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Characteristics of High Resolution Video Signals
| Number of Bits | Number of Colors |
|---|---|
| 8 | 256 |
| 15 | 32,768 |
| 16 | 65,536 |
| 24 | 16,777,216 |
| LOW | MEDIUM | HIGH | |
| Horizontal Scan Rate: | 15.75 KHz | 16 to 35 KHz | 36
KHz and above |
| Resolution: | 320 x 200 | 640 x 480 | 1024 x 768 or
more |
| Bandwidth: | 5 - 20 MHz | 30 - 50 MHz | 60 MHz and
above |
The resolution and horizontal scan rate of a computer signal are set parameters that determine the bandwidth required of all equipment in the data display system. When any part of the data display chain lacks sufficient bandwidth to pass the signal, the effects of reduced signal bandwidth are easily observed on the displayed image.
DISPLAY SYSTEM BANDWIDTH AND ITS EFFECTS ON DATA DISPLAY
The letter "E" is an excellent display tool that can be used
to observe the low and high frequency response of a system. In order to
visually check a display system's bandwidth, the user can simply display a
small letter "E" (preferably in Helvetica) using black type on a white
background. The vertical lines displayed define the horizontal resolution and
strongly depend on the bandwidth of the video signal and the display system.
The horizontal lines represent the low frequency part of the signal and are
used for reference to compare with the vertical lines. If the bandwidth of the
entire display system matches the bandwidth of the original video signal then
the horizontal and vertical portions of the letter "E" will have the same
intensity.
If the system bandwidth is not sufficient for the video signal, then the high
band portion of the video signal is removed and the edges become fuzzy and soft
(Diagram 2.10). The intensity of the vertical lines decreases as compared to
the horizontal lines. Reducing the bandwidth of the system even further
(Diagram 2.11) will result in a loss of the high and medium bands of the video
signal. The displayed image's vertical lines will become blurry, thin and low
in intensity compared to the horizontal lines.
When video equipment is selected for a system, it is very important that the
bandwidth of the equipment is at least the same or larger than the bandwidth of
the video signal. Cables play an important role in the overall quality of the
video image, and can degrade the performance of the best equipment.
SIGNAL DEGRADATION
Proper equipment selection is very important in designing an
audio visual system. Each piece of equipment will slightly degrade the video
image, and it is the system designer's responsibility to assure that the
equipment used minimizes the inevitable signal degradation.
Diagram 2.12 shows an input signal with a 50 MHz bandwidth
passing through a 30 MHz amplifier and a coax cable. The amplifier will
reduce the bandwidth of the signal to 30 MHz and the cable will reduce it even
further. Projectors and other display devices have bandwidth limitations which
may also limit the video signal. Therefore, the system shown in Diagram
2.12 would offer unsatisfactory bandwidth
performance, resulting in a visible degradation of the vertical lines.
Diagram 2.13 shows the same input signal used in Diagram 2.12 but
makes changes in the components of the system. By changing the equipment to a
100 MHz amplifier and high resolution coax cable, the original signal's
integrity is maintained. The displayed image will not show any of the effects
of reduced bandwidth.
Rule of Thumb: The bandwidth of video system equipment
should be 20 to 30% higher than the bandwidth of the video signal. Therefore,
to pass a signal of 100 MHz the bandwidth of amplifiers and other parts of the
display system should be around 120 to 130 MHz.
THE VIDEO SIGNAL PIPELINE
The transmission of a video signal through the video system
components is similar to water flowing through a pipe. In this analogy, we
will make water represent the video signal, with the amount of water
proportional to the signal's bandwidth. If all of the water (signal) is
allowed to flow through the pipe (video system), all of the video signal is
passed. However, if the water flow is restricted, some of the original
signal's bandwidth is lost.
Diagram 2.14 shows a typical video system and the corresponding "video
pipeline." We can see that all of the water is allowed to pass through the
system, resulting in a complete transmission of the entire video signal. On
the other hand, Diagram 2.15 below shows how the
signal gets limited by thin "pipes."
