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Video

Video is the technology of electronics|electronically Videography|capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing Scene (film)|scenes in motion.

History

Video technology was first developed for cathode ray tube television systems, but several new technologies for video display devices have since been invented. Standards for television sets and computer monitors have tended to evolve independently, but advances in computer performance and digital television broadcasting and recording are producing some convergence. Computers can now display television and film-style video clips and streaming media, encouraged by increased processor speed, storage capacity, and broadband access to the Internet. General purpose computing hardware can now be used to capture, store, edit, and transmit television and movie content, as opposed to older dedicated analog technologies.

Description of video

The term ''video'' ("video" meaning "I see", from the Latin verb "videre") commonly refers to several storage formats for moving pictures: digital (signal)|digital video formats, including Blu-ray Disc, DVD, QuickTime, and MPEG-4; and analog (signal)|analog videotapes, including VHS and Betamax. Video can be recorded and transmitted in various physical media: in magnetic tape when recorded as PAL or NTSC electric signals by video cameras, or in MPEG-4 or DV digital media when recorded by digital cameras. Video quality|Quality of video essentially depends on the capturing method and storage used. Digital television ''(DTV)'' is a relatively recent format with higher quality than earlier television formats and has become a standard for television video. ''(See List of digital television deployments by country.)'' 3D-video, digital video in 3-D film|three dimensions, premiered at the end of 20th century. Six or eight cameras with realtime depth measurement are typically used to capture ''3D-video'' streams. The format of ''3D-video'' is fixed in MPEG-4 Part 16 Animation Framework eXtension (AFX). In the United Kingdom, Estonia, Australia, Netherlands, Finland, Hungary and New Zealand, the term ''video'' is often used informally to refer to both Videocassette recorders and videotape|video cassettes; the meaning is normally clear from the context.

Characteristics of video streams

Number of frames per second

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 Persistence of vision|moving image is about fifteen frames per second.

Interlacing

Video can be interlaced or Progressive scan|progressive. Interlacing was invented as a way to achieve good visual quality within the limitations of a narrow bandwidth. The ''horizontal scan lines'' of each interlaced frame are numbered consecutively and partitioned into two ''fields'': the ''odd field'' (upper field) consisting of the odd-numbered lines and the ''even field'' (lower field) consisting of the even-numbered lines. 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 vertical line resolution, ''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. The result is a higher spatial resolution and a lack of various artifacts that can make parts of a stationary picture appear to be moving or flashing. A procedure known as deinterlacing can be used for converting an interlaced stream, such as analog, DVD, or satellite, to be processed by progressive scan devices, such as Liquid crystal display television|TFT TV-sets, projectors, and plasma panels. Deinterlacing cannot, however, produce a video quality that is equivalent to true progressive scan source material.

Display resolution

The size of a video image is measured in pixels for digital video, or horizontal scan lines and vertical lines of resolution for analog video. In the digital domain (e.g. DVD) standard-definition television (standard-definition television|SDTV) is specified as ''720/704/640×480i60'' for NTSC and ''768/720×576i50'' for PAL or SECAM resolution. However in the analog domain, the number of visible scanlines remains constant (486 NTSC/576 PAL) while the horizontal measurement varies with the quality of the signal: approximately 320 pixels per scanline for VCR quality, 400 pixels for TV broadcasts, and 720 pixels for DVD sources. Aspect ratio is preserved because of non-square "pixels". New high-definition televisions (high-definition television|HDTV) are capable of resolutions up to ''1920×1080p60'', i.e. 1920 pixels per scan line by 1080 scan lines, progressive, at 60 frames per second. Video resolution for 3D-video is measured in voxels (''volume picture element'', representing a value in three dimensional space). For example 512×512×512 voxels resolution, now used for simple 3D-video, can be displayed even on some personal digital assistant|PDAs.

Aspect ratio

Aspect ratio (image)|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

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 colours 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. 4:2:2#4:4:4|4:4:4, 4:2:2#4:2:2|4:2:2, 4:2:0#4:2:0|4:2:0/4:1:1#4:1:1|4:1:1).

Video quality

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 Scientific method|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)

A wide variety of methods are used to video compression|compress video streams. Video data contains spatial and temporal Redundancy (information theory)|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 and satellite television, and MPEG-4, used for home video.

Bit rate (digital only)

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 a bit rate of about 5 Mbit/s. HDTV has a still higher quality, with a bit rate of about 20 Mbit/s. Variable bit rate|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|constant bit rate (CBR) must be used.

Stereoscopic

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 polarization|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 overlayed 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 120Hz to 60Hz). Blu-ray Discs greatly improve the sharpness and detail of the two-color 3D effect in color coded stereo programs. See articles Stereoscopy and 3D film.

Video formats

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. There are a number of analog and digital tape formats, though digital video files can also be stored on a computer file system which have their own formats. In addition to the physical format used by the storage 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.

Video connectors, cables, and signal standards

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

Video display standards

Digital television

New formats for digital television broadcasts use the MPEG-2 video codec and include:
- ATSC Standards|ATSC - USA, Canada, Korea
- Digital Video Broadcasting|DVB - Europe
- ISDB - Japan, Brazil, Peru
- Digital Multimedia Broadcasting|DMB - Korea

Analog television

Analog television broadcast standards include:
- Field-sequential color system|FCS - USA, Russia; obsolete
- Multiplexed Analogue Components|MAC - Europe; obsolete
- Multiple sub-nyquist sampling Encoding system|MUSE - Japan
- NTSC - United States|USA, Canada, Japan
- PAL - Europe, Asia, Oceania
- PAL-M - PAL variation. Brazil
- 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 to cease