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In brief, analog video signals are in the form of waves
while digital video signals are in the form of 1s and 0s. However, before
either video format is displayed on your video display, video images first go
through a series of stages: The creation stage (video recording or
animation), the storage stage (on tape, DVD, or hard drive), and the
display stage (appearance on a TV or monitor). A video image may travel
through all stages as either analog or digital, or it may be converted from one
to the other and back again. Such analog/digital or digital/analog
conversions reduce the integrity of a video signal and degrade video quality
and should be avoided when possible, however most conversions are
unavoidable without the latest “expensive” video components. Because analog
video is displayed via a series of scans and digital video is displayed via
light and color intensity on fixed dots (pixels), conversions produce signal
loss and errors. Ideally a video signal would be analog or digital from one end
to the other. However, because digital signals can be copied, stored and
transmitted more accurately than analog signals, the ideal video system would be
digital from end to end.
Most computers create, store and transmit video signals
digitally, but before sending these signals to a video display (a monitor) they
are usually converted to analog signals and signal loss occurs in the
conversion. Once digital video display devices hit the market, two conversions
took place, one from digital to analog (within the computer) and another from
analog back to digital (within the video display). These two conversions create
a lot of signal loss that degrades video quality. However, with the recent
popularity of digital video ports on computers (digital video interface adapter
cards) and digital display devices such as projectors, LCDs and plasma displays,
a migration toward end-to-end digital video has begun. Ideally, conversions
between analog and digital video signals should be avoided when possible.
All
video signals start out as either analog or digital but usually end up as analog
signals on a display device because most display devices are analog. This is
because digital display devices are still new and very expensive. When it comes
to cabling for maximum video quality, adapting or converting between any of the
video display formats (listed below) should be avoided when possible.
As an example, even though your computer might have both composite video (one
RCA jack) and s-video (one 4-pin jack) and your video display has only composite
video (one RCA jack), it would be better to connect the two composite video
jacks than to “adapt” the s-video jack to a composite video jack on your display
device.
Below is a list of video cable types with a brief description:
Composite Video
– (Analog) All video signals are transmitted through a single 75-ohm coaxial
cable terminated with RCA, BNC or RF (F-type) connectors. Brightness (Y) and
color (C) signals are combined into one composite signal. Combining and then
separating video signals leads to some signals loss. Note that “composite” video
is often confused with “component” video (see below) because both commonly use a
three RCA connector cable, however where “composite” video uses yellow, red and
white RCAs (yellow for video and red and white for audio) “component” video uses
three RCAs (usually red, green and blue) that are dedicated to video alone.
S-Video
– (Analog) All video signals are transmitted through two miniature 75-ohm
coaxial cables that are usually joined into a single cable and terminated with
4-pin connectors. S-video improves video quality beyond composite video (above)
because the separate brightness and color signals are kept separate rather than
combined into the composite signal, and this reduces encoding and decoding and
therefore signal loss. However, the color signal must be further decoded and
demodulated into two other components to separate the RGB (red-green-blue)
signals. S-video provides better video quality than composite video unless
s-video is adapted to composite video.
Component Video (Y/Pb/Pr) – (Analog) Often confused
with RGB video (below). All video signals are transmitted through three 75-ohm
coaxial cables that are terminated with RCA, BNC or RF connectors. Component
video subtracts the brightness signal from the sub-components of the chrominance
signals (the blue signal and the red signal). Notice that blue and red will
produce green to achieve a completed RGB (red, green, blue) signal. Component
video is often described as Y/B-Y/R-Y or as Y/Pb/Pr and ports are usually marked
as such. The term "digital component video" is often used to describe component
video, however this is actually an "analog" display. Component video provides a
better quality picture than s-video.
RGB
Video – (Analog) Often confused with component video
(above). All video signals are transmitted on three, four or five 75-ohm coaxial
cables with either RCA or BNC connectors. RGB video is often confused with
component video (above). Within RGB cables, there are three sub types, depending
on the number of connections:
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RGB or RGSB
- Includes three separate connections: Red, Blue, and
Green+Synchronization. *Often confused with component video.*
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RGBS
- Includes four separate connections: Red, Blue, Green, and
Synchronization.
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RGBHV
- Includes five separate connections: Red, Blue, Green, Horizontal
Synchronization, and Vertical Synchronization.
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VGA/SVGA/XGA
- Includes one 15-pin connector (some with three rows of pins and some with
two rows of pins).
Digital Video
– (Digital) With the recent introduction of plasma displays, LCDs, digital
computer video interface cards, and digital projectors, digital video cables are
growing in popularity. Digital video cables generally use three connector types:
DVI (Digital Video Interface), DFP (Digital Flat Panel), and P&D (Plug &
Display) or EVC:
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DVI
– DVI cables come in three types: DVI-A cables which carry only analog
signals, DVI-D cables which carry only digital signals, and DVI-I cables which
can carry either digital or analog signals (so that analog devices can be
mixed with digital devices).
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DFP
– DFP cables carry only digital signals and cannot be connected to analog
devices. This means that you must change a computer’s video port to a digital
video port by installing a digital video interface adapter card in your
computer.
-
P&D
(Plug & Display) or EVC – P&D cables are also called EVC cables and can
carry digital or analog signals.
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