By Michael P. Woods, etown.com
Defining DTV So what exactly is DTV? Why is it better and how is it different from what we're watching right now? To answer those questions, we must first understand the basic difference between an analog television signal (which is what our current NTSC signal is) and a digital television signal (DTV). An analog signal contains only the actual video signal, nothing more. No codes, no bits, just plain video. It's the traditional signal type that is transmitted through the air. All things considered, it has served us very well for many years, but it has some significant disadvantages. In the course of being transmitted, an analog signal is subject to a wide variety of interference and distortion. Atmospheric conditions, terrain, obstacles (especially buildings, hills and even airplanes flying overhead) can distort the signal and thus distort the picture on your television. Perhaps more important is the strength of the signal, which makes for an enormous difference in picture quality. Stronger signals provide reasonably good picture quality (assuming the terrain allows it), but weaker signals appear "snowy" and distorted. On the other hand, a digital signal (which is used for DTV) is able to produce a good picture regardless of the atmospheric conditions, signal level or terrain. This is because the digital signal is actually a series of codes and instructions that are deciphered by the DTV receiver (the set-top box you need to display the DTV signal). Regardless of the strength of the DTV signal, the codes are still the codes -- and the receiver doesn't care if they are strong or weak (as long as they are strong enough to make it there). Therefore, the digital signal always remains clean and sharp. Basically, you'll never see a "snowy" DTV image -- you'll see one or you won't. Of course, nothing is perfect, and DTV is no exception. Tuning your antenna is just as important as it ever was. It is possible that a DTV signal can be so weak that the receiver can't read the codes. This appears on the television as the "cliff effect" -- the image will either be there or it won't be. This can sometimes be a real pain, especially if you don't have a signal strength meter to see how strong the signal is. At least with an analog signal, you can see the image getting better or worse as you turn the antenna. Getting the picture Now that we've covered the basic difference between the two types of signals, let's "tune in" on the specifics of DTV. We've just established why a digital signal is better that an analog signal, but what makes DTV a "high-definition" format? For starters, let's examine how the current NTSC image is created on your TV at home. A television image is created with a series of horizontal lines that are "drawn" by an electron gun that "shoots" a beam of energy onto the phosphor on the inside of the picture tube's face. When the beam hits the phosphor, the phosphor glows and a television image is created. In the current NTSC system, the total number of lines drawn by the electron gun is 480. These images are drawn 60 times a second, though not all of the 480 lines are drawn at this rate. The NTSC system is interlaced, meaning that the full 480-line frame is drawn in two separate fields consisting of 240 lines each. All of the odd-numbered lines are drawn on the first pass (the first 1/60th of a second). The even-numbered lines are drawn on the next pass (the next 1/60th of a second), completing the picture. This system can be referred to as "480i," where "480" is the total number of lines that make up the picture and "i" indicates that the lines are interlaced. There are a number of DTV formats that use the interlaced system. Many DTV formats use the progressive system. Progressive is much easier to understand, as all of the lines of resolution are displayed 60 times a second, not half and half. For example, the DTV format called "480p" ultimately gives you the same number of horizontal lines as 480i, but it does so by scanning all 480 lines every 1/60th of a second, rather than 240 (odd) lines and then 240 (even) lines. Thus, you get a full frame of video twice as often with 480p. As a result, motion in the image appears to be much smoother and better defined. As an aside, your computer monitor is a progressive-scan display device. Having mastered this, understanding DTV will be much easier. The ATSC (Advanced Television Systems Committee) has assigned 18 different formats for DTV. The most commonly used (for the time being, at least) are 1080i, 480p and 720p. Various networks have adopted certain DTV formats for their transmission of high-definition television (HDTV) programming. CBS, for instance, is transmitting video in 1080i, whereas ABC has elected to go with 720p. Which DTV format is best? So which DTV format is better? It really boils down to opinion. My personal favorite is 720p, because I like the fact that I get a full 720-line frame 60 times a second. My love for 720p started when Joe Kane gave me a demonstration of 720p video on a top-of-the-line projection television. The video was of a professional horse race. When the camera panned as the horses got near and then crossed the finish line, the resolution (720 lines) combined with the fact that I was getting a full new frame of video 60 times a second, made it seem as if I were looking through a window. Interlaced video leaves me with the feeling that I'm watching television (high-definition television, sure, but television nonetheless), as opposed to the more film-like experience I get from 720p. Another important thing to understand about DTV is the aspect ratio, or "shape" of the screen. All of our old televisions at home are 4:3 aspect ratio sets, meaning four units wide and three units high. DTV will be in a "widescreen" format known as 16:9 (16 units wide and nine units tall). The 16:9 aspect ratio is beneficial for various reasons, the most important of which is that more information can be provided on-screen in this format than on "square" (4:3) screens. Many cities are broadcasting DTV today, although most are broadcasting "upconverted" NTSC video. It's still in a DTV format, but the fact that the video was not recorded in 1080i, 480p or 720p (or in any of the other HD formats) means that the full potential of DTV has yet to be realized. These cities are, of course, transmitting terrestrial signals (that is, from an antenna somewhere in or near the city). Many other cities have yet to begin broadcasting DTV, and it may be a while before they do. Don't worry, though. Satellites can also broadcast DTV, filling the void for those cities that may not get terrestrial or cable DTV broadcasts in the near future. Later this year, some of the top satellite companies will start transmitting DTV material. You'll need a DTV-capable television and a new digital satellite system (as well as a digital tuner to decode the signal) to reap the benefits of DTV.
Copyright 2000, etown.com (taken from bestbuy.com)