Projector PWM Frequency Synchronization for LCD Uniformity
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Solution Overview
Problem
Conventional projectors face difficulties in displaying high-quality images in a wide dynamic range due to image persistence and non-uniformity caused by polarity reversal of the applied voltage in liquid crystal displays, especially when using Pulse Width Modulation (PWM) for light source control.
Innovation Solution
A projector design that sets the fundamental frequency of PWM based on the polarity-reversing frequency, ensuring equal ON-time for both positive and negative polarities, and optionally reversing the phase of PWM every polarity-reversing period, to offset brightness differences and reduce gradation shift and non-uniformity, using a light source unit with a solid-state light source and a liquid crystal display.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the light source unit is controlled by PWM with arbitrary frequency, then the dynamic range is widened, but brightness non-uniformity and gradation shift occur due to polarity reversal in the liquid crystal display
Solution Approach 1:
The patent applies parameter changes by setting the PWM fundamental frequency to an even multiple of the polarity-reversing frequency (e.g., 120 Hz for 60 Hz polarity reversal). This parameter relationship ensures that the light source remains ON for equal durations during both positive and negative polarity periods, eliminating brightness non-uniformity while preserving the wide dynamic range achieved through PWM control.
Solution Approach 2:
The patent employs periodic action by synchronizing the PWM modulation period with the polarity-reversing period of the liquid crystal display. The light source is modulated in a periodic manner where the ON/OFF pattern repeats every polarity-reversing period, ensuring that equal time is spent in ON state during both polarities, thus preventing display non-uniformity.
2Reliability
If the polarity of applied voltage is reversed to prevent image persistence, then liquid crystal degradation is prevented, but brightness changes become recognizable causing display non-uniformity
Solution Approach 1:
The patent applies the counterweight principle by balancing the ON-time of the light source during positive polarity with the ON-time during negative polarity. The PWM control is designed so that the light source contributes equal brightness during both polarity states, effectively counterbalancing the inherent brightness differences caused by polarity reversal and preventing recognizable display non-uniformity.
Solution Approach 2:
The patent changes the PWM control parameters to match the polarity-reversing frequency. By setting the fundamental frequency of PWM to an even multiple of the polarity-reversing frequency, the system ensures that the light source modulation is synchronized with polarity changes, maintaining equal contribution from both polarities and preventing display non-uniformity.
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 allows for high-quality image display in a wide dynamic range by offsetting brightness differences and minimizing gradation shift and non-uniformity, ensuring the image is viewed in average brightness, even with moving images.
Implementation Method 1
a liquid crystal display can be employed... a spatial light modulator that modulates light supplied from the light source unit according to an image signal
Implementation Method 2
the light source unit include a solid-state light source
Data Source
AI summary
A projector includes a light source unit that supplies light, and a spatial light modulator that modulates light supplied from the light source unit according to an image signal. The spatial light modulator is driven by an applied voltage whose polarity is reversed according to a polarity-reversing frequency which is specific to the spatial light modulator. The light source unit supplies light which is modulated according to pulse width modulation for which fundamental frequency is set based on the polarity-reversing frequency.