As with all equipment in a high resolution video display system, care must be taken to specify amplifiers and line drivers with sufficient bandwidth to pass the video signals with minimum degradation. The rule of thumb for equipment selection states that all high resolution video devices should have a bandwidth at least 20 - 30% higher than any signal they will carry. The bandwidth of the video signals presently in use should be used to make these computations, but the final selection of amplification equipment should also take into account possible expansion into higher bandwidth sources which may be added in the future.
CROSSTALK
Crosstalk is a measurement of an amplifier's tendency to let
one signal affect, or bleed over onto other signal(s) carried within the same
amplifier. In audio, crosstalk is a measure of the interference between the
left and right signals. Since high res video DAs and line drivers usually
carry three color components, crosstalk is a measure of the bleeding between
the red, green, and blue video signals. The visual effects of crosstalk might
be seen on the projected image as some of the green signal faintly showing up
also as red and blue images. Crosstalk increases with higher frequencies, so a
complete crosstalk specification will include the specific frequency where the
measurement was made. For most applications crosstalk should be below 40 dB.
Switchers
MANUAL SWITCHERS
A/V systems often include several video sources but only a
single display device. The diagram above shows how a video
switcher can be used to select which of the many video signals will be
routed to the display device. Switchers are either passive or
active depending on their design. Passive switchers use
special high frequency relays in the video signal path and are bi-directional
(a signal can be sent either way through the switcher). Active switchers use
solid state ICs and relays to route the video signals and are therefore
unidirectional.
In Diagram 3.14 above, two computer video signals are first sent to interfaces
in order to bring their signals into the convenient format of RGBS on 4 BNC
connectors. The output of each interface is then fed into one of the inputs on
the RGBS switcher. The switcher determines which of the two signals will be
sent to the video display. While the switcher in the diagram is a 6 In, 1 Out
switcher (also called a 6 X 1 switcher), other switchers are available, with
many A/V installations using a 4 In, 1 Out switcher a or 2 In, 1 Out switcher.
Selection of different inputs is accomplished either through the use of front
panel buttons, or by using a remote control device which controls the switcher
using contact closures or RS232 commands.
AUTOSWITCHERS
Some installations provide logistical problems requiring a
different type of switcher. Diagram 3.15 shows a presentation room with
multiple wall plates for RGBS video input. Depending on where the computer is
going to be set up the technician will hook up to the closest wall plate. In
this example, both wall plates are hooked up to the inputs of an autoswitcher.
Instead of using front panel buttons for switching, autoswitchers sense which
input has a signal present and switch to that input. This allows for fully
automatic switching in facilities like the one shown above. Autoswitchers are
available with 2, 4 or 6 inputs.
BANDWIDTH REQUIREMENTS
Video switchers should match the bandwidth performance of the
rest of the system. Due to their design, passive switchers are available in
very high bandwidths, whereas active switchers may tend to have slightly lower
bandwidth specifications.
MATRIX SWITCHERS
Matrix switchers are a special class of switchers that also
have some distribution amplifier characteristics. Matrix switchers (Diagram
3.16) typically have multiple input signals being routed to multiple display
devices. Each input can be selected to be sent to any output, and can even be
simultaneously sent to more than one output. Imagine a grid where the
horizontal lines represent the input signals and the vertical lines represent
the outputs. Each cross point on the grid can be selected so that any input
can be routed to any output. To enable an input to be sent to multiple outputs
simultaneously, the outputs are also amplified (similar to a distribution
amplifier), which means that matrix switchers are active.
Matrix switchers are specified by the number of inputs and outputs. For
example a 4 x 6 matrix switcher would have 4 inputs and 6 outputs. Matrix
switchers are available for composite video, S-Video (Y/C), RGBS, RGsB, RGBHV
and stereo audio.
Due to the many combinations that can be switched by a matrix switcher, a
control system or computer control is typically used to control the switching.
Most matrix switchers have a serial control port to accept RS-232 commands from
a computer or control system. In addition to centralizing control operations
for a complex routing system, matrix switchers can also simplify design and
installation tasks since a single matrix switcher can take the place of over a
dozen distribution amplifiers and switchers. While the first matrix switchers
were designed to have a set number of inputs and outputs, a new matrix switcher
design has recently been introduced which allows the user to change the number
of inputs and outputs using front panel controls. This new
reconfigurable matrix switcher is gaining great popularity for
permanent installations and especially for rental and staging companies who
assemble complex display systems requiring different numbers of input and
outputs on a daily basis. A reconfigurable matrix switcher helps system
designers add flexibility to their video routing systems, insuring
compatibility with a customer's changing communications needs.
