overscanning in LCD monitors
phil-new...@ipal.net
circuitry update the LCD grid at a faster rate than the video comes in from the
video input connector (either analog or digital)? One thing I am curious about
is why so many monitors don't operate below 50 Hz vertical frame rate. The LCD
grid can be updated slower, so why not just do so if the video is coming in at
a slower rate?
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Bob Myers
<phil-new...@ipal.net> wrote in message
news:fvbqq...@news1.newsguy.com...
> Are LCD monitors actually doing overscanning? What I mean is, does the
> monitor
> circuitry update the LCD grid at a faster rate than the video comes in
> from the
> video input connector (either analog or digital)? One thing I am curious
> about
> is why so many monitors don't operate below 50 Hz vertical frame rate.
> The LCD
> grid can be updated slower, so why not just do so if the video is coming
> in at
> a slower rate?
Per what you are calling "overscanning," not usually. (The term
"overscanning" is generally one used in the context of CRT monitors
or televisions, where the beam actually scanned beyond the usable
limits of the screen and so put some fraction of the image offscreen.)
LCD panels as used in monitors do, however, tend to operate over
a fairly narrow range of update rates (re the panel itself), most over
centered around 60 frames per second. If the incoming video is at
a signficantly different rate (say, 75 FPS), then a frame-rate conversion
will be performed by the monitor's "front-end" electronics (the
scaler/controller IC, etc.). There's really nothing to be gained by
operating the monitor at any such "non-native" rate (and the process
can introduce some motion artifacts), so it is best to always drive
the monitor at its preferred timing if this is possible.
And in most panels, there IS a lower limit on the update rate of
the panel. Once written, the LCD pixels/subpixels will not hold their
assigned value forever, and neither will certain other parts of the
drive electronics. You need to refresh them at a regular rate to
maintain acceptable image quality. This is NOT the same mechanisms
that caused CRT flicker, etc., but there is still a minimum guaranteed
rate below which the performance specifications may not be met.
Think of the panel as a big, slow dynamic RAM, and you're not all
that far off.
Bob M.