The fundamentals of video signals
• A color space is a mathematical representation of a
set of colors.
• The three most popular color models are RGB (used
in computer graphics), YIQ, YUV or YCbCr(used in
video systems) and CMYK (used in color printing).
• However, none of these color spaces are directly
related to the intuitive notions of hue, saturation and
brightness.
• All of the color spaces can be derived from the RGB
information supplied by devices such as cameras and
scanners.
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• Red, green and blue are three primary additive colors
(individual components are added together to form a
desired color) and are represented by a threedimensional, Cartesian coordinate system, Figure 1.
• The indicated diagonal of the cube, with equal
amounts of each primary component, represents
various gray levels.
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BLUE
CYAN
MAGENTA
WHITE
BLACK
RED
GREEN
YELLOW
Figure 1a. The RGB color cube.
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Figure 1b. The RGB color space.
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• Table 1 contains the RGB values for color bars.
Nominal
range
white
yellow
cyan
green
magenta
red
blue
black
R
0 to 255
255
255
0
0
255
255
0
0
G
0 to 255
255
255
255
255
0
0
0
0
B
0 to 255
255
0
255
0
255
0
255
0
Table 1. 100% RGB color bars.
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• The wavelengths our eyes can detect is only a small
portion of the electromagnetic energy spectrum.
• It is called the visible light spectrum.
• At one end of the visible spectrum are the short
wavelengths of light we perceive as blue.
• At the other end of the visible spectrum are the
longer wavelengths of light we perceive as red.
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• All the other colors we can see in nature are found
somewhere along the spectrum between blue and
red.
• Beyond the limits at each end of the visible spectrum
are the short wavelengths of ultraviolet light and
Xrays and the long wavelengths of infrared radiation
and radio waves, which are not visible to the human
eye.
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• If the visible portion of the light spectrum is divided
into thirds, the predominant colors are red, green and
blue.
• These three colors are considered the primary colors
of the visible light spectrum.
• Primary colors can be arranged in a circle, commonly
refered to as a color wheel.
• Red, green and blue (RGB) form a triangle on the
color wheel.
• In between the primary colors are the secondary
colors, cyan, magenta and yellow (CMY), which form
another triangle, figure 2.
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GREEN
60 o
YELLOW
120o
CYAN
0o
180o
RED
BLUE
300o
240o
MAGENTA
Figure 2. The color wheel.
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• The additive color system involves light emitted
directly from a source, before an object reflects the
light.
• The additive reproduction process mixes various
amounts of red, green and blue light to produce other
colors.
• Combining one of these additive primary colors with
another produces the additive secondary colors cyan,
magenta and yellow.
• Combining all three primary colors produces white.
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Source: Wikipedia
Figure 3. Additive color mixing: adding red to green yields yellow;
adding yellow to blue yields white.
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• Television and computer monitors create color using
the primary colors of light.
• Each pixel on a monitor screen starts out as black.
• When the red, green and blue phosphors of a pixel
are illuminated simultaneously, that pixel becomes
white.
• This phenomenon is called additive color.
• Thousands of red, green and blue phosphor dots
make up the images on video monitors.
• The phosphor dots emit light when activated
electronically, and it is the combination of different
intensities of red, green and blue phosphor dots that
produces all the colors on a video monitor.
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• All image capture devices utilize the additive color
system to gather the information needed to
reproduce a color image.
• However, none of these color spaces are directly
related to the intuitive notions of hue, saturation and
brightness.
• Hue is the perceptual attribute associated with
elementary color names.
• Hue enables us to identify basic colors, such as blue,
green, yellow, red and purple.
• People with normal color vision report that hues
follow a natural sequence based on their similarity to
one another.
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• Lightness corresponds to how much light appears to
be reflected from a surface in relation to nearby
surfaces.
• Lightness, like hue, is perceptual attribute that cannot
be computed from physical measurements alone.
• It is the most important attribute in making contrast
more effective.
• Saturation is the degree of color intensity associated
with a color’s perceptual difference from a white,
black or gray of equal lightness.
• Figure 3 summarizes these three attributes.
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Source: Wikipedia
Figure 3a. Hue, lightness and saturation, the three perceptual attributes of light.
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LOW LIGHTNESS
GREEN
60 o
120o
YELLOW
0o
CYAN
180o
RED
SATURATION
300o
BLUE
240o
MAGENTA
HUE
HIGH LIGHTNESS
Figure 3b. Hue, lightness and saturation, the three perceptual attributes of light.
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• The YUV color space is used by the PAL (Phase
Alternation Line), NTSC (National Television System
Committee) and SECAM (Sequentiel Couleur Avec
Memoire or Sequential Color with Memory)
composite color video standards, Figure 4.
• The black-and-white system used only luma (Y)
information, color information (U and V) was added in
such a way that a black-and-white receiver would still
display a normal black-and-white picture.
• Color receivers decoded the additional color
information to display a color picture.
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Figure 4. YUV color space.
