Encyclopedia
Video is the technology of capturing, recording, processing, transmitting, and reconstructing
moving pictures, typically using
magnetic tape, electronic signals, or digital media, primarily for viewing on
television or as
video clips on
computer monitors. It is to be differentiated from
cinema, which records images on celluloid film.
Description
The term
video commonly refers to several storage formats for moving pictures: digital video formats, including
DVD,
QuickTime, and MPEG-4; and 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.
Quality of video essentially depends on the capturing method and storage used. Digital television
' is a relatively recent format with higher quality than earlier television formats and has become a standard for television video. '
3D-video, digital video in
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 .
In the
UK,
Australia, and
New Zealand, the term
video is often used informally to refer to both
video recorders and
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 for old mechanical cameras to 120 or more frames per second for new professional cameras.
PAL and
SECAM standards specify 25 fps, while
NTSC specifies 29.97 fps. Film is shot at the slower frame rate of 24fps, which complicates slightly the process of transferring a cinematic motion picture to video. To achieve the illusion of a moving image, the minimum frame rate is about ten frames per second.
Interlacing
Video can be
interlaced or
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 consisting of the odd-numbered lines and the
even 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 per second.
In
progressive scan systems, each refresh period updates all of the scan lines. The result is a higher perceived 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 TFT TV-sets, projectors, and plasma panels. Deinterlacing cannot, however, produce a video quality that is equivalent to true progressive scan source material.
Video resolution
The size of a video image is measured in
pixels for digital video or horizontal scan lines for analog video. Standard-definition television is specified as
720/704/640×480i60 for NTSC and
768/720×576i50 for PAL or SECAM resolution. New high-definition televisions 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 . For example 512×512×512 voxels resolution, now used for simple 3D-video, can be displayed even on some
PDAs.
Aspect ratio
Aspect ratio describes the dimensions of video screens and video picture elements. The screen aspect ratio of a traditional television screen is 4:3, or 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 is around 1.37:1.
Pixels on computer monitors are usually square, but pixels used in digital video 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.
Color space and bits per pixel
Color model name describes the video color representation.
YIQ is used in NTSC television. It corresponds closely to the
YUV scheme used in 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 . A common way to reduce the number of bits per pixel in digital video is by
chroma subsampling .
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 to use for testing.
- Choose the settings of the system to evaluate .
- 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 . 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
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 and
satellite television, and MPEG-4, used for home video.
Bit rate
Bit rate is a measure of the rate of information content in a video stream. It is quantified using the
bit per second unit or
Megabits per second . 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 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 must be used.
Stereoscopic
Stereoscopic video requires either two channels — a right channel for the right eye and a left channel for the left eye or 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. New
HD DVD and HD
Blu-ray disks will greatly improve the 3D effect, in color coded stereo programs. The first commercially available HD players are expected to debut at the 2006
NAB Show in Las Vegas in April. See articles
Stereoscopy and
3-D film.
Video formats
See also
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