Encoders and Decoders
Up to this point most of the discussion has centered around interfacing,
distributing, and switching high resolution video, signals which are in the
RGBS format. Most A/V systems will integrate both high resolution video
signals and signals from traditional video sources such as VCR's, laser disc
players, video cameras, and tuners. Since most consumer and industrial grade
video sources output their signals in the composite or S-Video (Y/C) format,
this creates a potential problem when these sources must be routed to the
display devices along with the RGBS signals.
WORKING WITH COMPOSITE AND RGBS SIGNALS - TWO SOLUTIONS
Diagram 3.17 outlines a hypothetical A/V installation which
integrates two composite video sources, two high resolution sources, and a
video display device. All of the high res. sources are fed into Switcher A,
while all of the composite video sources are hooked up to Switcher B. The
output of Switcher A is connected to the RGB input on the RGB monitor, and the
output of Switcher B is connected to the composite input. In order to view the
desired source the user must first select the correct setting on Switcher A or
Switcher B and then set the monitor to either the RGB input or the composite
input.
Diagram 3.18 shows a simplified solution to the problem, achieved by
first decoding all the composite signals to the RGBS format. Once this has been
done, a single RGBS switcher can be used to switch all the signals and
all signals are routed to the data display's RGB input, eliminating the
need to switch between the monitor's composite and RGB inputs.
ENCODERS
Video signals are originally generated in the component format
(R-Y, B-Y, Y) which uses three discrete signal to represent the color
information. This is the best way to deal with broadcast video type signals,
resulting in the best possible resolution and color accuracy. Unfortunately,
the practical considerations of transmitting and storing video signals usually
forces the signal to be altered before final delivery to the viewing audience.
Most video transmission systems include a device somewhere in the chain which
encodes the component video signal into either a composite or
Y/C signal. This is done to save bandwidth, reducing the bandwidth required to
transmit the signal and also reducing the quantity of signal information which
must be stored and reproduced by various storage media such as laser discs and
video tape recorders.
Encoders may be built into video devices as part of the internal design or they
may be found as stand alone boxes. Encoders are a key component often included
in scan converters, devices which convert a high scan rate computer video
signal down to a composite or Y/C signal in the NTSC, PAL, or SECAM format.
For more information on scan converters see page 41.
DECODERS
At the other end of the chain come decoders.
All display devices which display broadcast video signals from a tuner, VCR, or
laser disc player use a decoder to split the composite video or Y/C signal back
into the separate color components which are then fed to the red, green, and
blue CRTs or LCD elements. Decoders (see Diagram 3.20) may be found built into
display devices or they may be stand alone units. While decoders built into
display devices often decode signals to a component format, stand alone
decoders as used in A/V systems (like the system shown in Diagram 3.18) often
decode signals to the RGBS format. Once all composite and Y/C signals have
been decoded to the industry standard RGBS format, switching and distribution
tasks are greatly simplified. Just as encoders are an integral part of scan
conversion devices, decoders are a key part of scan doublers, devices which
double a standard 15.75 KHz interlaced signal up to 31.5 KHz non-interlaced
signal. More information about scan doublers is included in the following
section.
Scan Doublers and Scan Converters
SCAN DOUBLERS
A specialized device has been perfected over the last few
years which takes a standard NTSC, PAL, or SECAM signal and converts it to an
Improved Definition Television (IDTV) signal. When the current video standards
like NTSC were developed (over 40 years ago!) they were designed within certain
parameters and limitations. First of all, the signal resolution was designed
for small screen sizes. NTSC signals look best when viewed at a distance of
eight screen heights, meaning a two foot high video image should be viewed from
no less than 16 feet away. The advent of large screen projection systems for
commercial and residential installations has generally thrown this requirement
out the window. People wanting to get a large screen, movie theater type
experience buy larger screen displays, and are now seated less than four screen
heights from the video display. As a result, large screen displays are
impressive for their picture size, but the picture quality suffers due to the
limited signal resolution of regular NTSC.
The NTSC format video signal uses a technique called interlacing to reduce the
rate at which data has to be read out of display memory and to double the
amount of information displayed without doubling the bandwidth requirements.