Our third video pipeline (Diagram 2.16) demonstrates the old proverb that "a
chain is only as strong as its weakest link." The amplifier and projector have
ample bandwidth to pass the signal nicely, but the coax cable is the weak link,
with its poor performance limiting the total system bandwidth. Once some of
the signal bandwidth is lost, it is not recoverable. This makes cable
selection an important issue, since poor quality cables can degrade the video
image of the best, most expensive routing and display equipment.
Cables for High Resolution Video
Since cables play such a major role in tying together data
display systems, this section will describe the three major types of cable
media used to transmit video information and discuss the merits and major
applications for each cable type.
TWISTED PAIR CABLE
Twisted pair is the type of cable found in home telephone
wiring systems. As the name suggests, twisted pair cables contain many sets of
small gauge wires, with each set comprised of a signal cable twisted with its
associated ground cable. Twisted pair cables are designed for use with digital
signals including digital control signals, data, and digital video signals.
Since digital video signals are susceptible to outside interference and cannot
be sent more than 6 to 12 feet without signal loss, digital signal line drivers
or amplifiers must be used to transmit digital signals over greater distances.
Even with a TTL line driver/amplifier digital video signals should not be
transmitted over 75 - 100 feet. Because of the problems inherent with digital
video signal transmission, it is suggested that digital video signals be
converted to an analog form using a video interface. This will allow for clean
signal transmission over hundreds of feet if necessary.
COAXIAL CABLE
Coaxial cables, commonly called coax cables,
are used to transmit analog video signals. These cables (Diagram 2.18) are
constructed in a special way to provide optimal signal transfer while insuring
adequate shielding from outside interference. The center conductor
(dielectric) of a coax cable carries the video signal and the outside conductor
(shield) is used as a return wire and shield.
Center conductor thickness is called gauge and is specified as
AWG (American Wire Gauge). The smaller the AWG number of a coax cable the
better it is at transmitting high bandwidth signals. A smaller AWG means a
thicker diameter wire, and in general a thicker wire gives better
performance.
Coax cable specifications rate characteristics such as impedance, attenuation,
velocity of propagation and shielding percentage. A coax cable's
impedance must match the impedance of the source and
destination equipment, which for most video systems is 75 Ohms.
Attenuation is specified as the amount of signal decrease per
100 feet of cable at certain frequencies and is measured in decibels (dB).
This specification correlates to the bandwidth of the cable. The
velocity of propagation is determined by the insulator type
used between the center conductor and the shield and is measure as a percent.
This number shows the speed of the signal in the coax cable as compared to the
speed of light in a vacuum. Shielding percentage shows the
effectiveness of the outside shield and / or foil braid. The higher the
percentage, the better the cable resists interference.
Some coax cable carries a special designation as plenum rated
cable indicating that the outer jacket of the cable is fire resistant
(often achieved through Teflon coating). While many city fire codes require
most wires to be run in conduit as a protection from fire, they usually allow
plenum rated coaxial cable to be run outside of conduit.
FIBER OPTIC CABLES
Summary
In summary, high resolution video signals from computer graphics cards are
composed of two main parts, the sync signals and the color components. A third
component, ID bits, may also be present which help the graphics card
communicate with the attached local monitor.
Sync signals are composed of a horizontal sync and a vertical sync which may be
combined together to form a composite sync signal. Composite sync may then be
combined on the green video signal or on all video signals. The color
components (red, green, blue) are used to create both primary colors and
various color shades
Analog and digital signals are the two types of video signals, each with its
own characteristics. Both signals can provide high resolution and high
bandwidth video information, but it is much easier to transmit analog signals
than digital signals. The impedance of analog video signals is 75 Ohms,
whereas digital video signals have a 10 K Ohm impedance. Because of this,
digital signals are much more susceptible to interference than analog signals.
With analog signals, coax cable is used and cable length may be extended as far
as 1000 ft. Digital signals use twisted pair cable and should not run more
than 12 ft. without special amplifiers.
Another aspect of high resolution video signals is color depth, or the number
of colors a computer graphics card can generate. While early graphics cards
were limited to 16 or 256 colors, the current crop of Hi-Color video cards can
reproduce 32,000 to 16.7 million colors, allowing for near photographic quality
image reproduction.
