DTV primer.........................................
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The 4-1-1 on DTV
Jan Ozer
EMedia Professional, August 1998
Copyright c Online Inc.
On April 4, 1997, the FCC decreed "let there be digital," and ushered in the digital television (DTV) era by giving 6MHz of spectrum to approximately 1,500 stations for DTV broadcasting. The decree requires the three commercial networks in the top ten markets to broadcast digitally by May 1, 1999, with markets 11 through 30 online by November 1, 1999. All stations must broadcast digitally by 2006, when their current analog spectrum is scheduled to revert back to the Fed.
Unfortunately, the decree doesn't specifically define DTV. In fact, it lists 18 formats for digital transmission. A concession to the computer interests that entered the standardization process late in the game, this lack of definition is the battleground for the war between the computer industry and the networks. At stake in this war is how soon DTV will deliver compelling content to the vast majority of us who don't buy High-Definition TV sets and whether your television set will ultimately sport the familiar Microsoft Windows logo.
THE FORMAT
Three characteristics define each DTV format: resolution, scan mechanism, and frame rate.
Four resolutions are acceptable under the FCC decree: 1920x1080, 1280x720, 720x480, and 640x480. The two largest resolutions are High Definition formats (HDTV), while the two smaller resolutions constitute Standard Definition TV (SDTV).
Scan mechanism refers to the difference between progressive scan and interlaced. Current NTSC video is interlaced-meaning that each frame is divided into two fields, one containing the even lines, the other the odd lines-and 60 fields are displayed each second. This contrasts with progressive scan mechanisms-like computer monitors-where every line of the entire frame is updated from top to bottom.
The final variable is frame rate (the number of times the screen is updated each second), which is integrally related to scan mechanism. Each resolution supports multiple frame rates and scan mechanisms. For example, 1920x 1080 supports 30 frames per second (fps) in interlaced and progressive modes, as well as 24fps in progressive mode.
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