LED Backlight Controller Eliminates Line Banding via Phase-Delayed Clocks
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Solution Overview
Problem
LED backlights in LCD devices suffer from visible line banding artifacts due to frequency differences between the local line clock signal and the modulated control clock signal, which worsen when the duty cycle of the modulated clock signal is varied to adjust brightness, leading to image defects like LED flicker and banding artifacts.
Innovation Solution
The LED backlight controller synchronizes the modulated control clock signal with the pixel clock signal and divides it down to match the line clock frequency, using multiple phase-delayed control signals to drive individual LED strings, ensuring the product of the control clock frequency and the number of LED strings equals the line clock frequency, thereby eliminating line banding artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the modulated control clock signal frequency is varied to adjust LED brightness, then brightness control is achieved, but line banding artifacts worsen due to frequency mismatch with line clock signal
Solution Approach 1:
The patent changes the frequency parameter of the modulated control clock signal to match the line clock frequency, eliminating line banding artifacts. By adjusting the frequency parameter rather than amplitude, the system maintains brightness control while avoiding the harmful frequency mismatch that causes banding.
Solution Approach 2:
The system uses feedback by comparing the modulated control clock frequency with the line clock frequency and adjusting accordingly. The controller monitors the frequency relationship and modifies the modulated clock frequency to maintain synchronization, preventing banding artifacts from occurring.
2Object-affected harmful factors
If multiple LED strings are driven with phase-delayed control signals, then line banding artifacts are reduced, but device complexity increases
Solution Approach 1:
The patent divides the backlight system into multiple LED strings, each driven by phase-delayed control signals. This segmentation allows the total illumination to be distributed across multiple channels, reducing the visibility of banding artifacts in any single region while maintaining overall brightness uniformity.
Solution Approach 2:
The system applies periodic phase-delayed control signals to multiple LED strings. Each string is activated in sequence with specific phase delays, creating a periodic illumination pattern that averages out the banding artifacts over time, reducing their visibility to the human eye.
Data Source
AI summary
The line banding image artifact that results from the interaction of LCD ripple and LED flicker in an LCD device that utilizes LED backlighting strings is substantially reduced by selecting a number of LED strings, individually driving the number of LED strings with a corresponding number of identical clock signals that are equally phase delayed, and selecting the frequency of the clock signals so that the product of the frequency of the clock signal multiplied by the number of LED strings is equal to the line clock frequency.