[G4] Displays LCD/CRT/Cinemas

[Garth Fletcher]

On Feb 2, 2005, at 10:49 AM, Polkadoter at aol.com wrote:

 > Are LCD display different than CRTs?  CRT Computer Monitors display in
 > square pixels - do LCD's display in square or non-square?  And more
 > importantly how about the Apple Cinema displays - square or
 > non-square?  Working with a variety of software programs and all the
 > pixel aspect ratios.  I need a new on my tower and want the correct
 > display.
 >  Thanks
 >  Linda

Generally speaking the LCD displays will be somewhat brighter and often
look "sharper" than a CRT.

Changes in viewing angle can cause very noticeable differences in both
color and brightness.  Cheap LCDs often show dramatic changes for really
small changes in angle - so that it becomes really difficult to keep your
viewing angle steady enough to do good color work. Other more expensive
LCDs do much better and are very usable.  If at all possible you *should*
sit in front of the model display you are considering to see if it is
suitable for your needs.

Good LCDs have a wide color range, though not quite as wide as a CRT, and
can be calibrated for good color work.  Note that the screen calibrator
must be LCD compatible - meaning that it doesn't attach via a suction
cup which can ruin the LCD!


The most often overlooked difference however is that LCDs have a "native
resolution" - the pixels are manufactured right into the display itself -
whereas CRTs don't in the same sense.

A CRT display uses a moving beam to "write" pixels on the phosphor coating
inside the tube.  Assume for now that the beam is moving at a constant speed -
if the beam is turned on and off very quickly it will "write" many small
dots; if it is turned on and off more slowly then the dots will be larger.

By adjusting the timing you can create 640, or 800, or 1024, or 1280, or..
pixels across the width of the display.

This is why the same CRT can be set to display, say, 800x600 or 1024x768,
or 1280x1024 - just by changing the timing (also in the vertical direction) -
and can be set to create square or rectangular pixels (though most video
boards are preset for resolution pairs which produce square pixels).

The key concept is that the CRT's resolution, up to some limit, is set
by the timing of the signals sent to it by the video board.

This can be very useful.  Those of us with ageing eyes often set the display
resolution to lower values, say 1024x768 instead of 1280x1024, to make
the pixels a bit bigger and easier to see.


LCDs are *very* different.  The resolution is built into the display itself
and cannot be altered.

Most LCDs are designed for around 100 pixels/inch, and all that I've
looked into provide "square" pixels.

Most 17" and 19" LCD displays are designed for 1280x1024 native resolution;
spaced at about 100/inch in 17" displays and 86/inch in 19" displays.

An LCD can "pretend" to work at some other resolution, but it does so by
scaling the image "on the fly" to fit the display's native resolution.

For example, if you want your 1280 wide LCD to display 1024 pixels, it can
only do so by either:
  a) using only the central 1024 pixels, leaving black 128 pixel margins on
     each side, which of course wastes a bunch of expensive screen space, or
  b) write 5 pixels in the display for every 4 in the image, so that the 1024
     image pixels get spread out over the 1280 display pixels.  This sort of
     scaling creates nasty artifacts - diagonal lines whose apparent thickness
     varies (pulsates) and/or blurring.

The effect is quite dramatic - just change the video board's settings from
the "native resolution" to something else and the quality of the LCD image
really goes down hill.

As a practical matter, you really need to run the LCD display at its "native
resolution". That means you must evaluate your needs to decide if 100 dpi
resolution will work for you when viewing items such as menus or toolbars
whose size you can't adjust.  Being a graybeard, I buy only 19" LCDs because
I really appreciate their easier to view 86 dpi (unique among LCDs).

Hope this helps...
-- 
Garth