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STN Technology
Kyocera proudly produces color STN liquid crystal displays. These displays are also
called passive matrix to distinguish them from a related LCD technology called
TFT or active matrix. In most discussions that compare these two
technologies, active matrix displays are judged a better because of their better
response time and better contrast. The price of this is the presence transistors
and a capacitor built on the glass in each subpixel. Each of these thousands of
transistors consumes power. They also consume surface area of the glass in each
subpixel, so there is less glass area to transmit light. Of course this reduction
in transparency is easily compensated by using a more powerful backlight, which
further increases power consumption.
In the case of reflective LCDs, there is no backlight. A transistor would simply get
in the way. Reflective LCDs almost always use passive matrix technology. Transflective
displays also do not want to waste light transmitting area on transistors, so again,
passive matrix is is often used for transflective displays.
Kyocera is the world leader in color passive matrix displays. The advantage of color
in communicating with people is well known and is driving the placement of color LCDs
into control panel and instrument applications. How many colors can be produced by a
color STN display? How many bits per pixel? "Bits per pixel" has meaning for
the controller, not for the
STN LCD. The
more bits per pixel in an image sent to the controller, then the more controller video
memory required to hold it. Fundamentally the LCD has 3 bits per pixel (red, green and
blue) for 8 colors. But all controllers do much better than 8 colors by changing these
bits over adjacent frames to achieve intermediate shades of the primary colors.
The very popular Epson
controllers achieve 4096 colors by modulating the 3 primary color bits over 16 frames.
It is most efficient to set up these controllers as 8 bpp which implies 256 colors out
of a palette of 4096 colors. 16 bpp images can also be sent which yield 64K colors if
you allow the controller to use spatial dithering, but spatial dithering does not look
so good on a LCD screen viewed up close. You can prohibit spatial dithering, which will
cause the controller to reduce the 16 bpp to 4096 colors. 4096 is also the typical
number of colors produced by controller function blocks inside highly integrated
microprocessors or SOC devices.
The very popular Asiliant
(formerly Chips and Technology) controllers achieve 16.7 million colors from 256
gray shades in each primary color by using a proprietary algorithm which they call
TMED. The input image can then be 24 bpp.
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