TECH ARTICLE
The method for adjusting brightness during the fully settled pixel period is simply by varying the amount of time the backlight is switched on during that period – a method more generally termed ‘pulsewidth modulation’ or PWM. Typically, there would be a default brightness level, with an associated PWM illumination period value, that is less than the full period during which the pixels are stable; this allows for higher or lower brightness levels by setting larger or smaller PWM values. Conventional PWM dimming turns the backlight on at the start of the stable pixel period and off after the PWM illumination period. But, as can be seen in Figure 2, there is a danger that a large PWM value may result in the backlight remaining on into the next pixel-writing period for that particular illumination zone (area A), which is clearly undesirable. To overcome this problem the Reverse PWM method aligns the end of the PWM period to the end of the fully settled pixel period as can be seen at the bottom of Figure 2 since it is better to always use area B and to start the illumination as early as necessary to achieve the desired brightness.
LED backlighting for LCD TVs using edge-lit display panels has rapidly become the technology of choice, providing both a highperformance and costeffective solution for the majority of the TV and desktop monitor market.
One point to note is the importance of referencing the timing of all illumination zones to the same V Sync that is associated with the start of that frame’s pixel writing process. Referencing some zones to a subsequent V Sync can result in brightness banding because of variations in frame duration that can result from video manipulation.
Conclusion LED backlighting for LCD TVs using edgelit display panels has rapidly become the technology of choice, providing both a high-performance and costeffective solution for the majority of the TV and desktop monitor market. The faster switching speed of LEDs coupled with advances in driver circuit design makes it possible to address some of the previous limitations of liquid crystal displays, namely motion blur and poor contrast. ■
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