Inside Digital TV

To create a digital broadcast, a television program's analog video and audio signals (as initially captured by a camera pickup or microphone) are first converted to a binary representation of the electrical equivalent of ones and zeros). That digital bitstream is then transmitted on an RF (radio frequency) carrier to your home, where the RF component is stripped away, and the bitstream is decoded and reconstructed into analog video and sound. As part of this process, digital TV relies on two now-familiar technologies--MPEG-2 video compression and Dolby Digital audio compression--to squeeze multiple high-definition or standard-definition programs, as well as ancillary data, into a conventional 6-MHz-wide broadcast TV channel. DTV Video Besides its use in DTV, MPEG-2 compression (as defined by the Motion Pictures Experts Group) is used to conserve space on DVDs and to allow digital-satellite providers to send high-resolution video through space. During the MPEG-encoding process, a computer program selectively throws out redundant data that appears from one frame to the next--information about stationary objects or backgrounds is discarded, and only the differences between frames caused by moving objects are recorded. A "variable bit rate" scheme also allows less data to be used for scenes that contain little movement or detail. This variable bit rate partially explains why the capacity of a 6-MHz digital channel isn't fixed. It changes with, among other things, the type of program material being broadcast and the desired resolution at the receiving end. For example, sports events and high-action material require a faster bit rate that reduces capacity and restricts the broadcaster's options to a single high-definition program or two to three standard-definition programs. Slow-action, or film-based material, on the other hand, can use a slower bit rate to allow up to two high-definition programs or as many as six standard definition programs to be sent on the same channel. Capacity is also affected by the video format used. There are a total of 18 interchangeable video formats attached to the DTV standard that vary in scanning method ("interlaced" or "progressive"), the density of picture elements (pixels), and the frame rate (the number of times per second an entire screen's worth of Information is painted by the raster). Six of these are HD formats based on two different pixel arrays: 1,080 horizontal scan lines by 1,920 pixels per line, and 720 lines by 1,280 pixels. The 1,080 by 1,920 signal can be sent at 24 or 30 frames per second (progressive) or 60 frames per second (interlaced). The 720 by 1,280 signal can be sent at frame rates of 24, 30, or 60, all in the progressive-scan method. What a broadcaster uses will depend, again, on the program material. Programs that originate in video--such as live news or sports broadcasts--are easier to send using an interlaced-scanning format, such as the oft-mentioned 1080I format, in which two slightly offset sets of raster lines are painted on the screen In rapid succession to create a full frame of video. Film-based programming, such as movies and 70 to 80 percent of primetime television programming, are best sent In a progressive-scan format, in which the entire frame of video is painted in one vertical swoop of the screen. Note that for the same number of scan lines, a progressively scanned picture offers a higher effective resolution than an interlaced picture. DTV Audio Dolby Digital, also used on DVDs, is used in DTV to encode a 5.1-channel, full-range digital soundtrack along with the video. (See "Learning the Logos.") During playback, the Dolby Digital data bitstream is fed to an audio decoder that's incorporated into the TV or an outboard component. This decoder reads the digital information and converts it into six separate analog audio channels that can be fed to an amplifier and speakers.