DTV->Digital TV. HDTV is High Definition (digital) TV. The new signals sent over the air or over cable are digital instead of analog. As long as you have a decent signal level, the picture and sound is digitally perfect. SDTV signal at 480i are essentially identical to standard analog TV in quality. Higher resolutions of 720i and 1080i are available. LCD style displays prefer a progressive (write the screen one time per frame instead of two interlaced images on a CRT). The 480p, 720p formats look better on a digital display device. 480i means 480 horizontal pixels. 1080i has 1080 horizontal pixels. The standard analog display uses a 4:3 aspect ratio (4 wide : 3 high). New widescreen digital display prefer 16:9 (16 wide : 9 high). The 1080i will be 1080 wide X 608 high.
Digital transmissions occupy less bandwidth than the analog signals. Many more channels can be packed into reduced frequency spectrum.