35 years of progress in digital magnetic recordingfiles.hisashikobayashi.com › articles ›...

1

35 Years of Progress inDigital Magnetic Recording

Hisashi KobayashiFrançois DolivoFrançois DolivoEvangelos Eleftheriou

Online:Disk Drives

File Systems300 Petabytes

Petabyte [1,000,000,000,000,000 bytes OR 1015 bytes]

Exabyte [1,000,000,000,000,000,000 bytes OR 1018 bytes]

How Much Data is Out There?

300 Petabytes

Offline:Magnetic Tape

CDs8 Exabytes

y y y

2 35 Years of Progress in Digital Magnetic Recording

5 Exabytes: All words ever spoken by human beings.

Analog Data:Paper – Film

Videotape200 Exabytes

http://www.sims.berkeley.edu/research/projects/how-much-info-2003/

2 Exabytes: Total volume of information generated worldwide annually.

0.5 x 1018 seconds: Age of Universe

2

Storage Yesterday & Today

1956 IBM RAMAC

b t $10 000Price per Mbyte:

1999 IBM Mi d iabout $10,000 1999 IBM Microdrive

about $ 0.4Price per Mbyte:

2005 Microdrive

about $ 0.03Price per Mbyte:


4.4 Mbyte340 Mbyte

8 Gbyte

PAPER/FILM

100

Cost of Storage

ents

(US)

per

Meg

abyt

e

2 5” HARD DISK DRIVES

FLASH CARDSMICRODRIVE

0.1

1

10


Ce

Availability, Year

2.5 HARD DISK DRIVES

00 01 02 03 04 05 06 07 08 09 100.01

3

Hard Disc Drives: 23B US$

High end productsPersonal storageMobile-laptopConsumer electronic


Recording “System”

Spindle Actuator

Hard-disk drive (HDD) basics

Rotating Thin Film DiskSuspendedGMR Head

Slid /GMR H d


Slider/GMR Head

A Recording Track

Track Density(Tracks/in.)

Linear Density (Bits/in.)

Areal density = Linear density x Track density

4

Magnetic Recording BasicsInductive Write

ElementGMR Read Sensor

Track

Grain Structure andMagnetic Transition

t

W

B


Direction of Disk Motion

Track of Recording Media

B

Magnetic Bit Sizes vs. Areal Density

15 nm

12 Gbits/in2

bpi/tpi= 10

80 nm

800 nm

50 nm

10 nm

35 Gbits/in2

bpi/tpi= 8

380 nm


5 nm

100 Gbits/in2

bpi/tpi= 4

40 nm

160 nm

380

Shrinking magnetized area needs highly sensitive read-back head1 Terabit/in2 : 25 nm x 25 nm

5

Grain Structure in Magnetic Media


Magnification = 1 million

Areal density ~ 10 Gbits/in2 Areal density ~ 25 Gbits/in2

Media Grain Size Scaling

8 nm

Particle energy Eparticle ∝ volume of grain


p

Thermal stability requires that Eparticle > 55 kBT tostore information for > 10 years

Superparamagnetic effect

6

Longitudinal vs. Perpendicular Recording

At high linear densities the transition region from one magnetization direction to the opposite one

Vwrite

Write elementRead elementGMR sensor

Longitudinalrecording

one magnetization direction to the opposite one becomes significant

At ultra-high linear recording densities the length of the transition region is the limiting factor

Magnetizations

N S S N N S S N N S S NN SS NN SRecordingmedium

Vwrite

Write element

Perpendicularrecording


Perpendicular recording “bits” do not face each other; hence can be written at closer distances

In March 2005 HGST reported a record of 238 Gbit/in2 using perpendicular recording

Recordingmedium

Softunderlayer

Track width

Return probe

HDD Areal Density Perspective

105

4

106

1st AFC Media

NPML

Superparamagnetic EffectPerpendicular Rec.

~17 Million X Increase

1st MR Head

1st GMR Head104

103

102

10

1

10 -1

25% CGR

60% CGR

100% CGR

Disk Drive Products

PRML

NPML

real

Den

sity

Meg

abits

/in2


1960 1970 1980 1990 2000 2010

IBM RAMAC (First Hard Disk Drive)

10 -1

10-2

10-3

Industry Lab Demos

Production Year

Ar

7

Technology Challenges

Track Position Control Head Sensitivity High Speed Writing


Media SNR Head Disk Spacing Signal Processing

Signal-Processing & Coding for HDDsThe "Channel Electronics" module in HDD

• Processes signal read from magnetic media• Figures of merit : bit error rate, linear recording density Channel module

Requirement: 1 bit in error in 1015 bits read

1970

1990

Theoretical foundation of digital recording • Partial-response shaping• Maxum likelihood sequence detection


Industry’s first hard-disk-drive based on: • Partial-Response Maximum Likelihood (PRML)

2000Introduction of new channel architecture:• Noise-Predictive Maximum Likelihood (NPML)

8

From Recorded “Magnets” to Electrical Signals

Electric currentElectromagnetcore

Pattern ofmagnetization

RecordingHead

Information is stored on the disc as small, permanently magnetized regions writtenby an inductive write head

Information is retrieved as a voltage change, when the magnetic field from magnetized regions modifies the MR sensor resistance

Magnetic field ofhead

TrackSpacing

RecordingDisc

(“Media”)

