What is High Definition Video?

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High-definition (HD) video generally refers to any video system of higher resolution than standard-definition (SD) video, most commonly at display resolutions of 1280×720 (720p) or 1920×1080 (1080i or 1080p). This article discusses the general concepts of high-definition video, as opposed to its specific applications in television broadcast (HDTV), video recording formats (HDCAM, HDCAM-SR, DVCPRO HD, D5 HD, XDCAM HD, HDV and AVCHD), the optical disc delivery system Blu-ray Disc and the video tape format D-VHS.

The current high definition video standards in North America were developed during the course of the advanced television process initiated by the Federal Communications Commission in 1987 at the request of American broadcasters. In essence the end of the 1980s was a death knell for most analog high definition technologies that had developed up to that time.

High definition video (prerecorded and broadcast) is defined threefold, by:

• The number of lines in the vertical display resolution. High-definition television (HDTV) resolution is 1080 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). However, since HD is broadcast digitally, its introduction sometimes coincides with the introduction of DTV. Additionally, current DVD quality is not high-definition, although the high-definition disc systems Blu-ray Disc and the defunct HD DVD are.
• The scanning system: progressive scanning (p) or interlaced scanning (i). Progressive scanning redraws an image frame (all of its lines) when refreshing each image. Interlaced scanning 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. 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 number of frames per second or fields per second. The 720p60 format is 1280 × 720 pixels, progressive encoding with 60 frames per second (60 Hz). The 1080i50 format is 1920 × 1080 pixels, interlaced encoding with 50 fields per second. Sometimes interlaced fields are called half-frames, but they are not, because two fields of one frame are temporally shifted; video engineers use the term 'picture' instead. Frame pulldown and segmented frames are special techniques that allow transmitting full frames by means of interlaced video stream.

Source: Wikipedia