definición de HIGH DEFINITION VIDEO (Wikipedia)
High-definition video or HD video is any video system of higher resolution than standard-definition (SD) video, and most commonly involves display resolutions of 1,280×720 pixels (720p) or 1,920×1,080 pixels (1080i/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, AVC-Intra, XDCAM HD, HDV, and AVCHD), the Blu-ray Disc, and the D-VHS video tape format.
From a historical perspective, the first electronic scanning format 405 lines was the first "high definition" television system as the previous mechanical systems had far fewer scanning lines.
From 1939, the US and other European countries experimented with 441 lines and 605 lines until the Federal Communications Commission (FCC) mandated 525 lines for the US from 1941. In wartime France, René Barthélemy experimented with higher definitions, reaching 1015 and even 1042 lines. Official French transmissions finally began with 819 lines from late 1949; however, this standard was abandoned in 1984 upon the adoption of 625-line color on the TF1 network.
Modern HD specifications date to the early 1970s, when Japanese engineers developed the HighVision 1,125-line interlaced TV standard (also called MUSE) that ran at 60 frames per second. The Sony HDVS system was presented at an international meeting of television engineers in Algiers, April 1981 and Japan's NHK presented its analog HDTV system at a Swiss conference in 1983.
The NHK system was standardized in the United States as Society of Motion Picture and Television Engineers (SMPTE) standard #240M in the early 1990s, but abandoned later on when it was replaced by a DVB analog standard. HighVision video is still usable for HDTV video interchange, but there is almost no equipment around to perform this function. Attempts at shoehorning in HighVision into a 6 MHz broadcast channel were mostly unsuccessful. All attempts at using this format for terrestrial TV transmission were forsaken by the mid-1990s.
Europe developed HD-MAC (1,250 lines, 50 Hz), a member of the MAC family of hybrid analogue/digital video standards; however, it never took off as a terrestrial video transmission format. HD-MAC was never designated for video interchange except by the European Broadcasting Union.
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.
The FCC process, led by the Advanced Television Systems Committee (ATSC) adopted a range of standards from interlaced 1,080-line video (a technical descendant of the original analog NHK 1125/30 Hz system) with a maximum frame rate of 30 Hz, and 720-line video, progressively scanned, with a maximum frame rate of 60 Hz.
In the end, however, the DVB standard of resolutions (1080, 720, 480) and respective frame rates (24, 25, 30) were adopted in conjunction with the Europeans that were also involved in the same standardization process. The FCC officially adopted the ATSC transmission standard (which included both HD and SD video standards) in 1996, with the first broadcasts on October 28, 1998.
In the early 2000s, it looked as if DVB would be the video standard far into the future. However, both Brazil and China have adopted alternative standards for high-definition video that preclude the interoperability that was hoped for after decades of largely non-interoperable analog TV broadcasting.
High definition video (prerecorded and broadcast) is defined threefold, by:
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.
|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||1,036,800||Interlaced||25 (50 fields/s), 29.97 (59.94 fields/s), 30 (60 fields/s)|
|1080p||1,920×1,080||2,073,600||Progressive||23.976, 24, 25, 29.97, 30, 50, 59.94, 60|
|Video mode||Frame size in pixels (W×H)||Pixels per image1||Scanning type||Frame rate (Hz)|
|2160p||3,840×2,160||8,294,400||Progressive||24, 25, 30, 50, 60|
|4320p||7,680×4,320||33,177,600||Progressive||50, 60, 120|
Note: 1 Image is either a frame or, in case of interlaced scanning, two fields. (EVEN and ODD)
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.
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 as IMAX 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.
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.
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 a movie was filmed in, 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:
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). Note that resolution values for 70 mm film are higher than those listed above.
A number of online video streaming/on demand and digital download services offer HD video, among them YouTube, Vimeo, Hulu, Amazon Video On Demand, Netflix Watch Instantly, and others. Due to heavy compression, the image detail produced by these formats are far below that of broadcast HD, and often even inferior to DVD-Video (3-9 Mbit/s MP2) upscaled to the same image size. The following is a chart of numerous online services and their HD offering:
|Source||Codec||Highest resolution (W×H)||Total bit rate/bandwidth||Video bit rate||Audio bit rate|
|Amazon Video On Demand (formerly "Unbox")||VC-1||1,280×720||2.5 Mbit/s|
|BBC iPlayer||H.264||1,280×720||3.2 Mbit/s||3 Mbit/s||192 kbit/s|
|CBS.com/TV.com (720p)||1,280×720||2.5 Mbit/s|
|CBS.com/TV.com (1080p)||1,920×1,080||3.5 Mbit/s|
|DaCast||VP6, H.264||7680×4320||5 Mbit/s|
|Hulu||On2 Flash VP6||1,280×720||2.5 Mbit/s|
|iPlayerHD||FLV, Quicktime H.264, MP4 H.264||1,920×1,080||5 Mbit/s|
|iTunes/Apple TV||QuickTime H.264||1,280×720||4Mbit/s|
|Netflix Watch Instantly||VC-1||1,280×720||5 Mbit/s||2.6 Mbit/s and 3.8 Mbit/s|
|PlayStationStore Movies & TV Shows||H.264/MPEG-4 AVC||1,920×1,080||8 Mbit/s||256 kbit/s|
|Vimeo||H.264||1,920×1,080||4 Mbit/s||320 kbit/s|
|Zune Video (formerly "Xbox Live Marketplace Video Store")||1,920×1,080||3 Mbit/s|
An increasing number of manufacturers of security cameras now offer HD cameras. The need for high resolution, color fidelity, and frame rate is acute for surveillance purposes to ensure that the quality of the video output is of an acceptable standard that can be used both for preventative surveillance as well as for evidence purposes. These needs, however, must be balanced against the additional storage capacity required by HD video.
The PlayStation 3 game console can output native 1080p through both component and HDMI cables. The Xbox 360 can output 1080p over HDMI, but games only run natively at 720p but can be upscaled to 1080p. The Wii can output up to 480p (enhanced-definition) over component, which while not technically HD, is very useful for HDTVs as it avoids de-interlacing artifacts. The Wii can also output 576i in PAL regions.
Native 1080p produces a sharper and clearer picture compared to upscaled 1080p. Besides increasing the visual quality of games, users can also download HD movies and video clips from the PlayStation Network or Xbox Live Marketplace services to their respective consoles. The PlayStation 3 can also play Blu-ray Discs which hold HD data.
Though only a handful of games available have the native resolution of 1080p, all games on the Xbox 360 and PlayStation 3 can be upscaled up to this resolution. Xbox 360 and PlayStation 3 games are labeled with the output resolution on the back of their packaging, although on Xbox 360 this indicates the resolution it will upscale to, not the native resolution of the game. Also, as the Xbox 360 did not originally support 1080p (it did not have an HDMI port), earlier games that said 720p on the box can now be upscaled to 1080p.
An increasing number of PC games can be rendered in 1,920×1,080 or higher.
Nintendo's new console, the Wii U, supports HD.
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