Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.

History [link]

Video technology was first developed for cathode ray tube (CRT) television systems, but several new technologies for video display devices have since been invented. Charles Ginsburg led an Ampex research team developing the first practical video tape recorder (VTR). In 1951 the first video tape recorder captured live images from television cameras by converting the camera's electrical impulses and saving the information onto magnetic video tape.

Video recorders sold for $50,000 in 1956, and videotape cost $300 per one-hour reel. However, prices steadily dropped over the years; in 1971, Sony began selling videocassette recorder (VCR) tapes to the public. After the invention of the DVD in 1997 and Blu-ray Disc in 2006, sales of videotape and tape equipment plummeted.

Later advances in computer technology allowed computers to capture, store, edit and transmit video clips.

Characteristics of video streams [link]

Number of frames per second [link]

''Frame rate'', the number of still pictures per unit of time of video, ranges from six or eight frames per second (''frame/s'') for old mechanical cameras to 120 or more frames per second for new professional cameras. PAL (Europe, Asia, Australia, etc.) and SECAM (France, Russia, parts of Africa etc.) standards specify 25 frame/s, while NTSC (USA, Canada, Japan, etc.) specifies 29.97 frame/s. Film is shot at the slower frame rate of 24photograms/s, which complicates slightly the process of transferring a cinematic motion picture to video. The minimum frame rate to achieve the illusion of a moving image is about fifteen frames per second.

Interlaced vs progressive[link]

Video can be interlaced or progressive. Interlacing was invented as a way to reduce flicker in early mechanical and CRT video displays without increasing the number of complete frames per second, which would have required sacrificing image detail in order to remain within the limitations of a narrow bandwidth. The horizontal scan lines of each complete frame are treated as if numbered consecutively and captured as two ''fields'': an ''odd field'' (upper field) consisting of the odd-numbered lines and an ''even field'' (lower field) consisting of the even-numbered lines.

Analog display devices reproduce each frame in the same way, effectively doubling the frame rate as far as perceptible overall flicker is concerned. When the image capture device acquires the fields one at a time, rather than dividing up a complete frame after it is captured, the frame rate for motion is effectively doubled as well, resulting in smoother, more life-like reproduction (although with halved detail) of rapidly moving parts of the image when viewed on an interlaced CRT display, but the display of such a signal on a progressive scan device is problematic.

NTSC, PAL and SECAM are interlaced formats. Abbreviated video resolution specifications often include an ''i'' to indicate interlacing. For example, PAL video format is often specified as ''576i50'', where ''576'' indicates the total number of horizontal scan lines, ''i'' indicates interlacing, and ''50'' indicates 50 fields (half-frames) per second.

In ''progressive scan'' systems, each refresh period updates all of the scan lines of each frame in sequence. When displaying a natively progressive broadcast or recorded signal, the result is optimum spatial resolution of both the stationary and moving parts of the image. When displaying a natively interlaced signal, however, overall spatial resolution will be degraded by simple line doubling and artifacts such as flickering or "comb" effects in moving parts of the image will be seen unless special signal processing is applied to eliminate them. A procedure known as deinterlacing can be used to optimize the display of an interlaced video signal from an analog, DVD or satellite source on a progressive scan device such as an LCD Television, digital video projector or plasma panel. Deinterlacing cannot, however, produce video quality that is equivalent to true progressive scan source material.

Aspect ratio [link]

Aspect ratio describes the dimensions of video screens and video picture elements. All popular video formats are rectilinear, and so can be described by a ratio between width and height. The screen aspect ratio of a traditional television screen is 4:3, or about 1.33:1. High definition televisions use an aspect ratio of 16:9, or about 1.78:1. The aspect ratio of a full 35 mm film frame with soundtrack (also known as the Academy ratio) is 1.375:1.

Ratios where the height is taller than the width are uncommon in general everyday use, but do have application in computer systems where the screen may be better suited for a vertical layout. The most common tall aspect ratio of 3:4 is referred to as ''portrait mode'' and is created by physically rotating the display device 90 degrees from the normal position. Other tall aspect ratios such as 9:16 are technically possible but rarely used. (For a more detailed discussion of this topic please refer to the page orientation article.)

Pixels on computer monitors are usually square, but pixels used in digital video often have non-square aspect ratios, such as those used in the PAL and NTSC variants of the CCIR 601 digital video standard, and the corresponding anamorphic widescreen formats. Therefore, an NTSC DV image which is 720 pixels by 480 pixels is displayed with the aspect ratio of 4:3 (which is the traditional television standard) if the pixels are thin and displayed with the aspect ratio of 16:9 (which is the anamorphic widescreen format) if the pixels are fat.

Color space and bits per pixel[link]

''Color model name'' describes the video color representation. ''YIQ'' was used in NTSC television. It corresponds closely to the ''YUV'' scheme used in NTSC and PAL television and the ''YDbDr'' scheme used by SECAM television.

