Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-04-21
2001-07-10
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S100000, C345S058000
Reexamination Certificate
active
06259425
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a matrix type display apparatus, such as active matrix type liquid crystal display apparatus, especially one which includes a video signal voltage compensating circuit for reducing distortion of a video signal waveform which may occur on the bus for supplying a video signal to sample hold circuits in a relatively large-scale matrix type display apparatus.
2. Description of the Related Art
In conventional large-scale liquid crystal displays having a diagonal size of 30 cm or more and configured to drive signal lines of liquid crystal elements by sample-holding a video signal sequentially, because of insufficient operation speeds of conventional sample-hold circuits, an increased number of video signal transmission lines is used to transmit video signals in parallel and to limit video signal band widths narrower.
For example, it has been proposed to increase the number of video signal transmission lines to 24 lines, as many as eight times three lines used conventionally, to transmit video signals in parallel and to limit the video signal bandwidth to one eighth. This proposal certainly alleviates the need for operation speeds of sample-hold circuits and can decrease distortion so much as the required frequencies become lower.
However, this approach results in increasing the wiring area around the display portion due to an increased number of video signal transmission lines and in increasing the manufacturing cost because of the need for an increased number of circuits for outputting video signals.
Another problem with the use of an increased number of video signal transmission bus lies in distortion of video signals and interference between signals on the video signal transmission bus. The video signal transmission bus is disposed in parallel with one side of the display portion of a matrix-type display device. Therefore, the larger the diagonal size of the matrix-type display device, the longer the transmission bus. It results in increasing distortion of video signals and interference between different video signals that are transmitted in parallel.
Especially, in a active matrix TFT (Thin Film Transistor) liquid crystal display apparatus, or the like, in which liquid crystal display elements and thin-film transistors are formed integrally in an array on a glass substrate, since a thin-film electrode is used as the video signal transmission bus, which is made of aluminum or the like, the resistance value of the wiring is as large as approximately 1 k&OHgr;, for example, and causes distortion of video signals beyond a negligible value.
FIG. 1
shows a construction of a conventional active matrix liquid crystal display device.
Connected to an input end of video signal transmission bus 53 are a gamma (&ggr;) correction table circuit
10
for correction of output characteristics, connected to an input terminal IN supplied with video signals as display data, a digital-to-analog converter (DAC)
40
connected to the output of the gamma correction table circuit
10
, and a video bus drive circuit
60
connected to the output of the digital-to-analog converter
40
. Signal lines are connected (tapped) to the video signal transmission bus
53
via switches in periodical intervals. These switches are controlled by a sample-hold control circuit
51
made up of shift registers, for example, and form a sample hold circuit group
54
together with capacitors C
1
associated with individual switches.
Scan lines controlled by a scan electrode drive circuit
52
extend across the signal lines. Provided at respective cross points are thin-film transistors, with gates connected to the scan lines, drains connected to signal lines and sources connected to liquid crystal pixels expressed in capacity, so that, when the thin-film transistors connected to the scan lines are driven by the scan electrode drive circuit
52
, video signals are applied to pixel electrodes, and an image can be displayed.
The video signal transmission bus
53
has a relatively large resistance. Assuming that each signal line has a resistance r&OHgr; and a floating (parasitic) capacitance CO as shown in
FIG. 1
, the resistance and the parasitic capacitance cause a distortion in wavelength of an input pulse signal.
FIG. 1
shows waveforms X
0
, Y
0
and Z
0
at three nodes, namely, node X nearest to the input end, node Y in an intermediate position and node Z at the end, and shows that the distortion increases as nodes become remoter from the input end. In greater detail, the originally input pulse signal waveform appears at the node X
0
nearest to the input end of the video signal transmission bus
53
; however, a distortion caused by the resistance of the video signal transmission bus
53
and a parasitic capacitance appears in the signal waveform at the node Y
0
positioned between the input end and the terminal end of the video signal transmission bus
53
, and the distortion cumulatively increases as nodes approach the terminal end where, at the node Z
0
, the distortion of the signal waveform increases further. Causes of the distortion include the time constant of each pixel and influence from a preceding pixel, and influence of reflection especially from the wiring end cannot be disregarded.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a display apparatus capable of controlling distortion of video signal caused mainly by a wiring resistance and a parasitic capacitance on a video signal transmission bus.
According the first aspect of the present invention, there is provided a display apparatus comprising:
a plurality of pixel capacitors in an matrix arrangement;
a video signal transmission bus for transmitting a video signal;
a plurality of sample-hold circuits connected to said video signal transmission bus to sample-hold said video signal in synchronism with a horizontal synchronizing signal;
a plurality of signal lines for delivering sampled video signals from said sample-hold circuits to said pixel capacitors via switching elements; and
a video signal voltage compensation circuit located at an input portion of said video signal transmission bus and provided with a filter for receiving said horizontal synchronizing signal and for varying frequency-amplitude characteristics of said video signal to be outputted to said video signal transmission bus in response to elapsed time from a reception of said horizontal synchronizing signal.
A display apparatus according to the invention uses a drive circuit having a sample-hold circuit block including a plurality of sample-hold circuits for sample-holding an entered video signal sequentially to sample the video signal sequentially and to drive signal lines of liquid crystal display devices, and connects a video signal voltage compensation circuit to the input end of a video signal transmission bus to which the sample-hold circuits are connected. The video signal voltage compensation circuit has a filter for compensating waveform distortion of the video signal which may occur on the video signal transmission bus while the video signal is supplied to the sample-hold circuits, and includes means for changing the coefficient for compensation by the filter depending upon the position on the video signal transmission bus where one of the sample-hold circuits currently under operation is connected.
Thus, the display apparatus can display high-quality and high-resolution images even in case of a large-scale liquid crystal display device, without increasing the number of video signal transmission bus lines at all, or so much, by changing the coefficient for signal compensation by the filter so as to minimize the distortion of the signal introduced into one of the sample-hold circuits under operation, depending upon the position of the sample-hold circuit on the video signal transmission bus and by alleviating waveform distortion of the video signal occurring on the video signal transmission bus of the matrix type display device and interference between three primary color
Kabushiki Kaisha Toshiba
Mengisteab Tewolde
Pillsbury & Winthrop LLP
Saras Steven
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