French said:
XP Home.
The LCD Screen flickers. You have an idea of some parts I can change to
fix it?
What would you do?
Thank you for your time.
Compaq Nx6125 AMD Turion 2Ghz
Thank you ~ Enjoy The Rest Of Your Cajun Crawfish
There are two kinds of flickering.
1) Irregular flickering. That could be a bad contact in the wiring
of the backlight and inverter. Or, the inverter could have a problem.
Or the high voltage could be leaking or arcing over. Or one of
the connectors could be loose in the panel. The inverter outputs 600V to 1000V,
at a frequency of 25KHz or so. Total power is about 3 watts per CCFL
backlight tube. So when I refer to high voltage, it is that voltage
coming from the inverter and feeding the backlight that I'm referring to.
The white light behind a lot of LCD monitors, comes from cold cathode
fluorescent lamps. Only a few monitors use LEDs for the light source.
In addition to running at 25KHz, to make the high voltage, the
inverter can also be pulse width modulated at a frequency in the
100+ Hertz range. The purpose of that, is to implement brightness
control. By varying the percentage of time the backlight is powered,
it allows the backlight to be brighter or dimmer, without affecting
the color balance of the tube. If they just changed the voltage on
the tube, the color would be "brownish" at low voltage. The color
would be thrown off. That kind of PWM control of brightness, is
not supposed to be visible to the user. So maybe if that was a
bit wacky, that could account for some flicker.
It could even be a video driver issue, as sometimes the video card
can be convinced to do crazy things by the software. Test the monitor
on a second computer, to see if the video card makes it do the same
thing or not. That will help you decide whether the feature is coming
from the monitor, or the video card.
If the refresh rate of the video card is set to 75Hz, try setting it
to 60Hz, as for a lot of monitors, 60Hz is the normal rate that
the frames are updated on the monitor.
2) Regular flickering. First of all, flicker on an LCD is different
than flicker on a CRT. The sweep on an old CRT monitor, begins to
decay almost immediately. The CRT phosphor has a limited persistence.
To combat this, people use higher "refresh rates" on a CRT, such
as 75Hz or 85Hz, which is starting to move above the frequency that
people can still see the CRT flicker.
An LCD is entirely different. The image on the screen, persists for
the whole frame time. And this is why, when the human eye looks at
an LCD screen at 60Hz, there normally isn't any flicker. So not as
high a refresh rate is needed, for ordinary LCD monitors, because
there isn't a phosphor that is decaying during a frame time. Because
the image is present for the entire frame time, before it changes,
there isn't the same flicker effect as a CRT.
OK, so why do people see "regular flicker" on an LCD ? Some
monitors contain an optimization, intended to improve the
responsiveness of the LCD monitor. It involves blanking portions
of the screen. This would be invisible to a lot of people, but some
people seem to see this optimization and it irritates them. In some
cases, this can be turned off in the OSD (On Screen Display used to
control the monitor). The following article contains lots of information
about tricks used in LCD monitors - it is worth reading the
entire article when you have time.
http://www.xbitlabs.com/articles/monitors/display/lcd-parameters_4.html
So first, you have to decide what kind of flickering you're seeing.
If the flicker is a regular and consistent effect, then look in the
OSD for an item to disable. If the flicker is irregular, it might
have something to do with the backlight. In some cases, it can be
a video driver problem, in which case, testing the monitor on a
second computer is recommended.
Paul
