Data Compression: The Complete Reference

648 6. Video CompressionIn addition to the 1080 × 1920 DTV format, the ATSC DTV standard calls fora lower-resolution format with just 720 scan lines, implying 16 9× 720 = 1280 pixelsper line. Each of these resolutions can be refreshed at one of three different rates: 60frames/second (for live video) and 24 or 30 frames/second (for material originally producedon film). The refresh rates can be considered temporal resolution. The result is atotal of six different formats. Table 6.7 summarizes the screen capacities and the necessarytransmission rates of the six formats. With high-resolution and 60 frames per secondthe transmitter must be able to send 124,416,000 bits/sec (about 14.83 Mbyte/sec),which is why this format uses compression. (It uses MPEG-2. Other video formats canalso use this compression method.) The fact that DTV can have different spatial andtemporal resolutions allows for tradeoffs. Certain types of video material (such as fastmovinghorse- or car races) may look better at high refresh rates even with low spatialresolution, while other material (such as museum-quality paintings) should ideally bewatchedinhighresolutionevenwithlowrefreshrates.total #refresh ratelines × pixels of pixels 24 30 601080 × 1920 2,073,600 49,766,400 62,208,000 124,416,000720 × 1280 921,600 22,118,400 27,648,000 55,296,000Table 6.7: Resolutions and Capacities of Six DTV Formats.Digital Television (DTV) is a broad term encompassing all types of digital transmission.HDTV is a subset of DTV indicating 1080 scan lines. Another type of DTVis standard definition television (SDTV), which has picture quality slightly better thana good analog picture. (SDTV has resolution of 640 × 480 at 30 frames/sec and anaspect ratio of 4:3.) Since generating an SDTV picture requires fewer pixels, a broadcastingstation will be able to transmit multiple channels of SDTV within its 6 MHzallowed frequency range. HDTV also incorporates Dolby Digital sound technology tobring together a complete presentation.Figure 6.8 shows the most important resolutions used in various video systems.Their capacities range from 19,000 pixels to more than two million pixels.Sometimes cameras and television are good to peopleand sometimes they aren’t. I don’t know if itsthe way you say it, or how you look.—Dan Quayle