• To appear in Journal of the Society for Information Display, 2009 

To appear in Journal of the Society for Information Display, 2009 
White LED Backlight Control for Motion Blur Reduction and
Power Minimization in Large LCD TVs
Wonbok Lee, Kimish Patel, Massoud Pedram
University of Southern California
Department of Electrical Engineering
Los Angeles CA 90089
Abstract- A 1-D LED backlight scanning and a 2-D local dimming technique for large LCD TVs are presented. These
techniques not only reduce the motion blur artifacts by means of impulse representation of images in video but also increase
the static contrast ratio by means of local dimming in the image(s). Both techniques exploit a unique feature of LED backlight
in large LCD TVs in which the whole panel is divided into a pre-defined number of regions such that the luminance in each
region is independently controllable. The proposed techniques are implemented in a Xilinx FPGA and demonstrated on a
Samsung 40-inch LCD TV. Measurement results show that the proposed techniques significantly reduce the motion blur
artifacts, enhance the static contrast ratio by about 3X, and reduce the power consumption by 10% on average.
Keywords- LED backlight, backlight scanning, backlight local dimming, motion blur, contrast ratio, low power.
1. Introduction
LCD (Liquid Crystal Display) TVs are the main stream products in the current FPD (Flat Panel Display) market and they are
present literally everywhere in our lives, e.g., living room, school/workplace, and grocery stores. In spite of their popularity
and excellent performance (e.g. vivid image representation and high native resolution) over the other types of TVs such as
PDP (Plasma Display Panel), LCDs suffer from a number of shortcomings such as motion blur artifact1 [1][2][3], low CR
(contrast ratio), and low brightness. For those reasons, CRT’s (Cathode Ray Tube) ‘motion blur free’ feature [4] and PDP’s
high CR feature pose continual threat to LCD’s dominance.
The biggest shortcoming of LCDs is the motion blur artifact due to three well-known phenomena: i) slow RT (response
time) of LC (Liquid Crystal), ii) scan-and-hold feature of LCDs, and iii) slow pursuit nature of the HVS (Human Visual
System). In order to overcome slow RT of LC, typically various kinds of over-driving techniques [5][6], are widely employed
by the LCD industry. Those over-driving techniques generally apply sufficiently high voltage levels to LCs so as to tilt them
1
Motion blur is the apparent streaking of rapidly moving objects in a still image or a sequence of images such as a movie.
1
To appear in Journal of the Society for Information Display, 2009 
faster, i.e., consequently get them to respond faster. The amount of over-driving voltages is typically determined at design
time and saved in a LUT (look-up table).
Furthermore to effectively tackle the scan-and-hold aspect of LCDs (illustrated in Fig. 1), together with the slow pursuit
nature of the HVS, LCD architects have devised various ‘eye-bleaching’ techniques such as BFI (black frame insertion),
backlight flashing/blinking, and backlight scanning [7][8][9][10]. In fact, although the response time may approach zero in
future LCDs, the motion blur artifact will continue to exist as long as the LCD panels display every image in the scan-and-
hold style. The aforementioned “fixes” attempt to re-create the impulse-type image display of CRT monitors in the LCD
panels so that the afterimages in an observer’s eyes are eliminated on a per frame basis.
Fig. 1 Hold-type image display in LCD vs. impulse-type image display in CRT.
Another shortcoming in LCDs is their relatively low CR compared to PDP. Contrast ratio is generally characterized in two
ways: i) static (or spatial) and ii) dynamic (or temporal). Static CR is defined as a (perceived) luminance difference between
the maximum and the minimum pixel values within an image frame whereas dynamic CR is defined as a luminance
difference between the maximum and the minimum pixel values across image frames.
As known, low CR in LCDs mainly originates from the backlight leak thru LC to the front side of the panel especially
when the pixel values are close to zero grayscale. One good solution to this problem is the dynamic dimming of the backlight,
which may be classified as follows: 1) 0-D (frame level) [11], 2) 1-D (line/segment level), and 3) 2-D (grid/tile level).
Relatively small LCDs, such as PC monitor, equipped with CCFL (Cold Cathode Florescent Lamp) backlight(s) simply
allows 0-D control while large LCDs, such as LCD TVs, allows 1-D control. In both cases, either a dynamic CR or a limited
range of static CR enhancements is merely feasible. True static CR enhancement is feasible only to the LCDs with 2-D
backlight modulation. The most popular backlight source, in this category, is the LED (Light Emitting Diode).
This paper presents two new backlight control techniques: a 1-D LED backlight scanning technique and a 2-D LED
backlight local dimming technique. The former reduces the motion blur artifact by effectively breaking the scan-and-hold
2
To appear in Journal of the Society for Information Display, 2009 
feature of LCDs while the latter enhances the static CR. In both cases, along with quality enhancement, power savings are
achieved as well. The aforesaid techniques are completely independent of one another and hence can be easily combined
together. In this paper, however, we do not consider this combination.
The remainder of the paper is organized as follows. In section 2, previous backlight scaling works are reviewed. Section 3
explains the proposed two techniques following the detailed analysis of the impulse-type image display in CRTs as well as
the HVS. Section 4 shows the methodology and experimental results. Lastly, section 5 is the conclusion.
2. Background
2.1 Previous Work
Previous work on backlight control can be categorized into two classes: 1) Reducing the power dissipation within a given
budget of image/video quality distortion, 2) Enhancing the dynamic/static CR while maintaining the overall luminance of
image(s).
