High-Definition Television (HDTV) refers to a television broadcasting system (or to a television set) that displays video at a high resolution of 720 horizontal lines or more, scanned from top to bottom.  Ordinary or Standard-Definition TV's (SDTV's) display video at a resolution of just 480 horizontal lines. HDTV's are digital broadcast systems, employing digital compression techniques to efficiently transmit the video at high definition.

HDTV specifications vary from one system to another, but usually an HDTV system is defined by three parameters:  

1)  Frame Size  - the number of horizontal pixels times the number of vertical pixels of each frame, e.g., 1280 x 720 means that each frame of the video is 1280 pixels wide and 720 pixels high;

2)  Scanning System - whether it uses progressive scanning (denoted by 'p') or interlaced scanning (denoted by 'i'); and

3)  Frame Rate - the number of video frames displayed per second.

These standard parameters make it is easy to describe an HDTV system by stringing the parameter values together in one word.  For example, if you see an HDTV described as 1920x1080p25, you know that it is a TV that has a frame size of 1920 pixels by 1080 pixels, uses progressive scanning, and displays 25 frames of video per second.

Standard scanning rates (frames per second) are as follows: 

-  for 50-Hz systems:  25i, 25p, 50p;

-  for 60-Hz systems: 23.98p, 24p, 29.97i, 59.94i, 29.97p, 30p, 59.94p, 60p.

Interlaced scanning ('i') means that half of the horizontal lines are displayed alternately with the other half.  Thus, in the case of a 1080i TV, 540 horizontal lines (say, all odd-numbered lines) are displayed on a frame first, which is followed by a second frame that displays the other 540 lines (all the even-numbered lines). Progressive scanning ('p') means that each frame displays all the lines at the same time. As such, progressive scanning requires a greater bandwidth than interlaced scanning. 

   

Figure 1.   A High-Definition TV (HDTV), with its sharper images, gives more viewing pleasure than an ordinary TV.

   

HDTV naturally provides a higher quality of image than SDTV because of its greater number of horizontal lines.  Since the gaps between the scan lines are smaller, each frame looks sharper. The picture improvement is most noticeable on HDTV's with larger screens.

Of course, the video displayed by HDTV will only be 'high-definition' if the broadcast was done at high-definition resolution.  Unfortunately, some service providers do not strictly follow HDTV specifications in order to save bandwidth so they can pack more (but lower-quality) channels into their services.  This practice defeats the purpose of HDTV.

Since HDTV programs are transmitted digitally, interferences will result in major image problems - frames may be frozen or skipped or they may simply display meaningless pixels, thereby making viewing not possible at all.  The use of good antennas, cables, and connectors is important in ensuring good signal quality. 

Common sources of HDTV programs include: 1) antenna reception from networks broadcasting in HDTV; 2)  cable services that offer HDTV programs; 3)  satellite HDTV;  4)  video gaming systems that offer HDTV entertainment capabilities; 5) graphics cards that can output HDTV video from the computer to an HDTV; and 6)  high-definition optical disc players (such as Blu-ray Disc players).

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