video là gì

This article is about the storage and reproduction medium. For their nội dung and production, see Film and Footage. For other uses, see Video (disambiguation).

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Not đồ sộ be confused with Vidio.

A one-minute animated đoạn phim showing stages of truyền thông production

Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual truyền thông.[1] Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) systems which, in turn, were replaced by flat panel displays of several types.

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 truyền thông, including radio broadcasts, magnetic tape, optical discs, computer files, and network streaming.


The word video comes from the Latin video (I see). [2]


Analog video[edit]

NTSC composite đoạn phim signal (analog)

Video was invented decades after film, which records a sequence of miniature images visible đồ sộ the eye when the film is physically examined. Video, by contrast, encodes images with electromagnetic waves.[3]

Video technology was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) television systems. Video was originally exclusively a live technology. Charles Ginsburg led an Ampex research team developing one of the first practical đoạn phim tape recorders (VTR). In 1951, the first VTR captured live images from television cameras by writing the camera's electrical signal onto magnetic videotape.

Video recorders were sold for US$50,000 in 1956, and videotapes cost US$300 per one-hour reel.[4] However, prices gradually dropped over the years; in 1971, Sony began selling videocassette recorder (VCR) decks and tapes into the consumer market.[5]

Digital video[edit]

Digital đoạn phim is capable of higher quality and, eventually, a much lower cost than thở earlier analog technology. After the invention of the DVD in 1997, and later the Blu-ray Disc in 2006, sales of videotape and recording equipment plummeted. Advances in computer technology allow even inexpensive personal computers and smartphones đồ sộ capture, store, edit and transmit digital đoạn phim, further reducing the cost of đoạn phim production and allowing program-makers and broadcasters đồ sộ move đồ sộ tapeless production. The advent of digital broadcasting and the subsequent digital television transition are in the process of relegating analog đoạn phim đồ sộ the status of a legacy technology in most parts of the world. The development of high-resolution đoạn phim cameras with improved dynamic range and color gamuts, along with the introduction of high-dynamic-range digital intermediate data formats with improved color depth, has caused digital đoạn phim technology đồ sộ converge with film technology. Since 2013, the usage of digital cameras in Hollywood has surpassed the use of film cameras.[6]

Characteristics of đoạn phim streams[edit]

Number of frames per second[edit]

Frame rate, the number of still pictures per unit of time of đoạn phim, ranges from six or eight frames per second (frame/s) for old mechanical cameras đồ sộ 120 or more frames per second for new professional cameras. PAL standards (Europe, Asia, nước Australia, etc.) and SECAM (France, Russia, parts of Africa etc.) specify 25 frame/s, while NTSC standards (United States, Canada, nhật bản, etc.) specify 29.97 frame/s.[7] Film is shot at the slower frame rate of 24 frames per second, which slightly complicates the process of transferring a cinematic motion picture đồ sộ đoạn phim. The minimum frame rate đồ sộ achieve a comfortable illusion of a moving image is about sixteen frames per second.[8]

Interlaced vs progressive[edit]

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 optimum spatial resolution of both the stationary and moving parts of the image. Interlacing was invented as a way đồ sộ reduce flicker in early mechanical and CRT đoạn phim displays without increasing the number of complete frames per second. Interlacing retains detail while requiring lower bandwidth compared đồ sộ progressive scanning.[9][10]

In interlaced đoạn phim, 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 thở 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 đoạn phim resolution specifications often include an i đồ sộ indicate interlacing. For example, PAL đoạn phim 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, overall spatial resolution is degraded by simple line doubling—artifacts such as flickering or "comb" effects in moving parts of the image which appear unless special signal processing eliminates them.[9][12] A procedure known as deinterlacing can optimize the display of an interlaced đoạn phim signal from an analog, DVD or satellite source on a progressive scan device such as an LCD television, digital đoạn phim projector or plasma panel. Deinterlacing cannot, however, produce đoạn phim quality that is equivalent đồ sộ true progressive scan source material.[10][11][12]

Aspect ratio[edit]

Comparison of common cinematography and traditional television (green) aspect ratios

Aspect ratio describes the proportional relationship between the width and height of đoạn phim screens and đoạn phim picture elements. All popular đoạn phim formats are rectangular, and ví can be described by a ratio between width and height. The ratio width đồ sộ height for 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.[13][14]

Pixels on computer monitors are usually square, but pixels used in digital đoạn phim often have non-square aspect ratios, such as those used in the PAL and NTSC variants of the CCIR 601 digital đoạn phim standard, and the corresponding anamorphic widescreen formats. The 720 by 480 px raster uses thin pixels on a 4:3 aspect ratio display and fat pixels on a 16:9 display.[13][14]

The popularity of viewing đoạn phim on mobile phones has led đồ sộ the growth of vertical đoạn phim. Mary Meeker, a partner at Silicon Valley venture capital firm Kleiner Perkins Caufield & Byers, highlighted the growth of vertical đoạn phim viewing in her năm ngoái Internet Trends Report – growing from 5% of đoạn phim viewing in 2010 đồ sộ 29% in năm ngoái. Vertical đoạn phim ads lượt thích Snapchat’s are watched in their entirety nine times more frequently than thở landscape đoạn phim ads.[15]

Color model and depth[edit]

Example of U-V color plane, Y value=0.5

The color model the đoạn phim color representation and maps encoded color values đồ sộ visible colors reproduced by the system. There are several such representations in common use: typically YIQ is used in NTSC television, YUV is used in PAL television, YDbDr is used by SECAM television and YCbCr is used for digital đoạn phim.[16][17]

The number of distinct colors a px can represent depends on color depth expressed in the number of bits per px. A common way đồ sộ reduce the amount of data required in digital đoạn phim is by chroma subsampling (e.g., 4:4:4, 4:2:2, etc.). Because the human eye is less sensitive đồ sộ details in color than thở brightness, the luminance data for all pixels is maintained, while the chrominance data is averaged for a number of pixels in a block and that 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[edit]

Video quality can be measured with formal metrics lượt thích Peak signal-to-noise ratio (PSNR) or through subjective đoạn phim quality assessment using expert observation. Many subjective đoạn phim 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 đoạn phim followed by an impaired version of the same đoạn phim. The expert then rates the impaired đoạn phim using a scale ranging from "impairments are imperceptible" đồ sộ "impairments are very annoying".

