DTV

1/9/99

Digital TV (DTV): A name for a collection of technologies that includes but is not limited to High Definition Television. It is not quite clear what is included in DTV and what is not. Different writers/speakers are likely to be talking about different things when using the term DTV.

Roughly, the United States and other countries have determined to replace their older analog television networks with digital television in the early 21st Century. The effort is quite controversial. Advantages include clearer pictures, truer color, freedom from ghosting, better sound, and lower bandwidth requirements. Objections include failure to establish international standards; optimistic conversion schedules; failure to define transmission format standards; failure to coordinate cable distribution digitation; and a change in screen shape to a "wide screen" format that is not compatible with existing television and computer viewing material.

DTV (in the United States) uses existing 6MHz channel allocations. Data is compressed using MPEG-2 compression. Channels can be Simplex (one signal) or Multiplexed. Modulation is Vestigial Sideband (VSB8) AM. Screens may have a 9:16 wide format as compared with a 4:3 ratio in older TVs and in computers. Consensus may be forming on 18 formats consisting of some permutations of three vertical resolutions, two aspect ratios, three scan rates, and interlace vs progressive scanning. Multiple low resolution pictures or digital data may be multiplexed.

The three resolutions proposed are 480 vertical (almost equivalent to the existing "525 vertical/484 displayed") in both 4:3 and 16:9 aspect ratios as well as 720 and 1080 lines 16:9. Scan rates are 60,30,24 screens per second. 30 and 24 screens are intended for use with motion pictures filmed at those rates. Audio uses multiple 5 channel (right, center, left, rear right, rear left) Dolby sound.

DTV is not subject to ghosting and some other problems that plague analog TV. It is reported that weak DTV signals do not degrade, they simply vanish when the level gets too low. However, DTV is reported to be subject to drops in resolution ("blockies") if there is inadequate bandwidth to transmit rapidly changing data such as high action motion picture sequences. Aircraft reflection or other temporary interference may cause signal dropout.

US over the air DTV does not use the same transmission standards as satellite TV or cable TV, and in all probability, will not use the same standards as those used in Europe, Africa or Asia.

Because of very aggressive marketing of HDTV there is a very large volume of probably wildly optimistic material published on DTV. The following site has some solid technical material:

~~http://www.tmo.hp.com/tmo/literature/English/VID_WhitePaper_005a.html~~