SPIE OE 49(11)117005 Wang

Optical Engineering49(11),117005(November2010)

On-chip frame memory reduction using a

high-compression-ratio codec in the overdrives of liquid-crystal displays

Jun Wang

Kyeong-yuk Min

Jong-Wha Chong

Hanyang University

Department of Electronics and Computer Engineering

718R&D Building

17Haengdang-Dong,Seongdong-Gu Seoul,133-791,Republic of Korea

E-mail:jwang@hanyang.ac.kr Abstract.Overdrive is commonly used to reduce the liquid-crystal re-sponse time and motion blur in liquid-crystal displays(LCDs).However, overdrive requires a large frame memory in order to store the previous frame for reference.In this paper,a high-compression-ratio codec is pre-sented to compress the image data stored in the on-chip frame mem-ory so that only1Mbit of on-chip memory is required in the LCD over-drives of mobile devices.The proposed algorithm further compresses the color bitmaps and representative values(RVs)resulting from the block truncation coding(BTC).The color bitmaps are represented by a lumi-nance bitmap,which is further reduced and reconstructed using median filter interpolation in the decoder,while the RVs are compressed using adaptive quantization coding(AQC).Interpolation and AQC can provide three-level compression,which leads ing a rate-distortion analysis,we select the three optimal schemes to compress the image data for video graphics array(VGA),wide-VGA LCD,and standard-definitionTV applications.Our simulation results demonstrate that the pro-posed schemes outperform interpolation BTC both in PSNR(by1.479to 2.205dB)and in subjective visual quality.C 2010Society of Photo-Optical Instru-mentation Engineers.[DOI:10.1117/1.3509370]

Subject terms:block truncation coding;color image compression;adaptive quan-tization coding;LCD overdrive.

Paper100491R received Jun.14,2010;revised manuscript received Sep.15, 2010;accepted for publication Sep.27,2010;published online Nov.9,2010.

1Introduction

Liquid-crystal displays(LCDs)have been widely used in television,desktop computers,and mobile devices.How-ever,LCDs have the important drawback of motion blur due to their long response times.Strong user demand for high-performance displays creates the need to reduce the liquid-crystal response time and to diminish any motion blur.Furthermore,many researchers have been studying3-D display implementation using LCDs for near-future appli-cations,which require even higher frame rates and,thus, shorter transition times.1–3To improve the display quality and to extend into new applications,new driving technologies have been developed.4–6Among them,the implementation of overdrive,an image-processing-based circuit,is an approach that is widely used to reduce the liquid-crystal response time.7 This approach also plays an important role in minimizing the motion blur.8,9To increase the speed of the liquid-crystal material reaction,overdrive detects and enlarges the desired change in the pixel value between the current and previous frames.In ideal overdrive,the memory of a full frame is needed to store and output the data of the previous frame in order to determine the desired change.In order to reduce the system cost of overdrive by reducing the cost of the frame memory,it is necessary to compress the image data stored in the frame memory in LCD overdrive.

There are two categories of applications with regard to memory reduction in LCD overdrive,off-chip and on-chip. 0091-3286/2010/$25.00C 2010SPIE Off-chip memory reduction is focused on reducing the over-drive system cost by reducing the data transfer rate used for the interface between the memory and the large-scale integration(LSI)circuit.Hence,off-chip memory reduc-tion compresses the image data by ratios of1/3or1/6for the high-end high-definition TV(HDTV)model.On-chip memory reduction focuses on reducing the overdrive sys-tem cost by reducing the memory capacity itself.The use of on-chip memory reduction negates the need for an external frame memory such as the synchronous dynamic random access memory(SDRAM)and the related problems of inter-face,data bit width,power consumption,and LSI pin size. Therefore,on-chip memory reduction uses1Mbit of on-chip memory for video graphics array(VGA)and wide VGA (WVGA)LCDs in mobile devices and standard-definition TV(SDTV),in which the image data must be compressed below ratios of1/7.03,1/8.79,and1/9.49,respectively.

Some previous research has been devoted to off-chip memory reduction in LCD overdrives.Someya et al.first de-veloped an overdrive technique in which an image compres-sion method was applied to reduce the amount of image data, and they then developed a motion-adaptive codec overdrive in which motion-adaptive processing was introduced into the image decoding section.10,11In these overdrives,block truncation coding(BTC)is commonly employed,due to its efficiency and low implementation cost.12More recently,Han et al.proposed an improved vector-quantization-based BTC (IBTC)using subblock vectors.8Wang and Chang proposed an advanced hybrid image codec based on the human visual system to improve the overdrive performance in LCDs.9