Bit Length


Write current Readback signalReadback signalWrite current

0 1 0-V

+V

0 1 0

Digital Magnetic Recording

…0 1 1 1 1 1 1 10 0 0 0 0

… +1 0 -1 +1 -1 +1 0 -1 +1 -1 0 +1 …

……

……


Intersymbol interference

Data-dependent media noise

Random electronic noiseIntersymbol

interference

9

Theoretical Foundation of PRML



10

Full Response

Regular data transmission system

Full response


Partial response signaling

Duobinary signaling

Partial- response


G(D)=1+D

11

Other examples of PR signaling

Modified duobinary (Lender) : G(D)=1-D

PR class 4 (Kretzmer): G(D)=1-D2


G(D)=1-D G(D)=1-D2

Linear System Model for Digital Recording


12

Principle of a PR-4 (or Interleaved NRZI) digital recording


PR-4

Shaping filter

T

TT

PR CHANNEL


an xn= an – an-2

Write current Noiseless readback signal

f

13

Partial Response Maximum Likelihood (PRML)Partial response

Shaping filter +an xn= an – an-2

noise

yn= xn + noise

+1 -1 -1 +1 +1 +1 -1

MAXIMUM LIKELIHOOD SEQUENCE DETECTOR

X Received samples yn


● Reconstructed samples xn

+1 -1 -1 -1 +1 +1 -1+1 -1 -1 +1 -1 +1 -1+1 -1 -1 +1 +1 +1 -1

+1 -1 -1 +1 +1 +1 -1

Most likely data sequence {an} is obtained from thesample sequence { xn} that minimizes

∑ (yn – xn )2

Viterbi Detection AlgorithmDirect maximum-likelihood detection requires a number of computations that increases exponentially with the length of the data sequenceRecursive minimization of the “squared distance”:

∑ (y (a a ))2

reduces the computational complexity dramatically∑ (yn – (an – an-2))2

‘1’

Time‘1’

‘0’


‘1’

‘0’

1

‘0’

14

1990: PRML Becomes Reality


Noise Predictive Maximum Likelihood (NPML)At high recording densities the signal-to-noise ratio is reduced dramatically

Magnetized area shrinks; less signal energy

Correlated data-dependent transition noise and head/electronics noise increase

An effective way to increase the signal-to-noise ratio and achieve near optimal performance, for given magnetic-recording components, is noise prediction

Predictorwn

Noisesample

Predicted Noise sample

wn

NPML is a noise prediction/whitening scheme combined with maximum likelihood detection

0 0 0 0

1 0 0 0

0 1 0 0

1 1 0 0

0 0 1 0

1 0 1 0

0 1 1 0

1 1 1 0

0 0 0 1

1 0 0 1

10


+Noise

sample

A decrease of the noise power by 2x3 orders of magnitude error rate improvement

1 0 0 1

0 1 0 1

1 1 0 1

0 0 1 1

1 0 1 1

0 1 1 1

1 1 1 1

10

15

Post-Processor Detector/DecoderTwo-stage detection strategy for retrieving the recorded information

• A primary NPML detector produces an initial estimate of the detected data• A noise-predictive post-processor detects and corrects errors in the primary detector

Primary NPMLDetector

Post-Processor

Readback signal

Detecteddata

First-passdata Error

signal


Post-processors: Reduced-complexity schemes to correct dominant error patterns at NPML detector output

Utilize the noise-predictive principle for detecting “error signals” in the presence correlative noise

“Soft-decoding” of combined modulation/parity inner coding schemes

NPML Channel Architecture

Sector data: 512 bytes

User dataDigital Communication: “Transmit from one point in space to another”Sector data: 512 bytes

User data

RSdecoder


Modulationdecoder

Transmit from one point in space to another

Digital Recording:“Record at one point in time and retrieve at another”

RSencoder


Parityencoder

Noise-predictivepost-processor

Modulationencoder


NPMLdetector

Low-passfilter

PRequalizer

Whiteningfilter

16

Then and Now

1 1 1 1 1 1 11 1 1 1 1 10 0 0 0 0 0 0 0Readback signal in the 70’s

1 1 1 1 1 1 11 1 1 1 1 10 0 0 0 0 0 0 0


… and we can still guarantee at most 1 error in 1015 bits read back

… readback signal today!

2000: NPML is Adopted by Industry


17

Recording Density GainsSNR requirements for bit error-rate 10-6

21

22

23

[dB

]

60 %

5.5 dB

15

16

17

18

19

20

21

gnal

-to-N

oise

Rat

io (S

NR

)


1990: Digital PRML leads to a 40-50 % increase in recording density2000: Digital NPML leads to a 50-60 % increase in recording density

1.8 2 2.2 2.4 2.6 2.8 3 3.2Normalized Linear Density (PW50/T)

13

14Sig

Information Theoretic Limits

SNR requirements for sector error-rate 10-4

25RS-MTR96/104

SN

R [d

B]

15

20

LDPC(4095/4376)


Normalized Linear density 2.4 2.6 2.8 3.0 3.2 3.4 3.6

10

18

Future Prospects

/in2

Atom Surface Density Limit

AtomLevelAtomLevel105

106

107

Area

l Den

sity

, Gbi

ts

HDD products

HDD LabDemos

Nanotechnology ProbeContact Area Limit

SuperparamagneticEffect

Probe-Like

Storage

Probe-Like

Storage

StorageStorage

10-1

100

10

102

103

104

bb

EnhancedMagneticEnhancedMagnetic


Signal processing and coding have been instrumental for the remarkable progress of Storage Densities over the last 35 years

They will be even more essential as we approach fundamental physical limits

Availability Year85

10-290 95 2000 05 10 15 20 25

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