pixels required for HDTV. HDTV Specification The HDTV specification provides for three different types of HDTV, denoted by the number of lines of resolution and the signal type. They are: 1080p (1080 lines, progressive scan) 1080i (1080 lines, interlaced) 720p (720 pixels, progressive) Any TV not capable of displaying at least 720 lines is not 'HDTV ready' and any signal with fewer than 720 lines is not an HDTV signal. Interlaced or Progressive? The 'i' and 'p' in the HDTV specification refer to interlaced and progressive scan. The analog TV we've been watching for years is interlaced. That simply means that each frame of video (of which there are 30 per second in NTSC and 24 per second in PAL) is split into two fields. Each field contains alternate lines of the signal (one has the odd, the other the even) and is displayed for 1/60th of a second (1/48th for PAL). Because it happens so fast, your brain interprets each frame as a single image. Progressive scan signals have no fields, they simply display each frame in its entirety for 1/30th (or 1/24th) of a second. This results in a higher quality image, particularly noticeable in fast-action broadcasts such as sporting events. So every TV that can display 720 lines is HDTV ready? No, and this is one of the key areas of confusion. The other important element of the HDTV specification is that signals must be broadcast in 16:9 widescreen. This is known as the aspect ratio, where '16' refers to the width of the image and '9' to the height. In other words, the height of the picture is 9/16ths of the width. Some manufacturers promote TVs with a maximum resolution of 1024 x 768 pixels as HDTV ready. This is incorrect. Although the total number of vertical pixels is greater than the 720 minimum necessary for HDTV, at an aspect ratio of 16:9 it would need 1280 pixels horizontally to display a 720-line HDTV picture. The only solution is to either squash or crop the picture, neither of which is satisfactory. So, in order to be properly HDTV capable, a TV needs to be able to display at least 1280 pixels horizontally as well as at least 720 vertically. About the author: Kenny Hemphill is the editor and publisher of The HDTV Tuner. Visit him at http://www.the-hdtv-tuner.com and discover why HDTV is the most exciting thing to happen to television since the advent of color TV. Better still, keep up to date with the latest developments in HDTV and high definition DVD by subscribing to the news feed. Click the link for details.