Lecture 6

Control and coding

Pickup head optics

Spindle motor

Servo control(tracking & focusing)

Disk

RF amp

User data

Error correction & interleaving

EFM coding/decoding

Speed control(CAV, CLA, ZCAV)

datacontrol

LD

Sync control(PLL)

PD

Power control

2004/5/14 2Lecture 6

Coding: representation of binary data

1 0 1 1 1 0 1data bits

NRZ (non RZ)

NRZI (NRZ inverted)

bi-phase

1 0 0 1 1 0 1 0 1 0 0 1 1 0bi-phasechannel bits

• lack of clock stability• large DC content

• self-clocking• 0% DC content

RZ (return to zero)

user data ↔ data bits ↔ channel bits

2004/5/14 3Lecture 6

Coding: RZ, NRZ, NRZI

– NRZ, NRZI: min pulse length = bit cell T– signal can remain high/low for arbitrarily long time– threshold detection– clock stability during readout questionable:

• sync between decoder clock and data pattern• scan velocity same as recording

– max DC content → 100%• signal affected by AC coupling, AGC, and high pass filtering

– min mark length Lmin= vT, v: scan velocity, Density ↑，Lmin↓

– Lmin > 0.6λ/ NA ~ 1um，ρ = 1 bit/Lmin= 1 bit/um

2004/5/14 4Lecture 6

Coding: bi-phase

– DC content = 0 – self-clocking codes– 2 channel bits = 1 data bit, 1 10, 0 01 – channel bits converted to signal using RZ or

NRZ, then record accordingly. – differential decoding: avoiding threshold drift,

DC content, signal modulation– Lmin = 0.5 T (v omitted) low data density

2004/5/14 5Lecture 6

RLL (run length limited) Codes

• Magnetic recording: – senses domains inductively– low-freq signal cannot be reproduced faithfully– codes prohibit long intervals between transitions

• Optical recording: – can record and reproduce arbitrarily long marks– have problems with DC contents and clock

synchronization– finding codes to support higher data density– RLL codes offers improvements in the above two areas

2004/5/14 6Lecture 6

– channel bit rate (1/ t) >> data bit rate (1/T)

– (d, k): number of 0’s following each channel bit 1 is at least d and at most k

– a practical RLL code map m data bits into n channel bits

– example: using NRZI code rules, a transition (land mark or mark land) identifies ‘1’ in the channel bit stream

– Lmin = (d+1)T

– Lmax = (k+1)T

– NRZ , NRZI: special case of (0, ∞) RLL, not self clocking

– Information/mark =ΣPi ln(Pi), Pi (i = d to k) = probability, Σpi = 1

– Lavg = ΣPi Li, Li = Lmin (i+1) / (d+1)

– Code efficiency (# bits / length) ρ = ΣPi ln(Pi) / (ln2 Σpi Li) bits

– ρ max = -ln2 (xd+1) bits/Lmin, xd+1(1 - xk-d+1) = 1 - x

RLL (run length limited) Codes

2004/5/14 7Lecture 6

3210

1.8601.6551.3871.000∞1.75091.7001.55881.6231.5521.35871.4981.4941.33861.2871.3951.30250.8931.2171.2350.975400.8631.1030.9473

00.8110.879200.6941

00

dkTheoretical D-M Code Efficiencies (bit / Lmin)

more clock

stability

Increase phase margin

RLL (run length limited) Codes

2004/5/14 8Lecture 6

data bits channel-bit representation (m, n)

Code Rules for MFM (1,3) and IBM(2,7) Modulation Codes

MFM 1 01 (1, 2) (1)0 00 (0)0 10

IBM(2,7) 10 0100 (1, 2) 11 1000 000 000100 010 100100 011 001000 0010 00100100 0011 00001000

RLL (run length limited) Codes

2004/5/14 9Lecture 6

RLL (run length limited) Codes

mark length (pulse width)- mark or space length represent

distance between adjacent ‘1’’s- higher data density- example: 3rd-generation MO

mark/pulse position (RZ)- all marks are identical and spacing

between marks represents distance between adjacent ‘1’’s

- easier for decoding- lower data density, higher data freq- example: 1st-generation MO

2004/5/14 10Lecture 6

Comparison of Selected D-M CodesCODE d k BITS/Lmin WINDOW WIDTH/BIT MAXIMUM DC

NRZ 0 ∞ 1 1 100% MFM 1 3 1 0.500 33 3-Φ 1 7 1.333 0.667 56 IBM(2,7) 2 7 1.500 0.500 40 EFM 2 10 1.412 0.471 0

RLL (run length limited) Codes

EFM (eight to fourteen) modulation• used in CD system• (2,10) code• Lmin = 3T, Lmax = 11T• 8 data bits are mapped into fourteen channel bits (m = 8, n = 14)• two 14-bit groups are joined by 2 merging bits to avoid coding

rule violation• additional 1 linking bit is added to remove DC content

2004/5/14 11Lecture 6

EFM

8 bits data is converted into 14 bits data meeting the boundary conditions:

• minimal 2 0’s between two 1’s (data density, inter-symbol interference)

• maximal 10 0’s between two 1’s (synchronization stability)

• conversion by table look-up

EFMconversion1101 0010

8-bits data 14-bits data

1001 0010 0100 01

2004/5/14 12Lecture 6

EFM land & pit marks

3T

4T

5T

6T

7T

8T

9T

10T

11T

1001

10001

100001

1000001

10000001

100000001

1000000001

10000000001

100000000001

0 0 1

0 0 0 1

0 0 0 0 1

0 0 0 0 0 1

0 0 0 0 0 0 1

0 0 0 0 0 0 0 1

0 0 0 0 0 0 0 0 1

1

0 0 0 0 0 0 0 0 1

0 0 0 0 0 0 0 0 0 0

1

1

1

1

1

1

1

1

0

1

0.833 µm

2004/5/14 13Lecture 6

Data Compression

The minimal readable pit size is LminThe space required to save an 8 bits number:• Direct data recording: 8 * Lmin• EFM data recording: 14 / 3 Lmin

1 1 0 1 0 0 1 0

100 100 100 000 11

Direct recording

EFM recording

Lmin

Lmin

2004/5/14 14Lecture 6

Principle of Error CorrectionTwo paraity bytes are saved with a series of 8 bytes. During read-back the parity bytes are recalculated and compared with the saved parity bytes. This way one erroneous bit can be located and be corrected.

