| 1.
What are LCD monitors and how do they work? |
The liquid crystal display (LCD) technology now used in flat panel
monitors has long been used in digital watches, calculators, and
many other devices. An LCD device (also called a cell) is made out
of two layers of very fine glass material called substrates that
form a "sandwich" around a thin layer of rod-shaped molecules
(liquid crystals) that flow like liquid. When a charge of electric
current passes through the layer of liquid crystals, they align
or twist, preventing or allowing the light to pass through.
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| 2.
How to choose a flat panel display: what features determine
quality? |
We recommend looking at the following major features when evaluating
the quality and price/performance ratio of an LCD monitor.
Resolution:
Make sure you measure the maximum true resolution--the resolution
achieved by the number of pixels the monitor contains--without scaling
the screen contents up or down.
Size:
Size is measured diagonally from one corner of the glass to the
opposite corner.
Aspect ratio:
This is related to resolution; it is the ratio of the monitor's
width to height. Most displays have a 1.25:1 ratio. Some, like Silicon
Graphics 1600SW, have a wider format, 1.6:1 ratio. The amount of
horizontal space is critical in determining how much information
can be displayed at once, particularly when viewing side-by-side
pages, or spreads.
Dot pitch:
This is a measure of the amount of space between each pixel. The
smaller the dot pitch, the sharper the image. The dpi, or dots per
inch (also called pixels per inch), is related to dot pitch: the
smaller the dot pitch, the higher the dpi is.
Analog/digital:
All LCDs are natively digital (unlike CRTs) but whether it is analog
or digital is determined by what kind of input it requires. Digital
monitors usually have a better picture quality, because the output
is not the result of an analog to digital conversion. Digital LCDs
require a digital-output graphics card in the CPU to drive the display.
Silicon Graphics 1600SW is bundled with a digital graphics card
so that users can take advantage of all-digital display performance.
Brightness:
Brightness is determined by the intensity and quality of the backlighting;
in displays, it is usually measured in luminance (candelas per square
meter). A brighter monitor will be easier to view and have greater
color intensity.
Color resolution:
The number of colors that can be represented on a display without
dithering. Higher quality LCDs have 24-bit color, allowing a color
gamut, or range, of 16.7 million.
Contrast:
Related to brightness, this is a measure of the range between the
lightest tones and the darkest tones that the LCD is capable of
producing. A higher contrast ratio makes the information on the
display more readable.
Viewing angle:
This determines how far above, below, or to either side of the display
a person can be and still accurately view the image on it.
Pixel response rate:
Measured in milliseconds or microseconds, this is the time it takes
for a pixel to respond to voltage (to be turned "on")
and then return back to its normal state. The shorter the pixel
response rate, the more quickly the panel will be able to display
and refresh images.
Additional features:
Additional features that are important to some users are color calibration
capability, a removable stand (for mounting the monitor on a wall
or adjustable arm), a pivoting head (for viewing in portrait mode),
and additional connectors (for USB, video inputs, etc.)
A critical consideration in determining a display's overall value
is TCO or total cost of ownership for the product.
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| 3.
How are digital LCDs different from analog ones? |
Unlike CRTs, all LCDs are inherently digital (that is, the pixels
that generate screen images are operated by digital signals) so
whether an LCD is analog or digital is determined by how it receives
information from the CPU, or computer. Because CRT technology has
been so ubiquitous prior to LCD adoption, many computer graphics
cards still convert digital data to analog for output to CRTs. For
this reason, many LCD manufacturers opt to make their displays analog-only
inputs in order to be compatible with older, analog graphics cards.
Converting the display information from digital to analog (in the
graphics card) and back into digital (in the monitor) can limit
the display's performance and compromise image quality. All-digital
displays, like Silicon Graphics 1600SW, require a digital-output
graphics card to drive them. SGI bundles the 1600SW with a high-performance
digital graphics card so that users can take advantage of all-digital
display performance.
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| 4.
What's the difference between resolution and size? |
Resolution usually refers to pixel resolution, or the number of
addressable pixels on a display, whereas size is a measure of the
diagonal distance from one corner of the display glass to the opposite
corner. Some monitors are very large, but not very high resolution
(most 20-inch and larger displays, for example, have only a 1280x1024
resolution). This means that the space between the pixels is larger
and results in a grainer screen image-much like a photograph loses
sharpness when it is enlarged. SGI monitors are built to have a
very high resolution relative to their size. This results in much
finer dot pitch (the space between pixels is much smaller) and a
much higher image quality. Even very small details in data or images
appear very crisp, and a greater number of pixels allows users to
comfortably display more information at once.
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| 5.
What is the difference between CRT and LCD size measurements? |
CRTs have two specifications for screen size: the CRT size (the
actual size of the picture tube) and the viewable screen size (the
usable screen area). Because the CRT picture tube is enclosed in
the plastic casing, the viewable screen size is smaller than the
overall CRT size. Though CRTs are commonly referred to by the picture
tube size, it is the viewable area that is important in comparing
a CRT to an LCD. Unlike CRTs, the viewing area of an LCD is the
only valid measurement of its size. This is why many LCD specifications
list a CRT equivalent size to identify the picture tube size that
is required to achieve the same viewable size display. For example,
the Silicon Graphics 1600SW has a diagonal screen size of 17.3 inches,
which is equivalent to a 19-inch CRT display's viewing area.
