A high definition television (HDTV) is a television that has a higher number of scanning lines per frame than a standard television (SDTV). Research on HDTV formats began in the 1970s while broadcasting began in the 1990s in Japan, Europe and the United States. Initially, HDTVs were very expensive and used analogue formats. Today, all HDTV broadcasts use digital signals.
The notation used for HDTV formats includes (1) a number denoting the resolution, (2) whether the frames are progressive (p) or interlaced (i) and (3) the frame rate in frames per second. For example, 720p60 denotes a HDTV set having a resolution of 1280 x 720 pixels with progressive signals and 60 fields or frames per second (60 Hz). Likewise a 1080i50 format denotes a HDTV set with a resolution of 1920 x 1080 pixels with interlaced signals (which are inferior to progressive signals) and with a frame rate of 50 fields or 25 frames per second.
If the frame rate is not specified, it is can assumed to be at 50 or 60 fps.
Comparision with SDTVs
HDTVs have higher resolutions (at least two times more) and display more detail than SDTVs. The maximum supported resolution in HDTVs can be 1280 x 720 (usually non-interlaced) or 1920 x 1080 (either interlaced or non-interlaced), whereas SDTVs have a resolution of only 640 x 480.
Most HDTVs have wide screens with an aspect ratio of 16 x 9. This eliminates the "letterbox" effect in which black lines fill up the empty spaces at the top and bottom of movies in their original formats. However broadcasts done in the SDTV aspect ratio (4 x 3) will have a “pillar box” effect in HDTV sets.
HDTV broadcasting is done using digital signals which provide excellent quality.
Initially HDTV sets were 720p or 1080i but not 1080p. The 1080p format was introduced in 2005 and provided output at better quality than the previous two formats.
HDTVs “upconvert” signals broadcast at the standard 480i to the level that the set supports (720p, 1080i or 1080p) by filling missing information using complicated algorithms. If the signal is in a higher quality format than supported, it will be downgraded.
"HD ready" or "HD capable" television sets refer to HDTV sets that do not have their own HD tuners but require HD set-top boxes from cable or satellite companies to receive broadcasts.
HD built in
These HDTV sets have built-in HD tuners through which broadcast signals are received.