High-definition video is a video of higher resolution and quality than standard-definition. While there is no standardized meaning for high-definition, generally any video image with considerably more than 480 horizontal lines or 576 horizontal lines is considered high-definition.
480 scan lines is generally the minimum even though the majority of systems greatly exceed that. Images of standard resolution captured at rates faster than normal (60 frames/second North America, 50 fps Europe), by a high-speed camera may be considered high-definition in some contexts.
By David Zvina
High definition is defined three fold by the following concept
The number of lines in the vertical display resolution. High-definition television (HDTV) resolution is 1,080 or 720 lines. In contrast, regular digital television (DTV) is 480 lines (upon which NTSC is based, 480 visible scanlines out of 525) or 576 lines (upon which PAL/SECAM are based, 576 visible scanlines out of 625).
The scanning system: progressive scanning (p) or interlaced scanning (i). Progressive scanning (p) redraws an image frame (all of its lines) when refreshing each image, for example 720p/1080p. Interlaced scanning (i) draws the image field every other line or “odd numbered” lines during the first image refresh operation, and then draws the remaining “even numbered” lines during a second refreshing, for example 1080i.
Interlaced scanning yields greater image resolution if subject is not moving, but loses up to half of the resolution and suffers “combing” artifacts when subject is moving.
The 720p60 format is 1,280 × 720 pixels, progressive encoding with 60 frames per second (60 Hz). The 1080i50/1080i60 format is 1920 × 1080 pixels, interlaced encoding with 50/60 fields, (50/60 Hz) per second.
Two interlaced fields formulate a single frame, because the two fields of one frame are temporally shifted. Frame pulldown and segmented frames are special techniques that allow transmitting full frames by means of interlaced video stream.
A frame or field rate can also be specified without a resolution. For example, 24p means 24 progressive scan frames per second and 50i means 25 progressive frames per second, consisting of 50 interlaced fields per second. Most HDTV systems support some standard resolutions and frame or field rates.
The most common are noted below. High-definition signals require a high-definition television or computer monitor in order to be viewed.
High-definition video has an aspect ratio of 16:9 (1.78:1). The aspect ratio of regular widescreen film shot today is typically 1.85:1 or 2.39:1 (sometimes traditionally quoted at 2.35:1)