High-definition video is video of higher resolution than is standard. While there is no specific meaning for high-definition, generally any video image with more than 480 horizontal lines (North America) or 570 lines (Europe) is considered high-definition. 720 scan lines is generally the minimum even though many 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.
Technical details[edit]
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 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). 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 (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 number of frames or fields per second (Hz). In Europe more common (50 Hz) television broadcasting system and in USA (60 Hz). 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.
Often, the rate is inferred from the context, usually assumed to be either 50 Hz (Europe) or 60 Hz (USA), except for 1080p, which denotes 1080p24, 1080p25, and 1080p30, but also 1080p50 and 1080p60.
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). Standard-definition television (SDTV) has a 4:3 (1.33:1) aspect ratio, although in recent years many broadcasters have transmitted programs "squeezed" horizontally in 16:9 anamorphic format, in hopes that the viewer has a 16:9 set which stretches the image out to normal-looking proportions, or a set which "squishes" the image vertically to present a "letterbox" view of the image, again with correct proportions.
Common high-definition video modes[edit]
| Video mode | Frame size in pixels (W×H) | Pixels per image1 | Scanning type | Frame rate (Hz) |
|---|---|---|---|---|
| 720p | 1,280×720 | 921,600 | Progressive | 23.976, 24, 25, 29.97, 30, 50, 59.94, 60, 72 |
| 1080i | 1,920×1,080 | 2,073,600 | Interlaced | 25 (50 fields/s), 29.97 (59.94 fields/s), 30 (60 fields/s) |
| 1080p | 1,920×1,080 | 2,073,600 | Progressive | 24 (23.976), 25, 30 (29.97), 50, 60 (59.94) |
Ultra high-definition video modes[edit]
| Video mode | Frame size in pixels (W×H) | Pixels per image1 | Scanning type | Frame rate (Hz) |
|---|---|---|---|---|
| 2000p | 2,048×1,536 | 3,145,728 | Progressive | 24 |
| 2160p | 3,840×2,160 | 8,294,400 | Progressive | 60, 120 |
| 2540p | 4,520×2,540 | 11,480,800 | Progressive | |
| 4000p | 4,096×3,072 | 12,582,912 | Progressive | |
| 4320p | 7,680×4,320 | 33,177,600 | Progressive | 60, 120 |
Note: 1 Image is either a frame or, in case of interlaced scanning, two fields. (EVEN and ODD)
HD content[edit]
High-definition image sources include terrestrial broadcast, direct broadcast satellite, digital cable, high definition disc (BD), digital cameras, internet downloads and the latest generation of video game consoles.
- Most computers are capable of HD or higher resolutions over VGA, DVI, and/or HDMI.
- The optical disc standard Blu-ray Disc can provide enough digital storage to store hours of HD video content. Digital Versatile Discs or DVDs (that hold 4.7 GB* for a Single layer or 8.5 GB* for a Double layer), look best on screens that are smaller than 36 inches (91 cm)[citation needed], so they are not always up to the challenge of today's high-definition (HD) sets. Storing and playing HD movies requires a disc that holds more information, like a Blu-ray Disc (which hold 25 GB* in single layer form and 50 GB* for double layer) or High Definition Digital Versatile Discs HD-DVDs which hold 15 GB* or 30 GB* in single and double layer. * = Gigabyte = 1,073,641,324 bytes, 1,048,576 killabytes, or 1024 megabytes.
Blu-ray Discs were jointly developed by 9 initial partners including Sony, Phillips (which developed CDs),and Pioneer (which developed its own Laser-disc previously with some success) among others. HD-DVD discs were primarily developed by Toshiba and NEC with some backing from Microsoft, Warner Bros., Hewlett Packard, and others. On February 19, 2008 Toshiba announced it was abandoning the format and would discontinue development, marketing and manufacturing of HD-DVD players and drives.