Resolution, scan rate and bandwidth are all interrelated. The higher the
resolution of a video signal the higher the scan rate and bandwidth. Proper
selection of video equipment and cables are important to minimize signal
degradation. The bandwidth of a system's components should be the same or
higher than that of the incoming video signal.
APPLICATIONS FOR HIGH RESOLUTION VIDEO EQUIPMENT
In this section we get down to the practical considerations of designing and
specifying video and data projection systems using high resolution video
equipment. Emphasis is placed on examples and application drawings which
demonstrate how different equipment can be used to design the best systems
solutions for real world video communications problems. This section is broken
down according to the equipment types listed above in Diagram 3.1.
Interfaces
The black box is commonly called an interface, a device which
was designed specifically for the type of application just mentioned. Most
interfaces use a monitor loop cable that allows for connection of the
computer's local monitor and then provides an output signal in the industry
standard signal format: RGBS on four BNC connectors. The finished installation
using an interface would look like this:
As Diagram 3.3 shows, the interface connects between the output of the graphic
card and the input of the local monitor. The output of the interface is used
to supply an RGBS signal to a second display device such as a data projector or
large monitor.
The first decision involved in choosing an interface revolves around the type
of interface needed for a particular application. The two major interface
types are dedicated interfaces and universal
interfaces (see Diagram 3.4). Dedicated interfaces are designed to
work with a particular computer signal, and thus, do not have to be as
sophisticated as a universal interface. The main reason to choose a dedicated
interfaces is cost savings. The most common dedicated interfaces are designed
for use with CGA/EGA, VGA, and MACII signals. These interfaces feature
permanently attached input cables and tend to have fewer user adjustable
controls than universal interfaces.
Universal interfaces are designed with a removable monitor loop cable and
sophisticated circuitry designed to operate with virtually any computer signal.
Universal interfaces such as the IN2000 can be used with over 75 different
monitor loop cables, each designed to work with a different computer. Thus,
there is one input cable for VGA, another for MACII, etc. Universal interfaces
typically cost more than dedicated interfaces but offer the user a greater
number of signal adjustments and flexibility when interfacing to multiple
computer types.
In addition to their key functions of splitting the video signal and providing
a standard type of output signal, interfaces provide multiple features
including: horizontal positioning, vertical positioning, color gain controls,
and various output signal sync formats including RGsB and composite
monochrome.
To better understand the complex functions an interface must perform, let's
take a look at some common computer signals.
CGA
The IBM Color Graphics Adapter has a maximum resolution of 640 x 200 and a
horizontal scan rate of 15.75 KHz. The video and sync signals are both digital
TTL signals, and because there are only 4 bits for video, a maximum of 16
colors are available. However, in the "high resolution" 640 x 200 mode only
three colors are available from a palate of 6 due to memory limitations. CGA
graphic cards output video on a 9 pin D connector, and are rarely seen today
except on very old desktop and laptop computers.
MDA
The IBM Monochrome Data Adapter (MDA) was designed to provide
better text resolution than the CGA could produce. Resolution was improved by
increasing the horizontal scan rate to 18.1 KHz. The video and sync signals
are both digital TTL signal, and only four shades of grey were available from
the two bit video. The IBM MDA card can not display graphics, so a company
called Hercules introduced its own MDA compatible card that could also display
the same graphics as the CGA card (but displayed in monochrome).
EGA
The IBM EGA card was designed to be downwardly compatible with
CGA and MDA cards but provide better graphics, more colors and better
performance. The resolution was further enhanced by increasing the horizontal
scan rate to 24.1 KHz, while the maximum colors available were increased to 64.
The 6 bit video and sync signals are both digital TTL signals, and the card can
be programmed to operate in low and high resolution modes.
