Video
From Freepedia
Video (from Latin, "I see") is the technology of processing electronic signals for representing moving pictures.
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Description
The term "video" commonly refers to related types of carrier formats — which can either be digital (DVD, QuickTime, MPEG-4) or analog videotape (VHS, Betamax). Video can be created with mechanical cameras (capture on celluloid film), video cameras (capture electric signal in PAL or NTSC formats) or digital cameras (commonly capture in digital MPEG-4 or DV formats). Quality of video commonly essentially depend on capturing and storing type. The last type of video — digital television (DTV) has a higher quality and now the become a mass standard (See List of digital television deployments by country)
New next generation of video was appearing at the end of 20 Century is 3D-video — digital video really in three dimensions. Such type of video was appearing with first cameras with realtime depth measurement. Six or eight such cameras can capture 3D-video stream. Format of this type of video was fixed in MPEG-4 Part 16 Animation Framework eXtension (AFX).
In the UK, 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 stream
Number of frames per second
Number of frames per second (FPS or frame rate): Can be from 6–8 fps for old mechanical cameras to 120 fps and more for new professional cameras. For PAL (Europe) and SECAM (France) standards it is 25 fps, for NTSC (North America) — 30 fps. Minimum number of frames for illusion of moving picture is about 10 fps.
Interlacing
Video can be Interlaced or progressive. Interlacing was originally conceived as a way to achieve good visual quality within the limitations of a narrow bandwidth. Every frame of interlaced video has 2 fields. One field contains only the odd-numbered lines (forming the odd field), and the next contains only even-numbered lines (forming the even field). NTSC, PAL and SECAM are interlaced formats. Interlacing commonly noted as i character near resolution (e.g. 576i50).
In progressive scan systems, each frame is complete (i.e. includes all scan lines). The result is a much higher perceived resolution.
A procedure known as deinterlacing can be used for converting an interlaced stream (analog, DVD, satellite) for progressive devices (TFT TV-sets, projectors, Plasma panels). Any deinterlacing inevitably decreases video quality.
Video resolution
Size of video image in pixels (for digital video) or lines (for analog video). Standard-definition television (SDTV) refer to 640×480i60 (NTSC) and 720×576i50 (PAL, SÉCAM) resolution. New High-definition television (HDTV) define resolutions up to 1920×1080i50.
Video resolution for 3D-video measured in voxels (volume 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 PDA's.
Aspect ratio
Aspect ratio describes the squareness 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 television uses an aspect of 16:9, or about 1.78:1. The aspect ratio of a full 35 mm film frame with soundtrack (also known as "Academy standard") is around 1.37:1.
Pixels on computer monitors are usually square, but pixels used in digital video have non-square aspect ratios, such 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 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 SÉCAM television.
The number of distinct colours that can be represented by a pixel depends on the number of bits per pixel (bpp). Very common way to reduce number of bits per pixel in digital video is chroma subsampling (for example 4:4:4, 4:2:2, 4:2:0).
Stereoscopic
Video with 2 channels — right and left for right and left eye. See articles Stereoscopy and 3-D film.
Video quality
Quality of video can be measured with formal metrics like PSNR or with experts subjective video quality).
Assume that you want to evaluate subjective video quality of a video processing system. To do this you should go through following steps:
- Choose video sequences for testing (they are often named SRC).
- Choose settings of system that you want to evaluate (often named HRC).
- Choose test method (how sequences are presented to experts and how their opinion is collected).
- Invite sufficient number of experts (not less than 15 are recommended).
- Carry out testing.
- Calculate average marks for each HRC based on experts' opinion.
Many of subjective video quality methods are described in ITU-T recommendation BT.500. One of standardized methods is DSIS — Double Stimulus Impairment Scale: expert is presented with unimpaired reference video, than with the same video impaired, and after that he is asked to vote on the second video using impairment scale (scale from "impairments are imperceptible" to "impairments are very annoying").
Video compression method (for digital only)
Compression method (standard) used for video. Video data contains spatial and temporal redundancy. Similarities can thus be encoded by merely registering differences within a frame (spatial redundancy, named intraframe compression and very close to image compression) and/or between frames (temporal redundancy, named interframe compression use motion compensation and other techniques). Most common standards are MPEG-2 — used for DVD and Satellite television and MPEG-4 — used for home video.
Bitrate (for digital only)
Bitrate is a measure of video stream "speed". It is quantified using the bit per second (bit/s) unit. Bigger bitrate allows better video quality. For example: 1 Mbit/s – VHS quality, 5 Mbit/s – DVD quality 10 Mbit/s – HDTV quality. Several types of bitrate strategies can be used. To maximize visual video quality variable bitrate (VBR) is used. On fast motion scenes it use more bits, on slow motion - less, we can see constant quality. For video streaming with limited channels (e.g. videoconferencing) constant bitrate (CBR) is used.
Video formats
| Video Display Standards | Video Connection Standards |
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Analog Tape Formats (See Analog television) |
Digital Tape Formats (See Digital video) |
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Optical Disc Storage Formats |
Digital Encoding Formats |
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See also
- Generally
- Video format
- Video usage
