Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-03-15
2001-07-24
Hjerpe, Richard (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S095000, C345S096000
Reexamination Certificate
active
06266039
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal apparatus, a driving method thereof, and a projection-type display apparatus and electronic equipment using the liquid crystal apparatus.
DESCRIPTION OF THE RELATED
For example, with an active-matrix type liquid crystal apparatus, action of writing data to the liquid crystal layer of each pixel is executed by point-at-a-time driving, via switching elements such as a plurality of TFTs (thin-film transistors) connected to a scanning signal line.
Also, in order to prevent unevenness in the display owing to imbalance in potential applied to the liquid crystal, and in order to prevent deterioration and so forth of the liquid crystal due to the direct current applied to the liquid crystal, polarity inversion driving is performed, wherein the polarity of the voltage applied to the liquid crystal is inverted at a certain time.
Polarity inversion driving is a method of driving wherein voltage is applied to one end of the liquid crystal, the polarity (positive or negative polarity) of this voltage being opposite to a reference potential applied to the other end of the liquid crystal. Incidentally, in the present Specification, the term “polarity” refers to the polarity of the voltage applied to both ends of the liquid crystal. In order to perform polarity inversion driving width an active-matrix type device using TFTs, either the potential applied to the common electrode opposing the pixel electrode across from the liquid crystal is changed, or the potential level of the image data signal is changed as to a reference center potential of the voltage amplitude of the image data signal applied to the pixel electrode.
Known types of polarity inversion driving methods involve inversion by line wherein polarity inversion is performed each time a scanning signal line is selected, or inversion by line combined with inversion by dot wherein polarity inversion is performed for each pixel connected to one scanning signal line.
FIG.
11
and
FIG. 12
are models for describing the polarity inversion driving method. With conventional active-matrix type liquid crystal apparatus, a polarity inversion driving method has been employed wherein point-at-a-time driving is performed and inversion driving is performed for each pixel (including for each line), and wherein pre-charging of the data signal lines is performed collectively during the blanking period immediately before.
In FIG.
11
and
FIG. 12
, S
1
through S
4
represent data signal lines, and H
1
through H
4
represent scanning signal lines. The “+” and “−” for each pixel represent the voltage applied to the liquid crystal of each pixel, and the polarity of the pre-charge potential supplied to the data signal lines immediately prior to the application of the voltage.
FIG. 11
represents the voltage polarity of each pixel at field N, and
FIG. 12
represents the voltage polarity of each pixel at field N+1. Regarding polarity inversion driving per pixel and per line, the arrangement is such that differing polarity voltage is applied to each neighboring pixel connected to the same data signal line (each neighboring pixel in the vertical direction in FIG.
11
and FIG.
12
).
In this case, even when waiting the same black data, for example, on the display to two neighboring pixels which are connected to the sane data signal line and connected to different scanning signal lines, the signal level for each of the pieces of black data differs, due to the polarity inversion driving. At this time, the data signal line itself has parasitic capacity, so time is required for changing the potential of the data signal line from the black level potential on the positive polarity side to the black level potential on the negative polarity side.
With reference to FIG.
13
and
FIG. 14
, a description will be given regarding change in the potential of the data signal line when writing the same black data to two neighboring pixels which are connected to the same data signal line.
In
FIG. 13
, C
10
represents the parasitic capacity of the data signal line S
1
(i.e., the equivalent capacity of the data signal line S
1
). Also, the “−” and “+” noted to the left side of
FIG. 13
represents the polarity of the voltage written to the pixels
22
and
24
. Incidentally, the pixels
22
and
24
are both to display “black”. The pixels are comprised of a storage capacity and a pixel electrode to which data signals are supplied via a switching element, and a liquid crystal layer to which voltage is applied between the pixel electrode and common electrode.
As shown in
FIG. 14
, during the horizontal scanning time T
1
, black level potential B
1
is applied to one end of the pixel
22
and black is displayed, and during the next horizontal scanning time T
2
, black level potential B
2
is applied to one end of the pixel
24
and black is displayed. In this case, a common potential set between the black levels B
1
and B
2
is applied to the other end of the pixels
22
and
24
, so that voltage of a negative polarity is applied to the pixel
22
, and voltage of a positive polarity is applied to the pixel
24
, thus inverting the polarity of the voltage applied to the liquid crystal for the same black display. Moreover, with a normally-white display such as described above, the difference in potential between the black level potentials B
1
and B
2
is greatest, as compared with display of other gray scale. Accordingly, in the event that pre-charging is not performed, the parasitic capacity C
10
of the data signal line S
1
must be charged (or discharged) by means of the image data signal itself, so as to change the potential of the data signal line from the black level potential B
1
to B
2
, as represented by “R1” in the Figure.
Conversely, by means of performing pre-charging of the same polarity as the polarity of the data signal before supplying the data signal i.e., by means of performing pre-charging before the horizontal scanning time T
2
so as to maintain the data signal line S
1
at the high-voltage second pre-charging potential PV
2
, as shown as “R
2
” in the Figure, all that is necessary is to change the potential of the data signal line from the pre-charging potential PV
2
to the black level voltage B
2
, so the amount of charging (discharging) of the parasitic capacity of the data signal line S
1
dose not have to be great. Accordingly, driving of the liquid crystal is increased in speed
Now, regarding conventional liquid crystal apparatus, the arrangement has been such that the black level potentials B
1
and B
2
are respectively set at 1V and 11V, the white level potentials W
1
and W
2
are respectively set at 5V and 7V, and the pre-charging potentials PV
1
and PV
2
are respectively set at 4V and 8V. That is to say, the pre-charge potentials PV
1
and PV
2
have been set symmetrically to the center potential (6V) between the black level potentials B
1
and B
2
, which are the video amplitude.
The 4V and 8V are voltages which are applied to one end of the liquid crystal via a switching element at the time of displaying intermediate gray scale, and are equivalent to the potential level at the time that the T-V curve, which represents the relation between the voltage applied to the liquid crystal (V) and the transmittance of the liquid crystal apparatus (T), becomes the steepest. In other words, 4V and 8V are equivalent to potential levels at the time that the change in transmittance corresponding to change in voltage applied to the liquid crystal is the greatest. By means of setting the pre-charging potentials PV
1
and PV
2
as such, the data signal line can be charged or discharged in a short time from the precharging potential to a potential for intermediate gray scale display, so accurate intermediate gray scale display can be realized even in the event that the sampling period is reduced.
Now, optical cross-talk is a problem with liquid crystal apparatus which perform liquid crystal display using light from a light source, e.g., projection
Hjerpe Richard
Nguyen Kevin M.
Oliff & Berridge. PLC
Seiko Epson Corporation
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