What is the Difference Between EDTV and HDTV?

Shopping for a new television can be a little complicated today. This article will attempt to clear up at least part of the picture by explaining the difference between enhanced definition TV (EDTV) and high-definition TV (HDTV). In the process we'll also define standard definition TV (SDTV).

A certain amount of technical information is necessary. To start, a television displays a moving picture by running a set of still pictures in quick succession to create the illusion of movement. These still frames flash by at 30 frames per second. But to display a single frame, the TV must "paint" the screen or picture, line by line.

A standard television screen is comprised of about 480 scan lines. The TV receives data for the even numbered lines first, then must make a second pass to fill in the odd numbered lines to complete each frame. Both passes together take place in the span of 1/30th of a second. Because each pass paints every other line, this type of display is referred to as interlaced, or 480i. It is also called 525i because there are 45 additional "blank lines" built into every frame transmission to give the display time to reset itself. Nevertheless, the proper designation is 480i, or standard definition TV (SDTV).

This technology was fine for smaller screens, but as displays became larger the 480 scan lines became visible, as did artifacts created by the interlacing technique. Objects in motion that created offset or jagged lines between the first and second passes became visible on large screens, resulting in poor picture quality. Simply put, large screen TVs only magnified SDTV's shortcomings. Enter progressive scanning.

A progressive scan TV paints the entire frame in one sequential pass, eliminating much of the distortion that two-pass interlacing creates. Although the TV still receives the video frame in two parts, it combines the data before painting it by using an internal processor called a deinterlacer, also known as a line doubler. Progressive scan displays are specified as 480p or 525p, more commonly called enhanced definition TV (EDTV).

EDTV represents a significant step forward in display technology, creating a much finer picture than interlaced TV. The deinterlacer not only assembles the frame but also cleans up any artifacts in the process. Additionally, EDTV can paint a complete frame in 1/60th of a second, which allows it to paint the same frame twice. This creates a cleaner, more stable picture. EDTV also features a 16:9 aspect ratio, which means the display is rectangular or theater-shaped. Finally, EDTV can render high-definition broadcasting, or HDTV, while SDTV cannot.

You might be asking, "If EDTV can render high-definition broadcasting, why get a HDTV?" The short answer is that HDTV can display HD broadcasts in better resolution than EDTV. Though a quality EDTV looks closer to a HDTV than it does to standard television, all else being equal, the HDTV will outshine the EDTV when it comes to high-definition broadcasts by a margin that some experts put at about 20 percent. How does HDTV do this? By adding more scan lines for finer resolution and more clarity.

While EDTV has 480 scan lines, HDTV supports two high-definition broadcasting formats: 720p and 1080i. 720p is a progressive scan with 720 scan lines, while 1080i is interlaced with 1080 scan lines. Generally speaking, the more scan lines, the better the resolution, particularly when considering large screens.

It might sound counter-intuitive that HD broadcasting would employ an interlaced signal, but with so many scan lines any artifacts created are too small to be noticeable and the higher resolution is breathtaking. Even so, 720p is preferred for fast motion, as the progressive scan technique is able to repaint the picture faster, making motion smoother. Hence, football games and other sports are broadcast in 720p, while "normal" HD broadcasting is in 1080i. HDTVs accept either format and do a good job of displaying them both, and many people can't tell the difference between the two. They are significantly clearer than the 480 scan lines of EDTV.

Hence, although high-definition broadcasts look great on EDTV, the signal is down-converted to 480 lines in order to display it, and in the process some of the extra clarity that a HDTV preserves is lost. This is the real reason to buy HDTV -- for that "extra 20 percent clarity" when watching HDTV programming. Some HDTVs (1080p models) upconvert 1080i broadcasts by deinterlacing them, displaying them as progressive scan pictures.

Not all HDTVs are superior to EDTVs when watching 480i standard broadcasting, VHS tapes, or watching a DVD on a non-progressive scan DVD player. A quality EDTV with a good deinterlacer might process these interlaced signals better than a low-end HDTV equipped with a poor deinterlacer. As time marches on, however, it makes sense to buy an HDTV.