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• The basic equations to convert between gammacorrected RGB (notated as R'G ' B)' and YUV are:
Y  0.299 R'  0.587G '  0114
. B
'
'
U  0147
. R'  0.289G '  0.436 B'  0.492( B'  Y )
V  0.615 R'  0.515G '  0100
. B'  0.877( R'  Y )
R'  Y  1140
. V
G '  Y  0.395U  0.581V
B'  Y  2.032U
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• For digital R'G ' B'values with a range of 0-255, Y has a
range of 0-255, U a range of 0 to 112 , and V a
range of 0 to 157.
• These equations are usually scaled to simplify the
implementation in an actual NTSC or PAL digital
encoder or decoder.
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• The transfer function of most displays produces an
intensity that is proportional to some power (referred
to as gamma) of the signal amplitude.
• As a result, high-intensity ranges are expanded and
low-intensity ranges are compressed.
• This is an advantage in combatting noise, as the eye
is approximately equally sensitive to equally relative
intensity changes.
• By ”gamma correcting” the video signals before
display, the intensity output of the display is roughly
linear, and transmission-induced noise is reduced.
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• The YCbCr color space was developed as part of ITUR BT 601 during the development of a world-wide
digital component video standard, Figure 5.
• The color spaceYCbCr is a scaled and offset version
of the YUV color space.
• Y is defined to have a nominal 8-bit range of 16-235, Cb
and Cr are defined to have a nominal range of 16240.
• There are several YCbCr sampling formats, such as
4:2:2 and 4:2:0.
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Figure 5a. YCbCr color space.
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Source: Wikipedia
Figure 5b. A colour image and the Y, Cb and Cr elements of it. Note that
the Y image is essentially a greyscale copy of the main image;
that the white snow is represented as a middle value in both
Cr and Cb; that the brown barn is represented by weak Cb and
strong Cr; that the green grass is represented by strong Cb and
weak Cr; and that the blue sky is represented by strong Cb and
weak Cr. The murkiness of the Cb and Cr elements (to the human
eye) demonstrate why many image compression codecs
downsample colour; details in Y are much more visible than in Cb or Cr.
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• The basic equations to convert between 8-bit digital
R'G ' B' data with a 16-235 nominal range and YCbCr
are:
Y601  0.299 R'  0.587G '  0114
. B'
Cb  0172
. R'  0.339G '  0.511B'  128
Cr  0.511R'  0.428G '  0.083B'  128
R'  Y601  1371
. ( Cr  128)
G '  Y601  0.698( Cr  128)  0.336( Cb  128)
B'  Y601  1732
. ( Cb  128)
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• When performing YCbCrto R'G ' B'conversion, the
'
resulting R'G ' Bvalues
have a nominal range of 16235, with possible occasional excursions into the 015 and 236-255 values.
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• Television services in Europe currently broadcast
video at a frame rate of 25 Hz.
• Each frame consists of two interlaced fields, giving a
field rate of 50 Hz.
• The first field of each frame contains only the odd
numbered lines of the frame (numbering the top
frame line as line 1).
• The second field contains only the even numbered
lines of the frame and is sampled in the video camera
20 msec after the first field.
• It is important to note that one interlaced frame
contains fields from two instants in time.
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• American television is similarly interlaced but with a
frame rate of just under 30 Hz.
• In video systems other than television, non-interlaced
video is commonplace (for example, most computers
output non-interlaced video).
• In non-interlaced video, all the lines of a frame are
sampled at the same instant in time.
• Non-interlaced video is also termed ’progressively
scanned’ or ’sequentially scanned’ video.
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• The red, green and blue (RGB) signals coming from
a color television camera can be equivalently
expressed as luminance (Y) and chrominance (UV)
components.
• The chrominance bandwidth may be reduced relative
to the luminance without significantly affecting the
picture quality.
• For standard definition video, ”Encoding parameters
of digital television for studios”, CCIR
Recommendation 601, later called Recommendation
YCbCr
ITU-R BT 601 defines how the component
video signals can be sampled and digitized to form
discrete pixels.
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• The terms 4:2:2 and 4:2:0 are often used to describe
the sampling structure of the digital picture.
• 4:2:2 means the chrominance is horizontally
subsampled by a factor of two relative to the
luminance.
• 4:2:0 means the chrominance is horizontally and
vertically subsampled by a factor of two relative to the
luminance.
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• The active region of a standard digital television
frame, sampled according to CCIR recommendation
601, is 720 pixels by 576 lines for a frame rate of 25
Hz.
• Using 8 bits for each YCb Cr pixel, the uncompressed
bit rates for 4:2:2 and 4:2:0 signals are therefore:
4:2:2-sampling: (720*576*25*8 + 360*576*25*8 +
360*576*25*8) bits/sec = 166 Mbits/sec.
4:2:0-sampling (720*576*25*8 + 360*288*25*8 +
360*288*25*8) bits/sec = 124 Mbits/sec.
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• MPEG-2 is capable of compressing the bit rate of
standard-definition 4:2:0 video down to about 3-15
Mbits/sec.
• At the lower bit rates in this range, the impairments
introduced by the MPEG-2 coding and decoding
process become increasingly objectionable.
• For digital terrestrial television broadcasting of
standard-definition video, a bit rate of about 4-6
Mbits/sec is thought to be a good compromise
between picture quality and transmission bandwidth
efficiency.
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