Video compression method (digital only)[edit]

Uncompressed đoạn phim delivers maximum quality, but at a very high data rate. A variety of methods are used đồ sộ compress đoạn phim streams, with the most effective ones using a group of pictures (GOP) đồ sộ 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 đồ sộ 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 đoạn phim 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 filters 90 degrees off-axis from each other on two đoạn phim projectors. These separately polarized channels are viewed wearing eyeglasses with matching polarization filters.
  • Anaglyph 3 chiều where one channel is overlaid with two color-coded layers. This left and right layer technique is occasionally used for network broadcast or recent anaglyph releases of 3 chiều movies on DVD. Simple red/cyan plastic glasses provide the means đồ sộ view the images discretely đồ sộ size a stereoscopic view of the nội dung.
  • One channel with alternating left and right frames for the corresponding eye, using LCD shutter glasses that synchronize đồ sộ the đoạn phim đồ sộ alternately block the image đồ sộ each eye, ví 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 effective đoạn phim framerate by a factor of two.


Different layers of đoạn phim transmission and storage each provide their own mix of formats đồ sộ choose from.

For transmission, there is a physical connector and signal protocol (see List of đoạn phim connectors). A given physical links 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 đoạn phim clips can also be stored on a computer tệp tin system as files, which have their own formats. In addition đồ sộ 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 đoạn phim coding format, of which a number are available.

Analog video[edit]

Analog đoạn phim is a đoạn phim signal represented by one or more analog signals. Analog color đoạn phim 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 đoạn phim. Analog đoạn phim may be carried in separate channels, as in two channel S-Video (YC) and multi-channel component đoạn phim formats.

Analog đoạn phim is used in both consumer and professional television production applications.

Digital video[edit]

Digital đoạn phim 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)

  • HDMI

  • DisplayPort

Transport medium[edit]

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 đoạn phim 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[edit]

Digital television[edit]

Digital television broadcasts use the MPEG-2 and other đoạn phim coding formats and include:

  • ATSC – United States, Canada, Mexico, Korea
  • Digital Video Broadcasting (DVB) – Europe
  • ISDB – Japan
    • ISDB-Tb – uses the MPEG-4 đoạn phim coding format – Brazil, Argentina
  • Digital Multimedia Broadcasting (DMB) – Korea

Analog television[edit]

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
    • PAL-M – PAL variation, Brazil
    • PAL-N – PAL variation, Argentina, Paraguay and Uruguay
    • PALplus – PAL extension, Europe
  • 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 đoạn phim format consists of more information than thở the visible nội dung 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[edit]

Computer display standards specify a combination of aspect ratio, display size, display resolution, color depth, and refresh rate. A list of common resolutions is available.


A VHS đoạn phim cassette tape.

Early television was almost exclusively a live medium with some programs recorded đồ sộ film for distribution of historical purposes using Kinescope. The analog đoạn phim tape recorder was commercially introduced in 1951. The following list is in rough chronological order. All formats listed were sold đồ sộ and used by broadcasters, đoạn phim producers or consumers; or were important historically.[22][23]

Digital đoạn phim tape recorders offered improved quality compared đồ sộ analog recorders.[23][25]

Optical storage mediums offered an alternative, especially in consumer applications, đồ sộ bulky tape formats.[22][26]

  • Blu-ray Disc (Sony)
  • China Blue High-definition Disc (CBHD)
  • DVD (was Super Density Disc, DVD Forum)
  • Professional Disc
  • Universal Media Disc (UMD) (Sony)
  • Enhanced Versatile Disc (EVD, Chinese government-sponsored)
  • HD DVD (NEC and Toshiba)
  • HD-VMD
  • Capacitance Electronic Disc
  • Laserdisc (MCA and Philips)
  • Television Electronic Disc (Teldec and Telefunken)
  • VHD (JVC)
  • Video CD

Digital encoding formats[edit]

A đoạn phim codec is software or hardware that compresses and decompresses digital đoạn phim. In the context of đoạn phim compression, codec is a portmanteau of encoder and decoder, while a device that only compresses is typically called an encoder, and one that only decompresses is a decoder.[27] The compressed data format usually conforms đồ sộ a standard đoạn phim coding format. The compression is typically lossy, meaning that the compressed đoạn phim lacks some information present in the original đoạn phim. A consequence of this is that decompressed đoạn phim has lower quality than thở the original, uncompressed đoạn phim because there is insufficient information đồ sộ accurately reconstruct the original đoạn phim.[27]

See also[edit]

Video format
Video usage
Video screen recording software


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  2. ^ "video", Online Etymology Dictionary
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External links[edit]

Wikimedia Commons has truyền thông related đồ sộ Video.

Wikiquote has quotations related đồ sộ Video.

Xem thêm: make off là gì

  • Video as Arts at Curlie
  • Video as Media Production at Curlie
  • Programmer's Guide đồ sộ Video Systems: in-depth technical info on 480i, 576i, 1080i, 720p, etc.
  • Format Descriptions for Moving Images