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|The Digital TV Standard|
The DTV Standard
On December 24, 1996, the U.S. FCC adopted the major elements of the ATSC DTV standard, mandating its use for digital terrestrial television broadcasts in the U.S. Within the DTV standard are 18 different picture formats.
The FCC did not mandate use of the specific HDTV and SDTV formats contained in the ATSC standard, but these have been uniformly adopted on a voluntary basis by broadcasters and receiver manufacturers.
All digital receivers (set-top boxes) and HDTV sets receive them all. A DTV receiver, which looks like a VCR or a cable or satellite receiver, gathers and translates the digital signal for the DTV monitor.
In 1997 the FCC adopted companion DTV rules, assigning an additional 6 MHz channel to approximately 1,600 full-power broadcasters in the U.S. to permit them to offer digital terrestrial broadcasts in parallel with their existing analog services during a transition period, while consumers made the conversion to digital receivers or set-top boxes. In accordance with the FCC plan, digital television service was launched in the U.S. November 1, 1998.
The two most commonly used signals by local broadcast stations are EDTV and HDTV. With the current analog system, TV images are created by interlace scanning, which uses two fields of alternating horizontal scanning lines to form a full picture.
This picture is referred to as “480 interlace,”
or 480i. With many DTVs, the number
of scanning lines are more than doubled to 1,080 (1080i). This is HDTV and delivers a more detailed image that practically jumps off the TV screen.
HDTV also may be broadcast and displayed as a “progressive image” (720p), like a computer monitor. Here, a full frame fills the screen from top to bottom, eliminating lines altogether so the picture has a more film like feel. EDTV quality is referred to as 480p for its 480 progressive lines of resolution. In the new digital era, broadcasters can offer free, over-the-air television of higher resolution and better picture quality than is possible under the current system.
If broadcasters so choose, they can deliver HDTV with theater-quality pictures and CD-quality sound. Or a broadcaster can offer several different TV programs at the same time (called “multicasting”), but in a lower resolution – SDTV. Even with fewer than 480 lines of resolution, the picture and sound quality of SDTV still is better than analog TV. The target date for completion of the analog-to-DTV transition is 2006, or 85 percent household penetration, whichever occurs later.