Video
Video is an
Video systems vary in display resolution, aspect ratio, refresh rate, color capabilities, and other qualities. Analog and digital variants exist and can be carried on a variety of media, including radio broadcasts, magnetic tape, optical discs, computer files, and network streaming.
Etymology
The word video comes from the Latin video (I see).[2]
History
Analog video
Video developed from facsimile systems developed in the mid-19th century. Early mechanical video scanners, such as the Nipkow Disk were patented as early as 1884, however, it took several decades before practical video systems could be developed, many decades after film. Film records using a sequence of miniature photographic images visible to the eye when the film is physically examined. Video, by contrast, encodes images electronically, turning the images into analog or digital electronic signals for transmission or recording.[3]
Video technology was first developed for
Video recorders were sold for US$50,000 in 1956, and videotapes cost US$300 per one-hour reel.
Digital video
Digital video is capable of higher quality and, eventually, a much lower cost than earlier analog technology. After the commercial introduction of the
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 standards (Europe, Asia, Australia, etc.) and SECAM (France, Russia, parts of Africa, etc.) specify 25 frame/s, while NTSC standards (United States, Canada, Japan, etc.) specify 29.97 frame/s.[7] Film is shot at a slower frame rate of 24 frames per second, which slightly complicates the process of transferring a cinematic motion picture to video. The minimum frame rate to achieve a comfortable illusion of a moving image is about sixteen frames per second.[8]
Interlaced vs. progressive
Video can be interlaced or progressive. In progressive scan systems, each refresh period updates all scan lines in each frame in sequence. When displaying a natively progressive broadcast or recorded signal, the result is the optimum spatial resolution of both the stationary and moving parts of the image. 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. Interlacing retains detail while requiring lower bandwidth compared to progressive scanning.[9][10]
In interlaced video, 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, 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 lifelike reproduction of rapidly moving parts of the image when viewed on an interlaced CRT display.[9][10]
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 described as 576i50, where 576 indicates the total number of horizontal scan lines, i indicates interlacing, and 50 indicates 50 fields (half-frames) per second.[10][11]
When displaying a natively interlaced signal on a progressive scan device, the overall spatial resolution is degraded by simple line doubling—artifacts, such as flickering or "comb" effects in moving parts of the image that appear unless special signal processing eliminates them.[9][12] A procedure known as deinterlacing can 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.[10][11][12]
Aspect ratio
The popularity of viewing video on mobile phones has led to the growth of
Color model and depth
The
The number of distinct colors a pixel can represent depends on the color depth expressed in the number of bits per pixel. A common way to reduce the amount of data required in digital video is by chroma subsampling (e.g., 4:4:4, 4:2:2, etc.). Because the human eye is less sensitive to details in color than brightness, the luminance data for all pixels is maintained, while the chrominance data is averaged for a number of pixels in a block, and the same value is used for all of them. For example, this results in a 50% reduction in chrominance data using 2-pixel blocks (4:2:2) or 75% using 4-pixel blocks (4:2:0). This process does not reduce the number of possible color values that can be displayed, but it reduces the number of distinct points at which the color changes.[11][16][17]
Video quality
Video quality can be measured with formal metrics like peak signal-to-noise ratio (PSNR) or through subjective video quality assessment using expert observation. Many subjective video quality methods are described in the ITU-T recommendation BT.500. One of the standardized methods 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)
Uncompressed video delivers maximum quality, but at a very high data rate. A variety of methods are used to compress video streams, with the most effective ones using a group of pictures (GOP) to reduce spatial and temporal redundancy. 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 compression standards are MPEG-2, used for DVD, Blu-ray, and satellite television, and MPEG-4, used for AVCHD, mobile phones (3GP) and the Internet.[18][19]
Stereoscopic
Stereoscopic video for 3D film and other applications can be displayed using several different methods:[20][21]
- 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 filters90 degrees off-axis from each other on two video projectors. These separately polarized channels are viewed wearing eyeglasses with matching polarization filters.
- Anaglyph 3D where one channel is overlaid with two color-coded layers. This left and right layer technique is occasionally used for network broadcasts 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 and right frames for the corresponding eye, using Cave Automatic Virtual Environment, but reduces effective video framerate by a factor of two.
