High-Definition Television. The high-resolution subset of the DTV system. The ATSC defines HDTV as a 16:9 image with twice the horizontal and vertical resolution of existing systems, accompanied by 5.1 channels of Dolby Digital audio. The CEA defines HDTV as an image with 720 progressive or 1080 interlaced active (top to bottom) scan lines. 1280:720p and 1920:1080i are typically accepted as high-definition scan rates.