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| 6.
How does the total cost of ownership (TCO) compare to an equivalently
sized CRT monitor (19 or 21-inch)? |
Total cost of ownership includes the actual purchase price for
the monitor as well as all other costs of owning and using the monitor.
Studies show that the purchase cost for computer-related equipment
often is less than a quarter of the total cost of ownership. LCD
panels offer excellent value compared to CRT monitors when viewed
from the total cost of ownership perspective. The 1600SW operates
on less than 20% of the power of typical 21-inch CRT monitors, in
addition to offering three times the brightness and up to five times
the contrast. This efficiency not only saves on power consumption
costs, but also translates into significant cooling power savings.
Flat panel displays take up far less space than CRT monitors and
can be easily mounted on flexible arms, saving on furniture expense
and allowing a higher density of both people and information displayed.
For applications requiring close communication of workers or large
amounts of information, flat panels can enable dramatic productivity
gains.
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| 7.
What is the difference between active matrix LCDs (AMLCD)
and passive matrix LCDs? |
For an LCD to work, each pixel must be energized to either let
light through or block light out. The difference between active
matrix and passive matrix displays is the way in which the pixels
are electrically addressed, or "energized." Passive matrix
flat panel displays consist of a grid of horizontal and vertical
wires. At the intersection of each grid is an LCD element that constitutes
a single pixel. Active matrix flat panels are a higher quality and
more expensive type of display in which transistors are built into
each pixel within the screen. For example, the 1600x1024 screen
size of the 1600SW requires over 14 million transistors, one for
each red, green, and blue subpixel. Active matrix, sometimes also
called TFT (thin film transistor) displays typically have higher
resolution, higher contrast, and much faster pixel response rates
than passive matrix LCDs.
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The number of colors an LCD monitor can display is dependent on
the number of grayscale levels that it can display, which is essentially
a measure of how well it is electrically addressed. Each subpixel
in the 1600SW is addressed by 8 bits of data, allowing it to have
256 (or 28) gray levels. In a color monitor, each pixel is made
up of three subpixels, one for each primary color. These red, green,
and blue dots are energized to different intensities (or grayscales)
to create a range of colors that we perceive as the mixture of these
dots. Because the shade of each of the three subpixels is determined
by 8 bits of data, the monitor displays in 24 (3x8) bit color. That
means the monitor is capable of displaying 256 (R) x 256 (G) x 256
(B), or over 16,700,000 colors! Imagine painting a picture with
only four colors. With such a limited selection, your level of expression
is greatly suppressed. With display devices, higher color depth
effectively gives you more colors with which to "paint"
your image. Some LCD monitors can only display 18 bits of color
and cannot show 24 bits of true color. While this may be fine for
some noncritical applications, serious professionals require the
full color range for their work. SGI flat panels all are 24-bit
color capable, giving you a fantastic color range, with no dithering.
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| 9.
What is color management? |
Color management, such as SGI ColorLock, consists of hardware and
software components that enable the monitor to achieve a very high
level of color accuracy and consistency. SGI ColorLock technology
is used exclusively with SGI flat panels and the Silicon Graphics
320 and Silicon Graphics 540 workstations. It allows the precise
control of gamma and color temperature over a wide dynamic range
and supports the adjustment of display characteristics to meet a
variety of industry display values, including sRGB, broadcast, graphic
arts D-50, Mac legacy, and Web viewing.
Color Calibration
The process of adjusting a display's output characteristics so as
to modify its appearance to conform to predetermined standards or
settings. Usually accomplished through the use of a color or luminance
measuring device and a lookup table (LUT) of values accessible to
the computer's graphics controller. See Gamma.
Color Filter
A red-, green-, or blue-dyed gelatin or pigment placed above each
LCD subpixel. Combinations of various light levels passing through
these color elements can produce all the visible spectral colors.
Color Gamut
The entire range of colors available on a particular device such
as a monitor. On an LCD with true 24-bit color, the color gamut
is 16,700,000 colors.
Color Resolution
The colors that can be represented on a display depending on the
number of Grayscales resolved by the LCD element.
Color Temperature
The definition of a monitor's white point, whose chromaticity coordinates
can be somewhat arbitrary, existing in color space from red-white
to blue-white. Expressed in Kelvins (K), color temperature refers
to the amount of light radiated by a perfect thermal radiator. Values
at or below 5000K appear reddish; higher numbers, for example 7000K,
appear bluish.
Column Drivers
Microelectronic circuits that provide the correct voltages to the
individual subpixels through the source lines. For example, 8-bit
drivers provide 256 gray shades, or 256 distinct colors per subpixel.
Contrast
The range between the lightest tones and the darkest tones in an
image. The lower the number value, the more closely the shades will
resemble each other. The higher the number, the more the shades
will stand out from each other.
