11 Using Checksum to Reduce Power Consumption of Display Systems for Low-Motion Content Kyungtae Han*, Zhen Fang, Paul Diefenbaugh, Richard Forand, Ravi R. Iyer, Donald Newell Intel Labs Oct. 5, IEEE International Conference on Computer Design, Lake Tahoe, CA

22 Contents Introduction Display subsystems Image change detections Evaluation & Prototype

33 Introduction Typical applications on mobile device tend to have static image contents –Web browsing, text editing –Each frame hardly changes from the previous one Efficiently detecting and handling no-motion scenes are critical to extend the battery life –Propose image change detection algorithms Optimize power consumption of display subsystems –Local display refresh –Remote display content compression

44 Local Display Refresh with Local Buffer Refresh screen at a fixed rate (e.g. 60 Hz) When images are not changed, refresh through local frame buffer in order to save power consumption in host* System DRAM System FB Gfx LCD controller Image hold LCD panel CPU Memory controller RGB data address Local FB * [Brakmo 2004]

55 Remote Display Encode data and transmit –Eliminate temporal redundancy between video frames Two steps of encoding 1. Find different pixel blocks between current and reference frames 2. Encode the difference Frame buffer X server/Win manager/3D driver Application Remote Display Interface: encoding Wireless network MPEG/H.264 decoding Frame buffer LCD controller (Thin display client)

66 Image Change Detection Current frame/block = = = = = = + Ref. frame/block = CRC Current frame/block Ref. frame/block Conventional Detection Proposed Detection 0: No change/ Zero-motion Others: Image changed/ motion * Cyclic redundancy check (CRC) is a type of data integrity checksum function that takes a data stream and produces a value

77 Pixel Fetch from DRAM Pixel-by-pixel Comp.Proposed method Frame n / MB n Frame n-1 / MB n-1 time compare Frame n+1 / MB n+1 compare Each pixel fetched from DRAM twice Frame n / MB n Frame n-1 / MB n-1 compare Frame n+1 / MB n+1 compare Each pixel fetched from DRAM once CRC1 CRC2 CRC3 (in DRAM) time (in DRAM)

88 Stream Image Change Detector Each pixel fetched from DRAM once System DRAM System FB Gfx CPU Memory controller CRC generator Delay CRC checker vsync pixel clock vsync LCD controller Image Hold LCD panel Stream Image Change Detector Local FB

99 Zero-Motion Detection Using Hardware CRC Generate CRC for current MB CRC of same MB in last frame Zero-motion detected Motion detected. Encode current MB as I-block ==? N Y from X server / win manager / 3D driver Current MB to frame buffer MB: macro block (16x16 pixels)

10 Additional Techniques Partial CRC –Ignore some of pixel components –E.g. Skip brightness –only use U and V and skip Y (Brightness) CRC Throttling –Turn On/Off CRC logic based on contents –Turn off CRC after zero motion vector has been generated for T successive frames, (no more than S) X server / window manager / 3D driver SRAM CRC Generation Baseline encoding process U, V for one blk RGB-to-YUV Y, U, V for one blk CRC throttle Q D Q D D Q Q D CRC(3) CRC(2) CRC(1) CRC(0) Pixel data in throttle clock Example CRC-4 Implementation for X 4 + X 2 +X +1 to frame buffer XOR CRC compare

11 Failure Recovery Failure of CRC-based image change detection –Slight possibility that motion between two blocks of data is missed by short CRCs –Result in repeated rendering of an obsolete image Overcome this failure –Periodically de-assert CRC-based image change detection output –Force into “image changed”

12 Quantitative Evaluation Total display system power Power consumption Total power savings : average fraction of frame contents that is not changing Motion Frames Power consumption baseline Image change aware display 0 60 (FPS)

13 Frame Change Characteristics & Estimated Power Savings

14 Extrapolated Amount of Motion at Macroblock Level

15 Prototype Proof-of-concept hardware illustrating CRC image change detection Components –PC mother board –Altera FPGA –1280x800 LCD When detector finds no image changes, then go to low-power mode System DRAM System FB Gfx CPU Memory controller CRC generator Delay CRC checker vsync pixel clock vsync LCD controller LCD panel Stream Image Change Detector Local FB

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17 Summary Propose and prototype a novel image change detection method using CRC Proposed method reduces significant percentage of frame buffer reads Prototype shows that proposed method detects image changes Future works –Experiment with checksum/hash functions rather than CRC –Investigate other application of the proposed image change detector

19 Backup slides…

20 Power Consumption of Key Components In Hypothetical battery-powered device