Active Matrix Display Grey-Level Generation With Simpler DACs
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Solution Overview
Problem
The complexity and cost of digital to analogue converter (DAC) circuits in active matrix liquid crystal displays (AMLCDs) increase significantly with higher bits/pixel, making it challenging to integrate high-accuracy DACs onto the glass substrate, which is necessary for achieving 6 bits/pixel to avoid unpleasant visual artefacts, especially in images with gradual color changes.
Innovation Solution
The integration of multi-level digital to analogue conversion with in-pixel level generation using sub-pixels with different areas or charge redistribution circuit elements, allowing for the reduction of DAC complexity by generating additional grey levels within each pixel, such as using area weighting or charge sharing techniques, enabling the use of less complex 5-bit DACs to achieve 6-bit output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high-bit DAC circuits are integrated onto the glass substrate to achieve 6 bits/pixel resolution, then image quality and grey scale accuracy are improved, but the complexity and area of the DAC circuits increase significantly
Solution Approach 1:
The pixel is divided into multiple sub-pixels with different areas (e.g., 1:2 area ratio). Each sub-pixel is driven by the same DAC output, but their different areas create intermediate grey levels through area-weighted averaging. This segmentation allows a lower-bit DAC to achieve higher effective resolution.
Solution Approach 2:
Different sub-pixels within the same pixel have different areas, creating local quality variations. The larger sub-pixel contributes more to the overall grey level than the smaller one, enabling fine grey scale control without requiring high-bit DAC precision.
2Measurement precision
If DAC circuits with high conversion accuracy are implemented in LTPS on glass, then 6 bits/pixel can be achieved to avoid visual artefacts, but the area occupied by DACs increases significantly
Solution Approach 1:
By segmenting the pixel into sub-pixels of different areas, the patent reduces the required DAC resolution. Instead of needing a full 6-bit DAC for each pixel, a lower-bit DAC combined with area-weighted sub-pixels achieves the same effective grey scale resolution, significantly reducing DAC area.
Solution Approach 2:
The patent changes the physical parameter of sub-pixel area to encode grey level information. Rather than using digital bit precision to control grey levels, the system uses continuous area variation of sub-pixels, which can be implemented with simpler, smaller circuitry.
3Device complexity
If one bit drive scheme is used to simplify driver circuits, then DAC complexity is reduced, but image quality and grey level resolution deteriorate
Solution Approach 1:
The patent segments each pixel into multiple sub-pixels that can be independently controlled. This allows multi-bit driving schemes to be implemented with simplified per-pixel circuitry, achieving high grey level resolution without complex driver circuits by distributing the control across multiple simpler sub-pixel pathways.
Data Source
AI summary
An active matrix display has a column driver for providing signals to the pixels for driving the display elements, the column driver comprising digital to analogue converter circuitry providing a first number of display element drive levels. Within each pixel, the first number of display element drive levels is converted into a second, greater number, of pixel grey levels. This combines multi-level digital to analogue conversion with in-pixel level generation and enables the complexity of the DACs to be reduced so that they can be integrated onto the display substrate, for example using low temperature polysilicon processing.