Contrast Ratio
The ratio between the amount of light transmitted by a pixel in
its unselected ("off") state and its selected ("on")
state. In an AMLCD, this ratio is usually greater than 150:1.
Controller
A digital signal source that puts data in the correct "order"
to the display. It provides H and V sync, data enable, clock and
8-bits each of R, G, and B information.
Convergence
The clarity and sharpness of each pixel.
LCD
(Liquid Crystal Displays). These displays are fabricated using semiconductor
processes, with each pixel comprising transistors set up in a grid.
LCDs are inherently digital displays. In their most common computer
application, notebook computers, video drivers take the digital
information in the graphics frame buffer and digitally interface
to the row and column drivers that set the colors at each pixel
in the display.
Pixel
The smallest addressable unit on a display screen. The resolution
of a monitor is determined by the number of pixels covering the
width and height of the complete on-screen image.
In storage, pixels are made up of one or more bits. The greater
this bit depth, the more shades or colors can be represented. Grayscale
and color displays typically use from 4 to 24 bits per pixel, providing
from 16 to 16 million colors.
On screen, pixels are made up of one or more dots of color (subpixels).
For grayscale, the pixel is energized with different intensities,
creating a range from dark to light. Color systems use a red, green,
and blue dot per pixel, each of which is energized to different
intensities, creating a range of colors perceived as the mixture
of these dots. Black is all three dots dark, white is all dots light.
Pixel Clock
The pixel clock that resides in the LCD is critical to ensure that
as each line is scanned, the red, green, and blue components of
each pixel precisely align with each other. If there is a slight
error in the clock, this error accumulates as the line is scanned
from the left to the right edge of the display.
Pixel Pitch
The distance from the edge of one pixel to the similar edge on an
adjacent pixel.
Pixel Resolution
The number of addressable pixels in a display. Several standard
display sizes are:
| VGA: |
640 x 480 |
| SVGA: |
800 x 600 |
| XGA: |
1024 x 768 |
| SXGA: |
1280 x 1024 |
| SXGA-Wide: |
1600 x 1024 |
UXGA:
|
1600 x 1200 |
| HDTV: |
1920 x 1080 |
| UXGA-Wide: |
1920 x 1200 |
| QXGA: |
2056 x 1536 |
Refresh Rate
How many times per second the screen is refreshed (redrawn).
Response Time
The speed at which the orientation of a liquid crystal material
can change in response to a charging/discharging cycle. Typically
noted as "rise plus fall," for twisted-nematic structures,
this time ranges from 20 to 50 ms.
SVGA
Super video graphics array is a set of graphics standards designed
to offer greater resolution than VGA. There are several varieties
of SVGA, each providing a different resolution. All SVGA standards
support a palette of 16 million colors, but the number of colors
that can be displayed simultaneously is limited by the amount of
video memory available. The SVGA standards are developed by a consortium
of monitor and graphics manufacturers called VESA.
SXGA
(Super Extended Graphics Adapter) A graphics standard offering a
display resolution of 1280x1024 pixels. SXGA-Wide has a display
resolution of 1600x1024.
TFT
(Thin Film Transistor) An a-Si, p-Si, or CdSe used as a switch to
a charge storage device beneath each subpixel of an AMLCD.. A type
of LCD flat-panel display screen, in which each pixel is controlled
by from one to four transistors. The TFT technology provides the
best resolution of all the flat-panel techniques, but it is also
the most expensive. TFT screens are sometimes called active-matrix
LCDs.
TMDS
(Transition Minimized Differential Signaling). TMDS is similar to
LVDS in concept but very different in execution. TMDS is a proprietary
specification defined by Silicon Image, Inc. TMDS is the generic
form of Silicon Image's PanelLink interface technology. VESA is
currently exploring the definition of TMDS as an open standard,
but today it remains proprietary to Silicon Image.
TTL
(Transistor-Transistor Logic). A common type of digital circuit
in which the output is derived from two transistors. The term is
commonly used to describe any system based on digital circuitry,
as in TTL monitor.
UXGA
(Ultra Extended Graphics Adapter). A graphics standard offering
a display resolution of 1600x1200 pixels.
VGA
(Video Graphics Array). VGA has become one of the de facto standards
for PCs. In text mode, VGA systems provide a resolution of 720 by
400 pixels. In graphics mode, the resolution is either 640 by 480
(with 16 colors) or 320 by 200 (with 256 colors). VGA remains the
lowest common denominator among PC displays.
Viewing Angle
The bounding angles generated from a point normal to the display
surface within which can be found acceptable contrast ratios and
linear grayscales.
XGA
(Extended Graphics Array). A high-resolution graphics standard introduced
by IBM in 1990. XGA was designed to replace the older 8514/A video
standard. It provides the same resolutions (640 by 480 or 1024 by
768 pixels), but supports more simultaneous colors (65,000 compared
to 8514/A's 256 colors). In addition, XGA allows monitors to be
noninterlaced.
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