In the first class, Chang et al. [12] scaled down the backlight luminance while compensating the (perceived) image
luminance loss by increasing the pixel values using two different mechanisms; 1) backlight luminance dimming with
brightness compensation, 2) backlight luminance dimming with contrast enhancement. Cheng et al. [13] improved this simple
approach by eliminating the pixel-by-pixel transformation of the displayed image thru minor hardware modifications to the
built-in LCD reference driver. Their key idea was to first truncate the image histogram on both ends to obtain a smaller DR
(dynamic range) of the image pixel values and then to spread out the pixel values. Iranli et al. [14] proposed a novel method
for image transformation whereby the DR of the original image is reduced such that the incurred image distortion is no more
than a pre-specified value. Later, the same authors [15] extended their frame-sensitive backlight scaling works to the video
domain such that the expected video flickering is minimized by considering both of spatial and temporal distortions.
In the second class, Oh et al. [16] proposed an adaptive dimming with CCFL backlights, which enhanced the dynamic CR
by a factor of two. Greef et al. [17] proposed to combine the 1-D dimming and boosting of the CCFL backlights to improve
the brightness, static CR, and still save power. Recently, Chen et al. [18][19] presented an idea that incorporated 2-D LED
local dimming and global dimming, both of which aimed at eliminating front-side backlight leak thru LC pixels. Though their
idea is effective to minimize the light leak in the dark regions (hence, improve the black-level and the static CR), aiming at the
light leak minimization even at their global dimming results in the luminance degradation in the overall image.
More recently, various backlight scanning based ideas have been proposed, which basically modulate the duty cycle of the
3
To appear in Journal of the Society for Information Display, 2009 
backlight source elements. Fisekovic et al. [20] proposed to consider parameters of i) proper timing, ii) number of backlight
segments, and iii) exposure time (i.e., duty cycle). They asserted that at least 6-7 segments and 25~30% of duty cycle is
effective in suppressing motion blur artifact while maintaining the reasonable brightness of the frame images. Hung et al. [21]
proposed to consider parameters of i) duty ratio, ii) lamp current, and 3) timing of CCFLs. Recently, Sluyterman et al. [22]
asserted a proper timing of backlight scanning in HCFL (Hot Cathode Fluorescent Lamp). Though their idea is novel, coarse
granularity and slow response of HCFL backlights makes their idea less attractive in practice. Greef et al. [23] combined the
backlight scanning, dimming and boosting techniques in HCFL backlights, which also has a similar limitation.
On this paper, we present two novel backlight control techniques. In addition,
The proposed techniques are implemented in a Full-HD 40” LCD TV equipped with white LED backlights (cf. Fig. 2).
Fig. 2 Test Environment and circuit blocks on the panel.
The proposed scanning technique balances the degree of motion blur reduction and luminance loss by determining the most
favorable backlight duty cycle.
In the proposed local dimming techniques, the reported power savings, together with the static CR enhancement, are
actually measured.
2.2 Circuit Architecture for LCD Panel Driving
Fig. 3 shows the typical circuit architecture for an LCD panel; RGB data generated from a VGA (Video Graphics Adapter) is
sent to TCON (Timing Controller) on LCD TVs in LVDS (Low Voltage Differential Signaling) format. The TCON transmits
the RGB pixel data to the source drivers while controlling the turning-on of each pixel lines via the gate drivers. By
synchronizing the data and the timing signals, each line on the LCD panel is refreshed from the top left to the bottom right
within a given frame display deadline, which is set to the inverse of the frame rate.
4
To appear in Journal of the Society for Information Display, 2009 
Fig. 3 Circuit architecture for LCD panel driving.
Note that LCDs are not self-emitting and need backlight(s) which is in fact the single most power consuming component in
the display subsystem [15]. The traditional backlight source is CCFL/HCFL, although white LED has recently become a
popular backlight source due to its merits such as relatively low power consumption and high NTSC gamut [24]. More
importantly, relatively short RT (say, less than 0.1msec) [18] of the LED and its finer granularity (i.e., a large array of white
LEDs rather a few CCFLs) makes it amenable to the fine-level backlight control for the image quality enhancement.
Fig. 4 Pixel granularity vs. LED granularity.
Fig. 4 shows an example of the LCD panel with LED array backlighting. Let us denote the total number of pixels for the
LCD panel as
N pixel = H ×V (1)
whereas the total number of LEDs as
N led = M ×Q (2)
Although, in general,
5
To appear in Journal of the Society for Information Display, 2009 
N pixel ≥ N led (3)
compared to typical CCFL backlighting with 1~8 CCFLs, the LED backlighting provides a much finer granularity of
backlight control (the white LED array on the test platform are shown in Fig. 9).
2.3 Crux of the Backlight Scaling in LCDs
To understand the fundamental issues in backlight scaling, we need to understand how each pixel is displayed in LCDs; each
pixel has an individual LC cell, a TFT (Thin Film Transistor), and a storage capacitor. The electrical field of the capacitor
controls the transmittance of the LC cell. The pixel transmittance, t(v(X)), is a function of the grayscale voltage, v(X), which
is in turn a function of the pixel value X. Although the LC luminance has in general a non-linear relationship with the
grayscale level and voltage, assuming a simple relational form (i.e., linear dependence) for simplicity of presentation, a pixel
with value X, the luminance L(X) of the pixel can be calculated as follows:
L ( X ) = b. t ( X ) (4)
where t(X) is the transmissivity of the LC cell for pixel value X and b∈[0, 1] is the normalized backlight illumination factor
with b=1 representing the maximum backlight illumination and b=0 representing no backlight.
The key idea of backlight scaling is to dim the backlight and compensate for the perceived luminance loss by adjusting the
grayscale of the image so as to increase its brightness or contrast. More precisely,
L(X) = β.t(Φ(X, β)) (5)
where 0