Differences Between Standard Definition, Enhanced Definition and High Definition TV

What we call high definition TV is just one type of digital television. All HDTV is digital, but not all digital TV is high definition. In fact, there are three primary types of digital television: standard definition (SDTV), enhanced definition (EDTV) and high definition (HDTV). And within HDTV, there are several variations, each with a different picture resolution. It's all a bit confusing, so settle back for a quick explanation.

Analog vs. Digital

Until very recently, all television broadcasts were analog. With analog transmission, programming is transmitted in a continuous signal that varies in amplitude, depending on the information contained in the picture. This signal can easily deteriorate over distance or suffer interference from other sources, which produces a lower-quality picture than the original. The primary advantage of digital transmission is that it slices the traditional analog television signal into a series of digital bits, which are then recombined to reproduce an exact copy of the original broadcast. These digital signals don't weaken with distance, as analog signals do. As long as the signal can be received, the picture is perfect, with no degradation. Since digital signals are comprised of binary bits, a 1 is always a 1 and a 0 is always a 0. Because of this exact end-to-end reproduction, digital means better picture and sound quality, no matter what is broadcast.

Different Types of Digital TV

Digital programming comes in many shapes and sizes. Not all digital broadcasts have the same resolution or aspect ratio. Here's a quick guide to all current digital formats:

• SDTV - Standard definition digital television has the same resolution and 4:3 aspect ratio as traditional analog television but is transmitted in digital fashion. The SDTV picture is 480 x 640 pixels, with interlaced scanning.

• EDTV - Enhanced definition television has the same resolution as SDTV but with progressive scanning instead of interlaced scanning, which creates a smoother overall picture. EDTV can be in either the 4:3 or 16:9 aspect ratio, for a resolution of either 480 x 640 or 480 x 720 pixels.

• HDTV (720p) - The first of the three current high definition television formats features 720 x 1280 pixel resolution with progressive scanning. This format is ideal for programming with lots of movement, such as sporting events. The aspect ratio is 16:9.

• HDTV (1080i). The second current high definition format features greater resolution than 720p (1080 x 1920 pixels), but with interlaced scanning. Like 720p, the aspect ratio is 16:9

• HDTV (1080p) - This is the ultimate high definition format, with 1080 x 1920 pixel resolution (in the 16:9 ratio) and progressive scanning. Because of the high bandwidth requirements, this format is not yet used for television broadcasts, although the new HD DVD and Blu-ray high-definition DVD formats will be capable of a 1080p picture.

Comparing the Formats

So which formats are used where? The following table provides a brief rundown of all the digital formats, as well as our current NTSC analog format:

Which Format Should You Buy?

Not all widescreen displays are high definition. Some low-cost plasma and LCD TVs offer an enhanced definition 480p picture -- not HDTV. While EDTV offers a better picture than what you get with an SDTV set, it's not as sharp as true high definition. EDTV sets aside, most flat panel, rear projection and front projection TVs today are compatible with at least some forms of HDTV. Most sets can reproduce both 720p and 1080i programming, and some high-end sets have a true 1080p display. Any of these sets are what you want if you're interested in the best quality picture today.

Remember, the greater the number of scan lines (from 480 to 720 to 1080), the more pixels are onscreen, which results in a sharper picture. And when comparing similar resolution, progressive scan delivers a smoother picture than interlaced scan. So look for the set in your price range that delivers the highest resolution -- and that looks best to your eyes.

Michael Miller is a writer and commentator on technology and digital lifestyle topics.