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The ideal resolution must lie within the compatible range of the display. For
example, some 27" to 36" presentation monitors are limited to input signals in
the 30 kHz to 50 kHz range. Check the Direct View Display Cross Reference
and the Projector Cross Reference on the Extron Web site (www.extron.com)
for the most up-to-date compatible ranges of most displays.
CRT displays — selecting the optimum resolution
While CRT displays do not have a native resolution, they do have a "sweet spot"
for input signal resolution. When the switcher is set to output at the sweet spot of
the CRT, it results in a detailed image with no visible scan lines. If the switcher is
set to output below the sweet spot, the displayed image has small scan lines
between the image lines. If the switcher is set to output above the sweet spot, the
image's lines overlap and the image loses detail because there are more lines and
pixels than the display can clearly resolve. When experimenting to find the sweet
spot, start with the output refresh rate set at 75 Hz and the resolution shown in the
table on the next page. Then, with a critical eye, try lower resolutions until you
achieve a solid image with optimum image detail.
CRT displays — selecting the optimum refresh rate
CRTs tend to flicker at refresh rates below 70 Hz. To achieve a flicker-free image,
try an output refresh rate of 75 Hz.
Note that as the refresh rate increases, the horizontal scan rate also increases,
placing greater bandwidth demands on the video distribution system and the
display. If the bandwidth is too high, you may see a softer image because the signal
exceeds the bandwidth of the distribution system and the display. An extremely
high refresh rate can also create a compatibility problem in which the video signal
is beyond the scan rate of the display.
Fixed pixel displays — selecting the optimum resolution and refresh rate
Display devices that are based on LCD, DLP, D-ILA, and plasma technology have a
specific number of pixels, a native resolution. These displays typically can show
higher or lower resolution signals, but can do so only by scaling the image up or
down to the native resolution. To avoid additional image scaling by the display
(and more processing is inherently bad for image quality), it is important to know
your display's native resolution.
Check the display's operation manual to determine its native resolution. Set the
switcher to match this native resolution. The switcher's refresh rate should be set
to 60 Hz for most LCD, plasma, D-ILA, and DLP projectors.
Higher refresh rates are not recommended for these display technologies. They
usually do not improve the image and may cause compatibility problems.
IN1508 Scaling Presentation Switcher • Operation
3-31
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