11 Using Checksum to Reduce Power Consumption of Display Systems for Low-Motion Content Kyungtae Han*, Zhen Fang, Paul Diefenbaugh, Richard Forand, Ravi.

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, 2009

2009 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 encoding1. Find different pixel blocks between current and reference

frames2. 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

=

CRCCRC

Current frame/blockRef. 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

GfxCPU

Memory controller

CRC generator

DelayCRC

checker

vsyncpixel 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)

1010

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 DQ Q D

CRC(3) CRC(2) CRC(1) CRC(0)

Pixel data in

throttle

clock

Example CRC-4 Implementation for

X4 + X2 +X +1

to frame buffer XOR XOR

XOR

CRC compare

1111

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”

1212

Quantitative Evaluation

• Total display system power

• Power consumption

• Total power savings

otherslerLCDcontrolrollerMemoryContrameBufferDispLocalFDRAMbusDRAMdevice PPPPPP

idleactive PPP )1(

dCRCoverhearameBufferDispLocalF

lerDynamicLCDcontrolmicrollerDynaMemoryCont

amicDRAMbusDynownDRAMpowerd

PP

PP

PP

)

(

: average fraction of frame contents that is not changing

Motion Frames

Power consumption

baseline

Image change aware

display

0 60 (FPS)

1313

Frame Change Characteristics & Estimated Power Savings

Video Creation Office Produc-tivity

E-Learning 3D Modeling Mobile Produc-tivity

movie "Ele-phant Dream"

CNN.com ac-tive browsing

0

50

100

150

200

250

300

350

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

Power savings % of no-motion frames

Po

we

r s

av

ing

s (

mW

)

% o

f n

o-m

oti

on

fra

me

s @

60

FP

S

1414

Extrapolated Amount of Motion at Macroblock Level

Video Creation

Office Productiv-

ity

E-Learning 3D Modeling Mobile Productiv-

ity

movie "Ele-phant Dream"

CNN.com ac-tive browsing

20%

30%

40%

50%

60%

70%

80%

90%

100%

% of no-motion frames @ 60 FPS (measured)

MB-level, if % of no-motion MBs in changed frame = 40%

MB-level, if % of no-motion MBs in changed frame = 60%

if % of no-MB-level, motion MBs in changed frame = 80%

% o

f C

on

ten

t w

ith

No

Ch

an

ge

(1

00

% =

ide

nti

ca

l co

nte

nt)

1515

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

GfxCPU

Memory controller

CRC generator

DelayCRC

checker

vsyncpixel clock vsync

LCD controller

LCD panel

Stream Image Change Detector

Local FB

1616

1717

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

1919

Backup slides…

2020

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