Active & Passive Matrix, technically

["Chris Schumann" <CHRISSC@leonardo.lmt.com>]

> From: krisdw@basegrp.com (Kris D. Williams)

> You typically won't see much motion video on a passive matrix
> screen because the pixels, since they are passively lit, don't 'change' fast
> enough.  
I'd really like to get technical here, just for fun.

Active matrix pixels are also passively lit. They are brighter because 
the driver for the pixel is right at the pixel. Since each pixel is 
basically a capacitor, this is faster because there is much less area 
to charge than a whole line of pixels driven by a chip off the edge of 
the screen. A larger voltage can also be applied because of the quick 
charge time. This also allows for a wider dynamic range, or more levels 
of brightness, or more colors. Because there are now several hundred 
thousand drivers, it is more likely that a few will be bad, hence the 
stuck-on or stuck-off pixels.

> You will also find that passive matrix screens suffer from ghosting (i.e. if
> you move your mouse to fast, you can lose it on the screen for a while).
This is the exact same effect. Namely, the image changing faster than 
then screen can accomodate.

> They also don't have as great a viewing angle as the active matrix screens.  
Just for education, does anyone have an explanation for this?

> You will also see a type of passive matrix called dual scan passive matrix.
> It is much better than standard passive matrix but is still not as fast and
> clean as active matrix.
Dual scan reduces many of the problems by using two controllers, one on 
each half of the screen. This means each line of pixels is only half as 
long, so has a lower capacitance, and each half of the screen is 
updated at the same time, effectively doubling the scan rate.