One complete frame of an image is made up of two fields. First, field one
displays all 262.5 of the even numbered lines in 1/60th of a second. Next,
field two displays all 262.5 of the odd numbered lines which also take 1/60th
of a second. The net result to the human eye is one frame of 525 lines
displayed in 1/30th of a second. The concept of interlacing creates an
illusion of viewing a 60 Hz signal (60 frames per second) when in reality a 30
Hz signal is all that is being displayed. However, the illusion is not
perfect, with a very noticeable side effect being thin black horizontal scan
lines displayed throughout the image (almost a "venetian blind" effect). Scan
doubling eliminates this illusion and replaces the image with an actual
60 Hz signal. The scan lines are virtually eliminated.
To achieve a solid, film-like image, scan doublers de-interlace the signal
(see Diagram 3.22) and display a full frame in 1/60th of a second.
Currently, line doubling and field overlay are the two methods of scan doubling
that INLINE uses. With line doubling, each line of a field is drawn twice so
that a full field is generated in 1/60th of a second. Two odd fields are
displayed on the screen at the same time, and then two even frames are
displayed. In field overlay, both even and odd fields are stored in memory and
then output at the same time. Thus line 1, 2, 3, 4, etc. are drawn in one
pass. However, this frame must be drawn twice so that the video information
going in to and out of the scan doubler are at the same rate. Line doubling is
better for video that has a lot of motion, while field overlay offers greater
detail for still video sources such as slide to video converters and
visualizers.
One additional effect of scan doubling is a higher horizontal frequency rate.
While regular NTSC sources have a horizontal scan rate of 15.75 KHz, scan
doubled video signals are output as an RGB signal at 31.5 KHz, much like a
standard 640 x 480 VGA signal. In order to display a scan doubled image, a
data grade monitor or video projector capable of displaying a 31.5 KHz VGA
signal must be used. Unfortunately, regular monitors and TV sets cannot
display a scan doubled image.
SCAN CONVERTERS
Now that computer graphic images are so popular for
presentation and training purposes, there is an increasing desire to display
these high resolution video signals on regular monitors. Some people also wish
to record computer graphic images on a video tape to demonstrate software for
sales or training purposes. Regular computer video interfaces, while useful at
providing the correct physical connections between a multi-scan RGB monitor and
a computer, do not help users who wish to connect a VGA or MAC video output to
a regular monitor. A scan converter is the correct piece of
equipment for this type of application.
Scan converters do two primary things to high resolution video signals. First
of all, they reduce the scan rate. The most common low cost scan converters on
the market today deal with scan rates of 31.5 KHz or 35.5 KHz, corresponding to
VGA or MACII signals at 640 x 480 resolution. The scan converter reduces these
signals down to a 15.75 KHz interlaced signal which can be viewed on any
regular video monitor or recorded on videotape. In addition to reducing the
scan rate, most scan converters also include an encoder so that the signal can
be output as a composite or s-video signal. Scan converters are also available
to convert frequencies as high as 90 KHz down to 15.75 KHz or 31.5 KHz but
these units are quite costly.
Part of the scan conversion process involves throwing out some of the video
information in order to display it at a lower scan rate. The ability to do
this throwing away of video information without loosing detail is a tricky
task, one which is made more difficult with more drastic scan conversions (from
very high frequencies down to 15.75 KHz). There will be some loss of detail in
all scan conversions, so the price / performance ratio of scan converters is a
critical issue - who can provide the best quality scan conversion at the best
price. A good scan converter is not cheap, and in some cases it might be less
expensive to upgrade the video display system to handle a higher scan rate
rather than reduce the scan rate to match the needs of a regular video display.
If a high scan rate image must be videotaped, broadcast over the air or sent
over a teleconferencing system the signal must be brought to 15.75 KHz, making
a scan converter the only viable solution.
Scan converters usually offer additional features such as a loop through for a
local computer monitor, controls for horizontal and vertical size and
positioning, and a flicker control. Some scan converters are dedicated for one
type of signal such as VGA or MAC, while others can be used with multiple
computer signals.
Sync Devices
In configuring a video system, it is sometimes necessary to convert signals to
a particular sync format. While a universal interface can accomplish many sync
conversion tasks, several specialized devices have been designed as economical
solutions for sync conversions such as RGsB to RGBS, RGBHV to RGBS, RGBS to
RGsB, etc.
SYNC STRIPPERS
A sync stripper "strips" the sync signal off of a monochrome
or video signal. It can be used to convert an RGsB signal to RGBS or with
monochrome signals it can provide three identical video signals to feed the
red, green, and blue display inputs along with a composite sync signal.
SYNC COMBINERS
The first type of sync combiner will combine the sync signal
with one or more color video signals. Typical conversions would be RGBS to
RGsB or RGBS to RsGsBs. A second type of sync combiner (also called a sync
compositor) combines the horizontal and vertical sync into a single composite
sync signal (RGBHV to RGBS).
