Road Test: Strong Technobeam, page 40 - PLSN.com

By VickieClaiborne
PROJECTION LIGHTS & STAGING NEWS
Components of a
Digital Video Signal
VIDEO DIGERaTI
Have you ever wondered what is actually
happening in that cable connecting
your video source to your
monitor or projector? If you’re like me, you
may be fascinated to know that the video
signal being transmitted down those cables
is comprised of many parts, all seamlessly
working together to provide the information
that our eye can use to get as much
detail as possible. Here’s a look at a few of
those components.
So, how do you transport an image from
the camera to your TV or projector? You
may be asking, “Why not just transmit the
RGB information as the camera captured
it?” The answer has to do with bandwidth.
Uncompressed RGB uses up a lot of expensive
bandwidth, so the first thing that typically
happens before transmitting a picture
is that the RGB signals are converted into a
more compact (compressed) format known
as “component video.”
Analog component video is made up of
three signals. The first is the luminance signal,
which refers to the brightness (or black and
white information) that is contained in the
original RGB signal. It is usually referred to as
the “Y” component and it could actually be
viewed as black and white video. The second
The composite signal is typically carried
through a cable terminated with an RCA connector,
and the signal can conform to NTSC,
PAL, or SECAM formats.
Digital video, on the other hand, uses a
different scheme for encoding chrominance
values. Instead of Pb and Pr, a different conversion
algorithm is used that allows for
more compression and bandwidth reduction.
The resulting red and blue components
are called Cb and Cr.
Why not just transmit the RGB information as the camera
captured it? The answer has to do with bandwith.
scheme, and in particular, understand the
tradeoffs in terms of signal quality
and compression, you’ll be able to
make better decisions when planning
a video workflow and obtaining the
proper equipment.
Vickie Claiborne is a freelance control systems
and digital media server training developer,
lighting director and programmer, and
an industrial training specialist. She can be
reached at vclaiborne@plsn.com.
It all comes down to RGB
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Three colors—red, green, and blue – are
the only colors that the human eye can actually
see. All the other perceptible colors and
shades of the spectrum are the result of your
brain interpreting the mix of red, green, and
blue signals coming from your eye’s optical
sensors. Therefore, a video system only needs
to capture red, green, and blue (a.k.a. RGB) in
order to capture images.
The camera must capture RGB on the front
end. That information must be delivered accurately
to your television or projector, which
then reproduces RGB in the display. By varying
the intensity of red, green, and blue, every
color of the spectrum can be reproduced. The
results are, hopefully, very close to what you
see in real life.
Bandwidth
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Composite video packages and sends RGB video as one compressed signal for broadcast.
Green (Y), blue (Pb) and red (Pr) cables capture RGB on the front end.
and third signals are called the “color difference”
signals, and they indicate how much
blue and red is present — relative to the luminance.
The blue component is “Pb” and the
red component is “Pr.” The color difference
signals are mathematical derivatives of the
RGB signal.
The green information doesn’t need to be
transmitted as a separate signal since it can
be derived from the “Y, Pb, Pr” combination.
The display device fills in the green information
based on how bright the image is (the Y
component), how much of that information is
blue, and how much of it is red.
In most professional video gear, analog
component video cables are terminated with
BNC connectors, although SCART connectors
are typically used in Europe. The cables are usually
colored green (Y), blue (Pb), and red (Pr).
Saving Bandwidth
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Once the video information is packaged
in component video format, the bandwidth
requirements are reduced by a factor of 1/3,
as compared to uncompressed RGB. But
more compression is required for broadcast
purposes, so with the introduction of the
color television in
1953, the National
Television Standards
Committee (NTSC)
developed a technique
that allowed
all of the component
video information to
be compressed into
one signal known as
“composite video.”
Composite video
provides a way of
packaging and sending
the luminance
( b r i g h t n e s s ) a n d
chrominance (color)
information through
a single cable, instead
of the three individual
cables required for
component signals.
By mixing the three
signals in a frequency-division
multiplexing
scheme and then
modulating them
with a radio frequency
carrier, the result is
a single composite
signal that can be
broadcast using less
bandwidth.
Chroma Subsampling
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In addition, digital uses chroma subsampling
to further reduce bandwidth. Subsampling
means that the color information is
sampled at a lower rate than the luminance
information. Since the human response system
is less sensitive to color information than
to luminance information, the eye can’t detect
the difference even though there is less
color information.
The “color sampling ratio” describes
the relationship between sampling rates
of the luminance and chrominance values
in a digital video signal. For professional
digital video, the color sampling ratio
is known as 4:2:2. Here, the two chroma
components are sampled at one half the
rate of the luminance component.
The resulting signal has a particular
data rate which depends on its compression.
The more compression present in
the signal, the faster the transfer rate —
but more compression has its drawbacks
in terms of a degraded visual image.
For this reason, a plethora of codecs are
available for compressing and decompressing
the data, hopefully minimizing
the signal degradation.
Video acquisition and encoding is a
complex subject. There are many encoding
schemes and video formats. Some of the
more important terms to remember are:
Component Video – A video signal in
which the luminance and chrominance
signals are kept separate, and three wires
are required for transmission. Component
requires a higher bandwidth, but yields a
higher quality picture.
Composite Video – the luminance and
chrominance signals are combined into
one signal, and one wire is required for
transmission. Signal quality is not as high
as component, but composite video facilitates
easy transmission in standard video
formats, such as NTSC, PAL or SECAM.
Luminance - the black and white, or
brightness, part of a component video signal;
also called the "Y" signal.
Chrominance – the part of a video signal
which carries the color information such
as hue and saturation.
As noted, there are many factors that
can affect the final result of a video signal.
If you understand the advantages and
disadvantages of each video transmission
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