VGA / SVGA
IBM's Video Graphics Adapter was a revolutionary step above EGA. The
horizontal scan rate was increased to 31.5 KHz resulting in a resolution of up
to 640 x 480 with 256 colors. The video signal is analog and the sync signal
digital TTL. The VGA card is capable of outputting four different resolution
modes, enabling it to be compatible with CGA, MDA, and EGA software. The
fourth mode is referred to as the 8514A mode. SVGA or Super VGA was introduced
by third party graphic card manufacturers. These cards support the four modes
of VGA plus many additional modes. Most SVGA cards also operate at high
resolutions, with some offering up to 1280 x 1024 resolution, and up to a
maximum of 16.7 million colors. VGA and SVGA cards output video on a 15 pin HD
connector.
MACII / QUADRA / POWERMAC
Apple computers are frequently used by trainers and presentation professionals,
and are the second most common computers interfaced to large screen displays.
The Apple MACII, Quadra, and PowerMac series output an analog video signal with
digital sync signals. One interesting aspect about MAC video cards is the fact
that their sync signals may be RGBS, RGBHV, or RGsB. The Apples are capable of
displaying various resolutions depending on the monitor used. Resolutions
range from 512 x 384 for the 12 inch color monitor to 1152 x 870 for the 21
inch color display, with horizontal scan rates of 24.48 KHz and 68.7 KHz,
respectively. The most popular mode is for the 13 inch color monitor running
at 640 x 480 resolution and 35.0 KHz. Apple MACII and Quadra computers output
video on a 15 pin D connector. Note: Some earlier MAC Powerbook
models do not have a video output. Some Powerbook models
and PowerMac 6100AV models have a 45 pin output requiring a special adapter to
convert to a standard MAC type 15-pin D video connector.
SPECIAL INTERFACING CONCERNS
It is important to understand that with certain types of
computers it makes a difference which monitor is attached to the video card at
boot up or whether any monitor is attached at all. Many common video cards
including VGA, MAC (especially Quadra), and Sun SPARC Station sense that
certain pins are shorted or use ID bits to go into different modes. For
example, a VGA card with no monitor at boot up will be set for monochrome. A
MAC Quadra senses the size of the attached monitor and adjusts the scan rate
accordingly.
Distribution Amplifiers and Line Drivers
Many audio visual installations require the use of multiple
monitors and / or projectors to display an image to a large audience. A common
application (Diagram 3.6) would involve the display of a computer's image on
two large screen monitors as well as the computer's own local monitor. First
an interface is used to loop through to the local monitor and provide an RGBS
output. Since there is a need for two RGBS signals (one for each large screen
monitor), another device called a distribution amplifier fills
that need. By providing multiple outputs of the same signal, distribution
amplifiers offer the ability to amplify and split video signals.
To better understand the functions performed by a distribution amplifier let's
examine the effects of impedence on video signal transmission.
Analog video signals have a 75 Ohm impedance, and must be terminated into 75
Ohms. To illustrate the importance of impedence matching, let's return to a
slightly modified version of the "video pipeline." In Diagram 3.7, water
(video signal) flows into the cup that is filled up to a certain water level
(representing video level). Water flows out of the cup at exactly the same
rate as it is coming in. Thus, the water level in the cup stays at the line
(equating to proper video level). The flow rate out of the cup represents the
75 Ohm termination, and as we can see, it is properly matched to the 75 Ohm
output impedance of the video signal.
Now, if the flow rate out of the cup is reduced (Diagram 3.8) by providing more
restriction to the flow, the water in the cup overflows. This condition would
occur when the impedence loading a video output is greater than 75 Ohms,
(usually an unterminated monitor). Since the impedence, or restriction to flow
has increased, the video level will go up and the monitor or video projector
will display an overdriven video signal, usually quite bright and blurry.
The opposite of an unterminated video output would be a double
terminated output. Diagram 3.9 demonstrates that if the flow rate out
of the cup is increased, the cup will empty. This is what happens if a video
signal is split passively (using no amplification) and the video output is
loaded with more than one monitor. When two monitors each apply their
own 75 ohm termination (or double terminate) a video
output, the effective impedence is reduced to 37.5 Ohms. This means
there is less restriction to video "flow" so the signal drains out of the video
pipeline. The video signal is lower than the regular level so each monitor
will receive a weak signal and display a poor, dark image.