The number of distinct colors that can be represented by a pixel depends on the number of ''bits per pixel'' (bpp). A common way to reduce the number of bits per pixel in digital video is by chroma subsampling (e.g. 4:4:4, 4:2:2, 4:2:0/4:1:1).

Video quality[link]

Video quality can be measured with formal metrics like PSNR or with subjective video quality using expert observation.

The subjective video quality of a video processing system may be evaluated as follows:

Choose the video sequences (the ''SRC'') to use for testing.

Choose the settings of the system to evaluate (the ''HRC'').

Choose a test method for how to present video sequences to experts and to collect their ratings.

Invite a sufficient number of experts, preferably not fewer than 15.

Carry out testing.

Calculate the average marks for each ''HRC'' based on the experts' ratings.

Many subjective video quality methods are described in the ITU-T recommendation BT.500. One of the standardized method is the ''Double Stimulus Impairment Scale'' (DSIS). In DSIS, each expert views an ''unimpaired'' reference video followed by an ''impaired'' version of the same video. The expert then rates the ''impaired'' video using a scale ranging from "impairments are imperceptible" to "impairments are very annoying".

Video compression method (digital only)[link]

A wide variety of methods are used to compress video streams. Video data contains spatial and temporal redundancy, making uncompressed video streams extremely inefficient. Broadly speaking, spatial redundancy is reduced by registering differences between parts of a single frame; this task is known as ''intraframe compression'' and is closely related to image compression. Likewise, temporal redundancy can be reduced by registering differences between frames; this task is known as ''interframe compression'', including motion compensation and other techniques. The most common modern standards are MPEG-2, used for DVD, Blu-ray and satellite television, and MPEG-4, used for AVCHD, Mobile phones (3GP) and Internet.

Bit rate (digital only)[link]

''Bit rate'' is a measure of the rate of information content in a video stream. It is quantified using the bit per second (''bit/s'' or ''bps'') unit or ''Megabits per second'' (''Mbit/s''). A higher bit rate allows better video quality. For example VideoCD, with a bit rate of about 1 Mbit/s, is lower quality than DVD, with maximum bit rate of 10.08 Mbit/s for video. HD (High Definition Digital Video and TV) has a still higher quality, with a bit rate of about 20 Mbit/s.

Variable bit rate (VBR) is a strategy to maximize the visual video quality and minimize the bit rate. On fast motion scenes, a variable bit rate uses more bits than it does on slow motion scenes of similar duration yet achieves a consistent visual quality. For real-time and non-buffered video streaming when the available bandwidth is fixed, e.g. in videoconferencing delivered on channels of fixed bandwidth, a constant bit rate (CBR) must be used.

Stereoscopic[link]

''Stereoscopic video'' can be created using several different methods:

two channels — a right channel for the right eye and a left channel for the left eye. Both channels may be viewed simultaneously by using light-polarizing filters 90 degrees off-axis from each other on two video projectors. These separately polarized channels are viewed wearing eyeglasses with matching polarization filters.

one channel with two overlaid color coded layers. This left and right layer technique is occasionally used for network broadcast, or recent "anaglyph" releases of 3D movies on DVD. Simple Red/Cyan plastic glasses provide the means to view the images discretely to form a stereoscopic view of the content.

One channel with alternating left/right frames for each eye, using LCD shutter glasses which read the frame sync from the VGA Display Data Channel to alternately cover each eye, so the appropriate eye sees the correct frame. This method is most common in computer virtual reality applications such as in a Cave Automatic Virtual Environment, but reduces the effective video framerate to one-half of normal (for example, from 120 Hz to 60 Hz).

Blu-ray Discs greatly improve the sharpness and detail of the two-color 3D effect in color coded stereo programs. See articles Stereoscopy and 3-D film.

Video formats[link]

There are different layers of video transmission and storage, each with its own set of formats to choose from.

For transmission, there is a physical connector and signal protocol ("video connection standard" below). A given physical link can carry certain "display standards" which specify a particular refresh rate, display resolution, and color space.

Many analog and digital recording formats are in use, and digital video clips can also be stored on a computer file system as files which have their own formats. In addition to the physical format used by the data storage device or transmission medium, the stream of ones and zeros that is sent must be in a particular digital "video encoding", of which a number are available.

Transport medium[link]

Video can be transmitted or transported in a variety of ways. Wireless broadcast as an analog or digital signal. Coaxial cable in a closed circuit system can be sent as analog interlaced 1 volt peak to peak with a maximum horizontal line resolution up to 480. Broadcast or studio cameras use a single or dual coaxial cable system using a progressive scan format known as SDI serial digital interface and HD-SDI for High Definition video. The distances of transmission are somewhat limited depending on the manufacturer the format may be proprietary. SDI has an negligible lag and is uncompressed. There are initiatives to use the SDI standards in closed circuit surveillance systems, for Higher Definition images, over longer distances on coax or twisted pair cable. Due to the nature of the higher bandwidth needed, the distance the signal can be effectively sent is a half to a third of what the older interlaced analog systems supported.

