Management Framework for Battery-Operated Multimedia Systems

The EDLS interface is a simple slider knob, similar to a brightness control knob on a monitor. The EDLS knob controls the trade-off between energy con-sumption and image quality, and not simply the back-light’s brightness. It provides users with a power management scheme that can extend battery life at the cost of whatever display degradation the user will accept. There is also an automatic mode that changes the power management setting, depending on the remaining battery energy.

When connected to an external power source, the backlight is fully on and exhibits its maximum lumi-nance. There should be no backlight power manage-ment so that users can enjoy the best image quality. When the system is battery powered, however, users might want to extend the battery life for future use, even if the battery is already fully charged. But users gener-ally aren’t ready to sacrifice appreciable picture quali-ty at that stage. As the remaining battery energy decreases, users might become increasingly willing to compromise image quality to extend battery life. This is the point at which EDLS applies DLS.

With a poor power budget, the user’s prime concern might well be to complete their current task within the remaining battery energy budget, even if the image quality decreases. This is the optimum time for EDLS to change from DLS to DCE mode. Although DCE might alter the original colors, a moderate degree of DCE does at least maintain a fixed distortion ratio. However, if the battery energy is nearly exhausted, the only remaining option is to turn off the backlight. Without the backlight, EDLS applies DCE to achieve the maximum possible contrast. In this case, EDLS cannot guarantee a fixed amount of image distortion, but the user should still be able to read the display and finish the task.

Formulation of DLS and DCE

To build the EDLS framework, we borrowed the DLS principles of brightness compensation and DCE princi-ples of contrast enhancement.2The EDLS process starts by building a red-green-blue (RGB) histogram of the image for display. The EDLS slider determines the panel mode (transmissive or reflective), the image processing algorithm (DLS or DCE), and the maximum allowed percentage of saturated pixels, S R, after image process-ing. (S R is a given input parameter determined by user preference.) Then, the EDLS process derives upper and

lower thresholds T

H and T

L

from S R and the histogram,

and calculates a scaling factor that controls the amount of backlight dimming, as Choi et al. have shown.2Let C denote the current color value. After brightness com-pensation, new color value C′is

C′=min(2n?1, S BC C),(1)

where n is the color depth of each color component, and S

BC

is the brightness compensation factor, which equals (2n?1)/T H. Similarly, after contrast enhance-ment, new color value C′is

C′=min[2n?1, S CE max(0, C?T L)],(2)

where contrast compensation factor S

CE

equals (2n?1)/(T H?T L). After image compensation, we reduce backlight luminance L B so that L B′=L B/S BC or L B′=L B/S CE, depending on the EDLS mode. The power reduction ratio of the backlight is 1 ?L B′/L B.1

Trade-offs in EDLS implementation Figure 2 summarizes how to add EDLS capability to typical multimedia applications. Such applications draw images in the frame buffer, and user preference sets the backlight luminance (Figure 2a). Because there are many ways to add EDLS to an application, we must consider application transparency and hardware-soft-ware partitioning. In addition, we must optimize the backlight energy, energy overhead, and the perfor-mance penalty of the EDLS process itself, balancing those costs against the resulting image quality.

Our first approach is to embed EDLS in an applica-tion (Figure 2b). The advantage of this approach is that it gives us many opportunities to reduce the EDLS over-head. For example, we can construct an approximate histogram in a compressed domain for an M PEG decoder. Sometimes, we can obtain the histogram before rasterization and thus avoid additional frame buffer accesses. Our first DLS implementation falls into this category; it was highly coupled with the application. (Choi et al. demonstrated the first DLS implementation at the SIGDA university booth at the 2000 Design Automation Conference and at the design contest dur-ing the 2002 International Symposium on Low Power Electronics Design.) However, such optimizations are ad hoc, and thus the necessary changes to the applica-tion discourage developers from using EDLS because of the heavy porting burden.

Our second DLS implementation introduced a stan-dard application programming interface (API) at the window management level.2A standard EDLS API makes porting systematic,2but this approach still

Embedded Systems for Real-Time Multimedia

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Embedded Systems for Real-Time Multimedia

IEEE Design & Test of Computers

PID controller

0 to 2.5 V control input

L u m i n a n c e f e e d b a c k

1,250 V rms AC (65 kHz)

PIC microcontroller Sensor amplifier

Reset circuitry

Backlight controller

AD/DA

CCFL inverter

NL6448BC33-50

18-bit color, 640 × 480-pixel

VGA-compatible transflective LCD panel with built-in CCFL backlight

Backlight luminance 8

16

3250 MHz

RGB, Hsyc and Vsync

SDRAM frame buffer

Power supply

PCI bus bridge

32-bit 33-MHz PCI bus

Configuration

PROM

SDRAM controller LCD controller with EDLS-4Local bus interface

(a)(b)(c)(d) (e)(f)(g)(h)

systems. Shim has a BS in computer engineering from Seoul National University. He is a student member of the IEEE and a SIGDA member of the ACM.

design and embedded systems design. Chang has a

BS, an MS, and a PhD in control and instrumentation

engineering from Seoul National University. He is a

methodologies and techniques for low-power design,

synthesis, and physical design. Pedram has a BS in

electrical engineering from the California Institute of

Embedded Systems for Real-Time Multimedia

IEEE Design & Test of Computers