Low-power color TFT LCD display for hand-held embedded systems

Abstract

An LCD (Liquid Crystal Display) is a standard display device for hand-held embedded systems. Today, color TFT (Thin-Film Transistor) LCDs are common even in cost-effective equipments. An LCD display system is composed of an LCD panel, a frame buffer memory, an LCD and frame buffer controller, and a backlight inverter and lamp. All of them are heavy power consumers, and their portion becomes much more dominant when running interactive applications. This is because interactive applications are often triggered by human inputs and thus result in a lot of slack time in the CPU and memory system, which can be effectively used for dynamic power management.In this paper, we introduce low-power LCD display schemes as a system-level approach. We accurately characterize the energy consumption at the component level and minimize energy consumption of each component without appreciable display quality degradation. We develop several techniques such as variable-duty-ratio refresh, dynamic-color-depth control and backlight luminance dimming with brightness compensation or contrast enhancement. Each method exhibits power reduction of 260mW, 250mW and 480mW, respectively. The aggregate energy reduction ratio is 28% out of total energy consumption including the CPU and the main memory system when we execute a document viewer. We also demonstrate that we can extend the battery life about 38% and 20% for a text editor and an MPEG4 player, respectively.

A Survey of Low-power Techniques for Liquid Crystal Display Systems with Light Emitting Diode Backlight Units

“…In [7], the authors introduced a backlight luminance dimming technique with brightness compensation or contrast enhancement. They modeled the intensity perceived by human eyes as a linear function of LCD transmittance, backlight luminance, and image luminance.…”

“…In the mean time, contrast enhancement is used for images with distinct color spectrums and it leads to more readable display and more aggressive power saving. [12] • Adaptive global backlight dimming [13] Low-power techniques with DVS • Variable duty-ratio refresh [7] • Variable dot clock and variable frame refresh [8] • Advanced DVS method [2] Software-based low-power techniques • Dynamic color depth control [7] • Liquid crystal orientation shift [8] Hardware-based low-power techniques • The use of LED instead of CCFL [14] • The use of advanced light guide [14] • The use of field sequential color system [14] • Chromatic encoding for digital visual interface (DVI) [15] From the experiments, the authors reported that the power saving reaches up to 200 mW, or around 6% of the total system power, while reducing the backlight luminance from 45 to 32 cd/m 2 . For contrast enhancement, the transformation of RGB values is found to be new_RGB min original_RGB* old_backlight_luminance new_bac 1,…”

“…The authors of [7] created a detailed energy model of a handheld embedded system with a high-quality LCD, as shown in Figure 7 To reduce LCD power consumption, they introduced a technique called variable duty-ratio refresh. They adjusted an LCD refresh rate to a minimum level where the flicker is invisible to user's eyes.…”

“…According to [7], most modern commercial color display systems allow us to modify a color depth. For example, the authors of [7] reduced the color depth from 16 to 8 bits.…”

“…For example, the authors of [7] reduced the color depth from 16 to 8 bits. Under a normal condition, this reduction does not contribute a significant power saving.…”