Formats
Different layers of video transmission and storage each provide their own set of formats to choose from.
For transmission, there is a physical connector and signal protocol (see List of video connectors). A given physical link can carry certain display standards that 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 coding format, for which a number is available.
Analog video
Analog video is a video signal represented by one or more analog signals. Analog color video signals include luminance, brightness (Y) and chrominance (C). When combined into one channel, as is the case among others with NTSC, PAL, and SECAM, it is called composite video. Analog video may be carried in separate channels, as in two-channel S-Video (YC) and multi-channel component video formats.
Analog video is used in both consumer and professional
-
Composite video
(single channel RCA) -
S-Video
(2-channel YC)
Digital video
Digital video signal formats have been adopted, including serial digital interface (SDI), Digital Visual Interface (DVI), High-Definition Multimedia Interface (HDMI) and DisplayPort Interface.
-
Serial digital interface (SDI)
-
Digital Visual Interface (DVI)
Transport medium
Video can be transmitted or transported in a variety of ways including wireless terrestrial television as an analog or digital signal, coaxial cable in a closed-circuit system as an analog signal. Broadcast or studio cameras use a single or dual coaxial cable system using serial digital interface (SDI). See List of video connectors for information about physical connectors and related signal standards.
Video may be transported over networks and other shared digital communications links using, for instance, MPEG transport stream, SMPTE 2022 and SMPTE 2110.
Display standards
Digital television
Digital television broadcasts use the MPEG-2 and other video coding formats and include:
- ATSC – United States, Canada, Mexico, Korea
- Digital Video Broadcasting (DVB) – Europe
- ISDB – Japan
- Digital Multimedia Broadcasting (DMB) – Korea
Analog television
Analog television broadcast standards include:
- Field-sequential color system (FCS) – US, Russia; obsolete
- Multiplexed Analogue Components (MAC) – Europe; obsolete
- Multiple sub-Nyquist sampling encoding (MUSE) – Japan
- NTSC – United States, Canada, Japan
- EDTV-II "Clear-Vision" - NTSC extension, Japan
- PAL – Europe, Asia, Oceania
- RS-343(military)
- SECAM – France, former Soviet Union, Central Africa
- CCIR System A
- CCIR System B
- CCIR System G
- CCIR System H
- CCIR System I
- CCIR System M
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 metadata and synchronization information. 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.
Computer displays
Recording
Early television was almost exclusively a live medium, with some programs recorded to film for historical purposes using Kinescope. The analog video tape recorder was commercially introduced in 1951. The following list is in rough chronological order. All formats listed were sold to and used by broadcasters, video producers, or consumers; or were important historically.[22][23]
- VERA (BBCexperimental format ca. 1952)
- 2" Quadruplex videotape (Ampex1956)
- 1" Type A videotape (Ampex)
- 1/2" EIAJ (1969)
- U-matic 3/4" (Sony)
- 1/2" Cartrivision (Avco)
- VCR, VCR-LP, SVR
- Robert Bosch GmbH)
- )
- Betamax (Sony)
- VHS (JVC)
- Video 2000 (Philips)
- 2" Helical Scan Videotape (IVC)
- 1/4" CVC (Funai)
- Betacam (Sony)
- HDVS (Sony)[24]
- Betacam SP(Sony)
- Video8 (Sony) (1986)
- S-VHS (JVC) (1987)
- VHS-C (JVC)
- Pixelvision (Fisher-Price)
- UniHi 1/2" HD (Sony)[24]
- Hi8(Sony) (mid-1990s)
- W-VHS (JVC) (1994)
Digital video tape recorders offered improved quality compared to analog recorders.[23][25]
Optical storage mediums offered an alternative, especially in consumer applications, to bulky tape formats.[22][26]
- Blu-ray Disc (Sony)
- China Blue High-definition Disc (CBHD)
- Super Density Disc, DVD Forum)
- Professional Disc
- Universal Media Disc (UMD) (Sony)
- Enhanced Versatile Disc (EVD, Chinese government-sponsored)
- NEC and Toshiba)
- HD-VMD
- Capacitance Electronic Disc
- MCA and Philips)
- Television Electronic Disc (Teldec and Telefunken)
- VHD (JVC)
- Video CD