Types of recorded media[edit]
The high resolution photographic film used for cinema projection is exposed at the rate of 24 frames per second but usually projected at 48, each frame getting projected twice helping to minimise flicker. One exception to this was the 1986 National Film Board of Canada short film Momentum, which briefly experimented with both filming and projecting at 48 frame/s, in a process known asIMAX HD.
Depending upon available bandwidth and the amount of detail and movement in the image, the optimum format for video transfer is either 720p24 or 1080p24. When shown on television in PAL system countries, film must be projected at the rate of 25 frames per second by accelerating it by 4.1 percent. In NTSC standard countries, the projection rate is 30 frames per second, using a technique called 3:2 pull-down. One film frame is held for three video fields (1/20 of a second), and the next is held for two video fields (1/30 of a second) and then the process is repeated, thus achieving the correct film projection rate with two film frames shown in 1/12 of a second.
See also: Telecine and Deinterlacing
Older (pre-HDTV) recordings on video tape such as Betacam SP are often either in the form 480i60 or 576i50. These may be upconverted to a higher resolution format (720i), but removing the interlace to match the common 720p format may distort the picture or require filtering which actually reduces the resolution of the final output.
Non-cinematic HDTV video recordings are recorded in either the 720p or the 1080i format. The format used is set by the broadcaster (if for television broadcast). In general, 720p is more accurate with fast action, because it progressively scans frames, instead of the 1080i, which uses interlaced fields and thus might degrade the resolution of fast images.
720p is used more for Internet distribution of high-definition video, because computer monitors progressively scan; 720p video has lower storage-decoding requirements than either the 1080i or the 1080p. This is also the medium for high-definition broadcasts around the world and 1080p is used for Blu-ray movies.
HD in filmmaking[edit]
Film as a medium has inherent limitations, such as difficulty of viewing footage while recording, and suffers other problems, caused by poor film development/processing, or poor monitoring systems. Given that there is increasing use of computer-generated or computer-altered imagery in movies, and that editing picture sequences is often done digitally, some directors have shot their movies using the HD format via high-end digital video cameras. While the quality of HD video is very high compared to SD video, and offers improved signal/noise ratios against comparable sensitivity film, film remains able to resolve more image detail than current HD video formats. In addition some films have a wider dynamic range (ability to resolve extremes of dark and light areas in a scene) than even the best HD cameras. Thus the most persuasive arguments for the use of HD are currently cost savings on film stock and the ease of transfer to editing systems for special effects.
Depending on the year and format in which a movie was filmed, the exposed image can vary greatly in size. Sizes range from as big as 24 mm × 36 mm for VistaVision/Technirama 8 perforation cameras (same as 35 mm still photo film) going down through 18 mm × 24 mm for Silent Films or Full Frame 4 perforations cameras to as small as 9 mm × 21 mm in Academy Sound Aperture cameras modified for the Techniscope 2 perforation format. Movies are also produced using other film gauges, including 70 mm films (22 mm × 48 mm) or the rarely used 55 mm and CINERAMA.
The four major film formats provide pixel resolutions (calculated from pixels per millimeter) roughly as follows:
- Academy Sound (Sound movies before 1955): 15 mm × 21 mm (1.375) = 2,160 × 2,970
- Academy camera US Widescreen: 11 mm × 21 mm (1.85) = 1,605 × 2,970
- Current Anamorphic Panavision ("Scope"): 17.5 mm × 21 mm (2.39) = 2,485 × 2,970
- Super-35 for Anamorphic prints: 10 mm × 24 mm (2.39) = 1,420 × 3,390
In the process of making prints for exhibition, this negative is copied onto other film (negative → interpositive → internegative → print) causing the resolution to be reduced with each emulsion copying step and when the image passes through a lens (for example, on a projector). In many cases, the resolution can be reduced down to 1/6 of the original negative's resolution (or worse).[citation needed] Note that resolution values for 70 mm film are higher than those listed above.
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