The invention relates to a display device as defined in the pre-characterizing part of claim 1.
The invention also relates to a method of operating a display device as defined in the pre-characterizing part of claim 9.
An embodiment of the display device of the type mentioned in the opening paragraph is known from WO 00/38163.
The known display device comprises a display panel having picture elements and selection means for addressing, i.e. activating and deactivating, the picture elements by applying voltages to the picture elements. A picture element is a location where light can be decoupled from an optical waveguide plate. Picture elements are arranged in substantially parallel lines of the display device. When a picture element is activated, a movable element is locally brought into contact with the optical waveguide plate and light is decoupled from the optical waveguide plate. The picture element remains in this state until the picture element is deactivated, i.e. the contact is interrupted, and vice versa Picture elements are addressed one line at a time. Multiline operation can be applied, because more than one line can simultaneously be active.
The picture elements can only be active and inactive. In order to create gray levels at picture elements, the image information concerning an image is decomposed into a number of subfields. Picture elements arranged in a number of lines are displayed in consecutive lines in each subfield. These lines are part of the number of lines on the display device. Each subfield has its own predetermined number of active standard periods of time, i.e. periods of time in which picture elements arranged in a line are active after having been activated in an immediately preceding activating standard period of time, so as to be deactivated again in an immediately following deactivating standard period of time. Hereinafter standard period of time is also denoted as interval. A line is activated, meaning that picture elements arranged in the line are activated, in one interval and deactivated in one interval. By displaying all subfields consecutively in one frame time, the percentage of time during which light is decoupled from each picture element is regulated. In this way gray scales are created.
The execution of a subfield modulation scheme generates a sequence of activating, active, and deactivating intervals to activate, to keep active, and to deactivate picture elements arranged in lines. The sequence starts with an activating interval to activate a first line of a first subfield and ends with a deactivating interval to deactivate a last line of a last subfield.
A drawback of the known display device is that the sequence of activating, active, and deactivating intervals results in a time-inefficient addressing of the lines because intervals available for addressing are present in which no line is activated or deactivated. Therefore, relatively few gray scales can be displayed in one frame time. If the number of gray scales is relatively low, artifacts will appear in the image, substantially adversely affecting the image quality.
It is a first object of the invention to provide a display device of the kind mentioned in the opening paragraph which is able to display a relatively large number of gray scales.
It is a second object of the invention to provide a method of operating a display device of the kind mentioned in the opening paragraph, which enables the displaying of a relatively large number of grayscales.
The first object is achieved in that the display device in accordance with the invention is constructed as specified in claim 1.
Under predetermined conditions of the predetermined number of active standard periods of time and the predetermined second number of lines, available intervals in the first subfield are not used for activating a line because activation of a first line in a second subfield is started after the deactivation of a last line in the first subfield. The inventors have realized that in this way available intervals in the first subfield are not used for activating a line. It can now be achieved that the activation of a first line in the second subfield starts already after the activation of a last line in the first subfield. In this way gray scales are displayed in a time-efficient way. It is not always possible to activate a first line in the second subfield immediately in the next consecutive interval after activating a last line in the first subfield. This depends on the predetermined number of active intervals and the number of lines remaining to be deactivated in the first subfield. As a result, it may happen that deactivating of a line in the second subfield will coincide with deactivating of a line in the first subfield. In this case the activation of a first line in the second subfield must be postponed for one or more intervals to avoid this coincidence. Some time-efficiency is lost in this way.
Using this insight, one or a number of advantages can be obtained. The number of gray scales can be increased, and/or the length of the interval can be increased, and/or the frame time can be decreased, and/or the number of lines can be increased.
An embodiment of the display device in accordance with the invention is claimed in claim 2. In this case the predetermined number of active intervals of the first subfield incremented by one is less than the number of lines. Now active lines have to be deactivated before a last line in the first subfield is activated. If, furthermore, the second number of lines is unequal to an integer divisor of the predetermined number of active intervals of the first subfield incremented by one, available intervals in the first subfield are used for activating a line in the second subfield. It is not always possible to activate a first line in the second subfield immediately in the next consecutive interval after activating a last line in the first subfield. This depends on the predetermined number of active intervals in the first subfield, the predetermined number of active intervals in the second subfield, and the number of lines remaining to be deactivated in the first subfield.
A special embodiment of the display device in accordance with the invention is claimed in claim 3. Electrodes are simple and reliable.
An embodiment of the display device in accordance with the invention is claimed in claim 4. A possible coincidence of deactivating of a first line in the second subfield with deactivating of a line in the first subfield is avoided by arranging the subfields in order of increasing predetermined number of intervals. Before a first line in the second subfield has to be deactivated, a last line in the first subfield has already been deactivated. The activation of a first line in the second subfield now appears in the next interval after activation of a last line in the first subfield. This way of addressing is very time-efficient.
A modification of the preceding embodiment is claimed in claim 5. In binary weighted subfields the predetermined number of active intervals in a subfield equals a power of two, giving the largest number of gray scales for the lowest number of subfields. Now the image information is decomposed bit-wise, which is well-suited for displaying and manipulating the image information.
An embodiment of the display device in accordance with the invention is claimed in claim 6. This embodiment enables the application of subfield modulation schemes generating a regular sequence of activating, active, and deactivating intervals, resulting in a time-efficient addressing of the lines.
A special embodiment of the preceding embodiment is claimed in claim 7. The activating intervals are separated by an odd number of intervals. Now there is no need for a fixed order of bits, and furthermore a regular sequence of activating, active, and deactivating intervals is generated, at the cost of some time efficiency. In between the activating interval for activating a line and the activating interval for activating a next consecutive line there is one unused interval. Except in the start and the end phase of the sequence, the unused intervals are filled with deactivating intervals. This is due to the constraint that the predetermined number of active intervals of all subfields is even. A collision of activating and deactivating intervals is avoided thereby. The ordering of the activating and deactivating intervals is very smooth and deterministic. The generation of the sequence of activating, active, and deactivating intervals can thus be readily realized in hardware.
A special embodiment of the previous embodiment is claimed in claim 8. Compared with the previous embodiment, the activating intervals are separated by one interval, and therefore this embodiment is the most time-efficient one of the embodiments as claimed in claim 7.
The second object is achieved in that the method of operating a display device in accordance with the invention is arranged as specified in claim 9.
A special embodiment is claimed in claim 10.
These and other aspects of the invention will be further elucidated and described with reference to the drawings, in which:
a is a diagrammatic cross-sectional view of the display panel;
b is a diagrammatic front elevation of the display panel;
The Figures are schematic and not drawn to scale, and the same reference numerals refer to corresponding parts in all Figures.
In
In
In
In the next Figures the subfield modulation scheme of the first two subfields of
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It will be obvious that many variations are possible within the scope of the invention without departing from the scope of the appended claims.
Number | Date | Country | Kind |
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01204541.5 | Nov 2001 | EP | regional |
Filing Document | Filing Date | Country | Kind |
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PCT/IB02/04774 | 11/12/2002 | WO |