From the Source

The most important factor in determining the quality of the entertainment experience that any home theater system can provide is the original source material. Today, everyone understands that DVDs offer better picture quality than VCR tapes. The reason is because DVDs are a higher resolution source material than videotapes—in other words, they have more audio-visual "information" on them and this increased data leads directly to better quality picture and sound. Standard definition television signals, whether they're received from a cable TV provider, satellite provider or via a regular antenna, have more resolution than VHS tapes, but less than a DVD. High-definition television signals, on the other hand, have even more resolution than DVDs, and this translates into even higher-quality signals. Forthcoming higher definition DVD formats (including both HD-DVD and Blu-Ray) have HD-like or even potentially higher quality signals.

It's important to be aware that even within the same "category" of quality there are differences. For example, while cable TV, digital cable and satellite TV all deliver standard definition television channels, digital cable and satellite offer higher-quality versions of those signals and, as a result, generally offer better image quality than regular cable. Even within each of these systems there are differences in quality between different channels, based on how much bandwidth, or signal resolution, each one is given. For example, you could show the exact same movie on a premium movie channel like HBO and, say, USA Network, and more than likely, the premium movie channel will look better on the same display than USA Network (or any other "non-premium" channel) because the satellite or cable company will give more "room" to the premium movie channel within the defined limits of its system. While visual quality difference won't be as dramatic as the difference between the VHS and DVD versions of a movie, you can definitely see it. Unfortunately, however, there's nothing you can do about an example like this—it's fixed by your cable or satellite provider.

Table 1 summarizes the relative quality of various videos signal types, showing them translated to computer monitor resolutions (for comparison's sake) as well as lines of resolution. The lines of resolution specification, which is often used to measure the quality of video signals, refers to how many vertical lines of the signal can be "resolved" or actually seen using test equipment. One interesting thing to note is that the high-definition television signals have resolutions that are similar to PC monitor resolutions. As a direct result, many (though not all) PC monitors can function as high-quality, high-definition TVs. Equally important, many high-definition TVs can function as PC monitors, or at least, provide a high-quality display for PC-type information such as web sites. In other words, in many ways, high definition televisions and monitors are becoming one and the same. This has important implications for the future as the worlds of computers and consumer electronics begin to converge.

VIDEO FORMAT

RESOLUTION (Pixels or Equivalent)

LINES OF RESOLUTION

VHS Tape

640 x 480 (Interlaced)

320

Standard Definition (SD) TV

640 x 480 (Interlaced)

525

DVD

800 x 480 (Interlaced or Progressive)

525

Enhanced Definition (ED) TV

640 x 480 (Progressive)

480

High Definition (HD) TV

1,280 x720 (Progressive) or 1,920 x 1,080 (Interlaced)

720 or 1,080

HD-DVD or Blu Ray

1,920 x 1,080 (progressive and interlaced), 1,280 x 720 1,080 or 720

To fully understand some of the specifications in Table 1, it helps to know a little bit about how TVs and monitors work. Until recently, one of the primary differences between televisions and monitors was that televisions worked with interlaced video signals, whereas monitors all worked with progressive video signals. The difference lies in how an image on a screen is created. With interlaced video, a single frame, or full-screen image, is actually made up of two "half" images that are combined together so quickly that our eyes see them as one image.

Specifically, with NTSC (National Television Standards Committee) TV signals—the kind used in the US, Japan, and several other regions around the world—there are 525 individual lines that go into an image and the first half, or "field" as it's technically referred to, of a frame consists of all the odd lines (line 1, 3, 5, 7, etc.) while the second field consists of all the even lines (2, 4, 6, 8, etc.). Standard definition televisions show the first field and then the second field and these two matching fields are interlaced to create a single image. Each field only remains visible for 1/60th of a second, but through persistence of vision, our eyes typically see a single frame that lasts 1/30th of a second. Progressive scan displays, on the other hand, draw all the lines in a frame sequentially (1, 2, 3, 4, 5, etc.) until the full frame is complete and then move onto the next frame. The end result is a higher-quality, smoother image.

As you can gather, progressive scan displays are "simpler" in many ways, but when televisions were first developed in the 1930s, the technology required to broadcast a signal in this manner was too complicated and too expensive. That's why all televisions—until the age of high-definition—used interlaced scanning to present a signal. PC monitors, on the other hand, have always used progressive scanning—and that's part of the reason why they've always been more expensive than televisions at similar sizes (such as 20" monitors vs. 20" TVs).

Progressive scan DVD players use similar principles to take the otherwise interlaced DVD signal off a DVD disc and turn it into a progressive scan signal before it reaches the DVD player's outputs. (For more information than you'll probably ever want to know about the DVD format, see the very thorough DVD FAQ.)


 

 
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