Multiple Pulse Width Modulation for Liquid Crystal Gray-Scale Control
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Solution Overview
Problem
Existing digital modulation methods for liquid crystal micro displays, such as PWM and DFM, face challenges in achieving monotonic gray-scale control due to complexities, high data rates, and the need for fast LC response times, which are not feasible in many display systems.
Innovation Solution
A new digital driving method using multiple pulse-width modulation (MPWM) with two or more pulse-width modulated pulses separated in time to allow for LC turnoff, reducing data rate and bandwidth requirements, and allowing for monotonic electro-optic behavior even with differing rise and fall response times.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pulse width modulation (PWM) is used to achieve excellent gray-scale control, then gray-scale linearity is improved, but device complexity and data rate requirements increase significantly
Solution Approach 1:
The patent divides the single PWM pulse into multiple sub-pulses separated by turn-off periods. This segmentation allows the liquid crystal to fully relax between pulses, eliminating the need for complex compensation circuits while maintaining gray-scale linearity. The gray level is determined by the cumulative duration of sub-pulses within each frame period.
Solution Approach 2:
The patent employs periodic pulse sequences where sub-pulses are distributed throughout the frame period with regular intervals. This periodic structure simplifies the timing control logic compared to continuous PWM, as the controller only needs to manage discrete pulse events at predetermined time slots rather than continuous duty cycle adjustment.
2Device complexity
If duty factor modulation (DFM) is used to reduce complexity, then device complexity is reduced, but monotonic gray-scale behavior deteriorates due to non-monotonic response at certain gray levels
Solution Approach 1:
The patent proactively prevents the non-monotonic behavior by inserting turn-off periods between voltage pulses. This preliminary action ensures the liquid crystal fully returns to its relaxed state before the next pulse, eliminating the cumulative effect that causes non-monotonic gray-scale response in conventional DFM schemes.
Solution Approach 2:
The turn-off period acts as an intermediary element between voltage pulses. This intermediate state allows the liquid crystal to completely relax and reset, serving as a buffer that prevents the interaction between rising and falling edges that causes non-monotonic behavior, while still allowing the pulses to contribute to the overall gray level through cumulative exposure.
3Measurement precision
If fast LC response times are required for PWM operation, then gray-scale control is improved, but the feasibility of implementation deteriorates due to unrealistic speed requirements
Solution Approach 1:
The patent performs the liquid crystal relaxation action in advance by inserting turn-off periods before the frame ends and before subsequent pulses occur. This preliminary relaxation ensures that each pulse starts from a known baseline state, eliminating the need for the liquid crystal to respond extremely fast to achieve accurate gray levels.
Solution Approach 2:
The patent dynamically distributes the voltage application throughout the frame period using multiple sub-pulses rather than applying the full voltage in a single continuous pulse. This dynamic approach allows the liquid crystal to operate within its natural response time characteristics, as each sub-pulse only needs to drive the crystal from a relaxed state, avoiding the need for extremely fast response times.
4Measurement precision
If multiple pulses are used in DFM to achieve gray levels, then gray-scale range is improved, but data rate requirements increase drastically
Solution Approach 1:
The patent merges the gray-level encoding function with the temporal distribution of sub-pulses. Instead of requiring separate data signals for each pulse in conventional DFM, the invention encodes the gray level information in the cumulative duration and positioning of sub-pulses within the frame, reducing the data rate by utilizing the time dimension for information encoding.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach simplifies the implementation, reduces memory and data rates, and achieves monotonic gray-scale modulation with lower logic and bandwidth requirements, making it more economical and feasible for high-density displays.
Implementation Method 1
modulating the polarization rotation characteristics (and thus the net optical transmission) of a liquid crystal micro display
Data Source
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AI summary
A method, device and computer program are detailed for modulating write light. For a plurality of pixel locations of an electro-optic layer of an optical write valve and across each of a plurality of consecutive frames, a set of pixel data bits is modulated across a first (102) and a second pulse width period (103) of the frame. The first and second pulse width periods, and adjacent pulse periods of sequential frames, are separated from one another by a pulse-off (104)that is at least equal to a response time of the electro-optic layer during which no bits are modulated. Separately in each frame, write light is output from each of the plurality of pixel locations according to the modulated pixel data bits in the frame. In an embodiment, the set of pixel data bits are modulated by applying a volatage at a pixel location of the electro-optic layer in synchronism with illuminating a light source that illuminates that pixel location.