Video connectors, cables, and signal standards[link]

See List of video connectors for information about physical connectors and related signal standards.

Video display standards[link]

Digital television[link]

New formats for digital television broadcasts use the MPEG-2 video codec and include:

ATSC - USA, Canada, Korea

Digital Video Broadcasting (DVB) - Europe

ISDB - Japan

*ISDB-Tb - Uses the MPEG-4 video codec. Brazil, Argentina

Digital Multimedia Broadcasting (DMB) - Korea

Analog television[link]

Analog television broadcast standards include:

FCS - USA, Russia; obsolete

MAC - Europe; obsolete

MUSE - Japan

NTSC - USA, Canada, Japan

PAL - Europe, Asia, Oceania

* PAL-M - PAL variation. Brazil, Argentina

* PALplus - PAL extension, Europe

RS-343 (military)

SECAM - France, Former Soviet Union, Central Africa

An analog video format consists of more information than the visible content of the frame. Preceding and following the image are lines and pixels containing synchronization information or a time delay. This surrounding margin is known as a blanking interval or blanking region; the horizontal and vertical front porch and back porch are the building blocks of the blanking interval.

Many countries are planning a digital switchover soon.

Computer displays[link]

See Computer display standard for a list of standards used for computer monitors and comparison with those used for television.

Recording formats before video tape[link]

Phonovision

Kinescope

Analog tape formats[link]

1" Type B video tape (Robert Bosch GmbH)

1" Type C videotape (Ampex, Marconi and Sony)

2" Quadruplex videotape (Ampex)

2" Helical Scan Videotape (Rank Cintel)

Betacam (Sony)

Betacam SP (Sony)

Betamax (Sony)

S-VHS (JVC) (1987)

W-VHS (JVC) (1994)

U-matic 3/4" (Sony)

VCR, VCR-LP, SVR

VERA (BBC experimental format ca. 1958)

VHS (JVC)

VHS-C (JVC)

Video 2000 (Philips)

''(See List of video recording formats.)''

Digital tape formats[link]

Betacam IMX (Sony)

D-VHS (JVC)

D-Theater

D1 (Sony)

D2 (Sony)

D3

D5 HD

Digital-S D9 (JVC)

Digital Betacam (Sony)

Digital8 (Sony)

DV

HDV

ProHD (JVC)

MicroMV

MiniDV

Optical disc storage formats[link]

Blu-ray Disc (Sony)

China Blue High-definition Disc (CBHD)

DVD (was Super Density Disc, DVD Forum)

Professional Disc

Universal Media Disc (UMD) (Sony)

Discontinued[link]

Enhanced Versatile Disc (EVD, Chinese government-sponsored)

HD DVD (NEC and Toshiba)

HD-VMD

Laserdisc (old, MCA and Philips)

Digital encoding formats[link]

CCIR 601 (ITU-T)

H.261 (ITU-T)

H.263 (ITU-T)

H.264/MPEG-4 AVC (ITU-T + ISO)

M-JPEG (ISO)

MPEG-1 (ISO)

MPEG-2 (ITU-T + ISO)

MPEG-4 (ISO)

Ogg-Theora

VP8-WebM

VC-1 (SMPTE)

Standards [link]

System M

System B

See also [link]

General

* Audio

* List of video topics

* Video clips

* Video editing

Video format

*Analog television

*Cable television

*Color space

*Digital television

*Digital video

*Film formats

*Interlaced

*Progressive scan

*Satellite television

*Telecine

*Television

*Timecode

*Video codec

Video usage

*Closed-circuit television

*Fulldome video

*Optical feedback

*Video art

*Interactive video

*Video production

*Video projector

*Video synthesizer

*Video teleconference

*Video communication

Video screen recording

*Screencast

Projects

*Redesign project

References [link]

External links [link]

Programmer's Guide to Video Systems: in-depth technical info on 480i, 576i, 1080i, 720p, etc.

Category:Digital television Category:Film and video technology Category:High-definition television Category:Video formats Category:Display technology Category:Television terminology Category:Video signal Category:History of television

ar:فيديو be-x-old:Відэа bg:Видео bs:Video ca:Vídeo cs:Video cy:Fideo da:Video de:Videotechnik et:Video es:Vídeo eo:Video eu:Bideo fa:ویدیو fr:Vidéo ga:Físeán gl:Vídeo gan:視頻 ko:비디오 hr:Video id:Video is:Myndband it:Video he:וידאו jv:Vidéo ku:Wîno ky:Видео mk:Видео ms:Video my:ဗီဒီယို nl:Video ja:映像信号 no:Video pl:Wideo pt:Vídeo ro:Video qu:Kuyuq rikch'a waqaychana allwiya ru:Видео scn:Video simple:Video sk:Videotechnika sl:Video sr:Видео sh:Video fi:Video sv:Video ta:நிகழ்படம் th:วิดีโอ tr:Görüntü Sinyali uk:Відео ur:منظرہ vi:Video wa:Videyo yi:ווידעא zh:视频