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List:       linux-sony
Subject:    [Linux-sony] noisy screen with sony laptops
From:       "Raphael Langerhorst" <raphael-langerhorst () gmx ! at>
Date:       2002-12-24 10:36:48
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As for the noise the screen makes when not turned to full bright, that's
ok if it is just above level of recognizing it. If it is really loud
then of course it is not ok.

Technical reason for making noise:
the backlight of the LCD is probably driven by a PWM module (pulse width
modulation). So if turned to full bright the voltage for the backlight
over time would look like this:

on    __________________________________
off

if not turned to full bright, it just turns off for a percentage of the
time in a period:

     |oneperiod|         |         |

on    _____     _____     _____     _____
off  |     |___|     |___|     |___|

this would be about 60% high and 40% low;

Now that's of course some frequency and we know that such impulses have
a large spectrum (of sine waves) so there is likely to be some of them
in the <20kHz area which human ears can hear -> that's the sound.

Since our eyes can't follow this high frequency of changing between high
and low states for the screen they level it out and that makes about 60%
of the maximum (I know the eyes have a logarithmic recognition but that
would only make things complicated).


Hope that helped someone,

Raphael

PS: my model is the R600HFPD




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