Video Basics
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Analog video is produced by `scanning' in televisions, for example
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2D info from the camera input is converted to 1D electrical signal
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This signal is scanned from LHS top to RHS bottom of the screen
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Three terms are important, Resolution, Flicker & Bandwidth:
Let N be the number of lines/picture, R the number of lines scanned
per second. The spatial resolution increases with N, temporal resolution
increases with R, and flicker decreases with R.
High resolution, low flicker needs high bandwidth, there is usually
a tradeoff between these factors.
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Progressive & Interlaced Scanning
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In Progressive scanning, one image is scanned on the screen at a time.
This is the kind of scanning that is used in computers and HDTV (usually),
because they require high spatial and temporal resolution. This kind of
scanning is also called non-interlaced scanning. Computer screen refresh
rates are always greater than 60 fps nowadays, typical values are about
72 fps
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In Interlaced scanning, two images are scanned at a time (the required
bandwidth is therefore halved, which is desirable). This technique is used
in TV broadcasting
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MPEG-2 encoding of HDTV allows interlaced scanning (this was originally
called MPEG-3, but was folded into MPEG-2 because the MPEG-2 standard could
easily fulfill these requirements itself)
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Colour components
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The camera produces three colours: red, green and blue, this is the RGB
colour system, which is processed for transmission and storage. A new set
of colour signals is generated. There are three main colour systems used
globally
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The 3 standard Source Image Formats (SIFs) for TV broadcasting:
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NTSC (National Television Standards Committee) - North America
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PAL (Phase Alternating Line) - UK, Europe and Commonwealth countries
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SECAM (Systeme Electronique Couleur Avec Memoire) - France, East Europe
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The principle of colour systems coding: the human eye is less sensitive
to chrominance (changes in colour) than luminance (change in brightness)
so that the former can usually be encoded with lesser resolution - this
is a smart way of data compression without actually decreasing image quality
as far as human eyes are concerned!
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Example of a colour system: The YUV system Y is the luminance, U and V
are the chrominance components. The formula for YUV is very similar to
the Stokes Parameters in radiative physics:
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Y=aR+bG+cG, where a,b,c are rational numbers such that a+b+c=1, R,G and
B are the red green and blue colours of the pixel/group
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U=R-Y
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V=B-Y
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Now the U and V values can be sampled with lower resolution - this means
that instead of writing down the values for EACH pixel, we can write the
values of U and V for groups of four pixels each, while Y can be sampled
for each pixel!
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To digitize analog video: the images are filtered, then sampled and quantized.
For more information on these processes, go to the related
links section
Go
to the next page, or use one of the following quick links:
Main
Page
Motivation
For Compression - Some True Stories
A
Brief History of Compression (heheh - brief, compress... get it?)
Requirements
From Any Compression Algorithm
Data
Compression Fundamentals
Some
Compression Techniques
Video
Compression Techniques: The MPEG-1 Standard
The
Future: MPEG-4 And MPEG-7
Related
Links
Contact Me: sundar@pha.jhu.edu
Copyright © Sundar Srinivasan
2002