Appeal No. 99-0872
Application 08/885,399
matrix liquid crystal displays. First, the appellants state
(Br. at 4):
In order to reduce the number of DACs or video
amplifiers required, and hence lower system cost,
the number of input lines to the DACs or video
amplifiers can be reduced. The problem with doing
this is that the width of the integrated
multiplexers, required to deliver data on a small
number of input lines to a much larger number of
display data lines, must be increased. That is,
each input line must serve more display data lines.
All the data lines must be charged to the required
data voltage within each horizontal line time.
Thus, if fewer input lines are used (i.e., more
display data lines per input line) the time
available to charge each individual data line is
reduced. The charging must take place through at
least one polysilicon TFT. These devices have
significantly lower channel conductance than
conventional single crystal transistors, and thus
the short charging time tends to degrade the
accuracy with which the data lines are charged. The
final voltages on the data lines at the end of each
line time are transferred into the addressed row of
pixels, and thus a lower precision of the data line
voltage is reflected directly in less accurate pixel
voltages. This degrades the ability of the display
to render gray scales accurately, and reduces image
quality.
In order to maintain high image quality, the
number of input lines can be increased. However,
for each additional input line, an additional DAC or
video amplifier is required, along with its
associated support circuitry, and the cost and
complexity of the external drive system goes up
significantly.
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