To summarize, an analog video signal should not be split using a passive split
like a "Y" cable since the resulting double termination will result in a very
poor quality image on both monitors. If a single video signal must be split to
two or more monitors/projectors a distribution amplifier must be used in order
to split the signal in a way which maintains full signal integrity. A video
distribution amplifier will provide the correct 75 Ohm termination while
splitting the signal, providing the desired number of buffered (isolated)
outputs.
FIXED GAIN DISTRIBUTION AMPLIFIERS
One important specification of video distribution amplifiers
(DA's) is their gain characteristics. Video Amplifiers come
in two basic varieties, fixed gain and variable
gain. Fixed gain DA's always provide the same amount of gain. If a
fixed gain amplifier is designed to provide unity gain, then
it will always output the same video signal voltage presented to the input,
meaning that a 1.0 volt signal coming into the DA would leave the DA as a 1.0
volt signal. Other fixed gain DA's are designed to provide a slight signal
boost to compensate for signal losses inherent in long coaxial cables. An
amplifier with a gain of 1.1 would increase a 1.0 volt input signal up to 1.1
volts, compensating for the voltage loss found in about 75 feet of coax
cable.
VARIABLE GAIN DISTRIBUTION AMPLIFIERS
Variable gain distribution amplifiers (see Diagram 3.10)
provide users with the option of adjusting the gain characteristics to meet
their needs. With a typical gain adjustment range of .7 to 3.0, adjustable
gain DAs can provide enough signal boost to compensate for cables 300 to 400
feet long. On an RGBS distribution amplifier with variable gain, each video
output connector is fitted with a gain adjustment pot so that each individual
signal can be adjusted. This allows technicians installing a large system to
set the gain differently for each output, a situation which may be encountered
if some output cables are fairly short while others are long. Individual gain
pots also provide a way of balancing the red, green, and blue signal levels.
PEAKING CONTROLS
Variable gain DAs may also provide a second set of controls to
adjust peaking. When video signals are transmitted over long
cables, they suffer both a loss in voltage and a loss of high frequency
components due to cable bandwidth limitations. While a distribution
amplifier's gain controls work to compensate for the voltage loss, the peaking
controls apply an equalization curve to help restore the high frequency signal
components. Since DAs designed for high resolution video signals usually
center this peaking equalization "bump" at around 100 MHz, peaking controls
generally offer no positive effects for low bandwidth video signals. On the
other hand, high bandwidth signals may receive a significant restoration of
signal detail when the peaking controls are properly adjusted.
LINE DRIVERS
A video line driver (Diagram 3.12)
provides two of the same features found in an adjustable gain
distribution amplifier, namely adjustable gain and peaking. The main
difference between a distribution amplifier and a line driver is the fact that
a video distribution amplifier provides multiple video outputs while a line
driver has only one output. Since line drivers amplify the signal but do not
split it, they are specialized devices designed to compensate for signal losses
found in long cable runs.
Perhaps the most common question regarding line driver usage revolves around
when to use one in a cable run and how often to use additional line drivers.
To answer this question accurately a system designer must first know the
bandwidth characteristics of the signal being transmitted and the bandwidth
capabilities of the specified coax cables. At only 31.5 KHz, a garden variety
VGA signal has only moderate bandwidth requirements and can be sent at least
100 feet using high bandwidth cables without a line driver (especially
when used with an interface which provides a 10% signal boost). Video signals
from SGI, Sun SPARC or other graphics workstations run at 61 to 71 KHz or
higher and have great bandwidth requirements. They may need a line driver
after only 75 feet of high resolution coax cable.
If a very long cable length is required for high bandwidth signals, additional
line drivers should be placed at intervals down the cable run to re-boost the
signal. These should be placed where the signal level has dropped back to
regular frequency response. If the line driver is placed too far down the line
after high frequency loss has become excessive, there is no good signal left to
boost. There are practical limits to the distance a high bandwidth video
signal can be carried on coax cables, even with line drivers, so an alternative
transmission medium such as fiber optic should be considered for very long
cable runs.
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notes
