Electronic moving image
Not to be confused with
Vidio
.
Video
is an
electronic
medium for the recording, copying, playback, broadcasting, and display of
moving
visual
media
.
[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 media, including
radio broadcasts
,
magnetic tape
,
optical discs
,
computer files
, and
network streaming
.
Etymology
[
edit
]
The word
video
comes from the
Latin
video
(I see).
[2]
History
[
edit
]
Analog video
[
edit
]
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
mechanical television
systems, which were quickly replaced by
cathode-ray tube
(CRT)
television
systems. Video was originally exclusively
live
technology.
Charles Ginsburg
led an
Ampex
research team to develop one of the first practical
video 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 video is capable of higher quality and, eventually, a much lower cost than earlier analog technology. After the commercial introduction 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
to capture, store, edit, and transmit digital video, further reducing the cost of
video production
and allowing program-makers and broadcasters to move to
tapeless production
. The advent of
digital broadcasting
and the subsequent
digital television transition
are in the process of relegating analog video to the status of a
legacy technology
in most parts of the world. The development of high-resolution video 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 video technology to converge with film technology. Since 2013,
[update]
the use of
digital cameras
in
Hollywood
has surpassed the use of film cameras.
[6]
Characteristics of video streams
[
edit
]
Number of frames per second
[
edit
]
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
[
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 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
[
edit
]
Aspect ratio
describes the proportional relationship between the width and height of video screens and video picture elements. All popular video formats are
rectangular
, and this can be described by a ratio between width and height. The ratio of width to 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 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. The
720 by 480 pixel
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 video on mobile phones has led to the growth of
vertical video
. Mary Meeker, a partner at Silicon Valley venture capital firm
Kleiner Perkins Caufield & Byers
, highlighted the growth of vertical video viewing in her 2015 Internet Trends Report – growing from 5% of video viewing in 2010 to 29% in 2015. Vertical video ads like
Snapchat
's are watched in their entirety nine times more frequently than landscape video ads.
[15]
Color model and depth
[
edit
]
The
color model
uses the video color representation and maps encoded color values to 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 video.
[16]
[17]
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
[
edit
]
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)
[
edit
]
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
[
edit
]
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 filters
90 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
LCD shutter glasses
that synchronize to the video to alternately block the image for 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 effective video framerate by a factor of two.
Formats
[
edit
]
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
[
edit
]
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
television production
applications.
Digital video
[
edit
]
Digital video
signal formats have been adopted, including
serial digital interface
(SDI),
Digital Visual Interface
(DVI),
High-Definition Multimedia Interface
(HDMI) and
DisplayPort
Interface.
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 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
[
edit
]
Digital television
[
edit
]
Digital television
broadcasts use the
MPEG-2
and other
video coding formats
and include:
Analog television
[
edit
]
Analog television
broadcast standards include:
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
[
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.
Recording
[
edit
]
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]
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]
Digital encoding formats
[
edit
]
A video codec is
software
or
hardware
that
compresses
and
decompresses
digital video
. In the context of video 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
. The compressed data format usually conforms to a standard
video coding format
. The compression is typically
lossy
, meaning that the compressed video lacks some information present in the original video. A consequence of this is that decompressed video has lower quality than the original, uncompressed video because there is insufficient information to accurately reconstruct the original video.
[27]
See also
[
edit
]
- General
- Video format
- Video usage
- Video screen recording software
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[
edit
]
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External links
[
edit
]
Wikimedia Commons has media related to
Video
.
Wikiquote has quotations related to
Video
.
|
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Television
|
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Analog
| 405 lines
| |
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525 lines
| |
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625 lines
|
- System B
,
C
,
D
,
G
,
H
,
I
,
K
,
L
,
N
- Color systems:
PAL
,
PAL-N
,
PALplus
,
SECAM
- MAC
|
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819 lines
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1125 lines
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1250 lines
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Audio
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Hidden signals
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Historical
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Digital
| |
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Free
| |
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Rental and purchase
| |
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Others
| |
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Discontinued
| |
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International
| |
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National
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Other
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