1 0 1 0 1 0 0 00011101

1111001

1000110

1110001

0110101

1010100

1011001

1101010

10000001

1 1 0 0 1 1 0 0 0

1 0 1 0 1 0 0 00011101

1111001

1000110

1110101

0110101

1010100

1011001

1101010

10000101

1 1 0 1 1 1 0 0 0

parity byte

parity byte

2004/5/14 15Lecture 6

Error Correction on CD formats

The error correction of CD formats is more advanced:

•In a block of 24 bytes the CD Error Correction Decoder can locate and correct 1 or 2 erroneous bits. More than 2 bit-errors can not be corrected

•With the CD error correction small reading errors randomly spread over the disc can easily be corrected. Larger local errors can not be corrected.

•To make the CD format less sensitive to local damages the CD is equipped with an extra Error Correction technique known as interleaving.

2004/5/14 16Lecture 6

Interleaving implies a controlled scrambling of the data to spread out error clusters over a large number of data blocks.

Interleaving

2004/5/14 17Lecture 6

C1 interleaving C228 bytes32 bytes

4 parity bytes

24 bytes

scrambleddata

scrambleddata

originaldata

originaldata

reading

writing

4 parity bytes

28 bytesdataDisk

1 erroneous bit corrected by C1 E112 erroneous bits corrected by C1 E21> 2 erroneous bits no correction E31

Interleaving1 erroneous bit corrected by C2 E122 erroneous bits corrected by C2 E22> 2 erroneous bits no correction E32

(uncorrectable)

CD Error Correction Procedure

2004/5/14 18Lecture 6

Performance of Error Correction Strategy

Comparison of three ECC strategies;each one has two strategies for C1 and C2.

2004/5/14 19Lecture 6

Interleaving Specification

• 1 EFM frame = 588 bit.• To make it possible to correct complete destroyed

data of 14 EFM frames the data is interleaved over 108 EFM frames.

• 14 EFM frames ~2 mm on the disc.• 108 EFM frames ~18 mm on the disc.• Max interpolatable burst length ~ 8.5 mm on the

disk

2004/5/14 20Lecture 6

CD error correction procedure: encoding

2004/5/14 21Lecture 6

CD error correction procedure: decoding

2004/5/14 22Lecture 6

CD frame structure

2004/5/14 23Lecture 6

Signal measurement – CD-R

• Signals from which the parameters are derived

• Parameters for the unrecorded disks (before recording)

–disk design, replication quality

• Parameters of the recorded disks (after recording)

–writing process, drive compatibility• All CD-R parameters are specified in Orange Book Part

II: CD-WO, Latest version: version 3.0, December 1997• A large part of the specifications are similar to the

specifications of the conventional CD, described in the Red Book

2004/5/14 24Lecture 6

CD-R signals

Almost all CD-R parameters are derived from the following 4 signals:

• The reflectance level of the land area and the groove area

• The push pull signal• The wobble signal• The EFM signal (for recorded CD-R only)

2004/5/14 25Lecture 6

land

groove

land

Ilb

Igb

groove

land

land

grooveland

groove

land

Iga

Ila

pit marks

CD-R signals: Land & Groove Reflection

• The groove reflectance level (Ig) is the photo-diode detector signal (ISUM)when the read-out spot is focused in the middle of the groove.

• The land reflectance level (Il) is the photo-diode detector signal (ISUM) when the read-out spot is focused in the middle of the land area between two grooves.

2004/5/14 26Lecture 6

groo

ve

groo

ve

groo

ve

land

land

land

land

TES

SUM

Ig

Il

pp21 II −

track pitch

workingareaTES

ppmIIII 211.021 19.0 −⋅=− µ

CD-R signals: Push Pull Signal

• The Push Pull signal is the TES when the optical pick-up unit is not tracking.

• The parameter Push Pull is the push pull signal at 0.1 µm from the centre of the groove

• With a track pitch of 1.6 µmthis is:

2004/5/14 27Lecture 6

10 -

30 k

Hz

TES

45.3 µs

frequency = 22.05 kHz

IW

CD-R signals: Wobble Signal

The wobble is detected as small fluctuations on the Tracking Error Signal (TES). To isolate the wobble the TES is passed through a 10 to 30 kHz band pass filter.

2004/5/14 28Lecture 6

average groove center actual groove center

wobble amplitude

wobble period

Wobble

The pregroove in CD-R and CD-RW is slightly wobbled:

• Spatial period typically 60 µm

• Amplitude typically 30 nm

The wobble can be detected as small fluctuation in TES

2004/5/14 29Lecture 6

Speed locking

• Locking frequency: 22.05 kHz at 1x speed

• Spatial wobble period determines the linear velocity (CLV)

Time encoding: Absolute Time In Pregroove (ATIP)

• ATIP points are modulated in wobble frequency at 1 kHz (jumps to 21.05 and 23.05 kHz are decoded as 0s and 1s)

• Series of 42bits = 1 ATIP frame = 1 / 75 second

• An ATIP frame consists of minutes, seconds, frames (mm/ss/ff)

• 10% of the ATIP frames in the lead-in area is used for encoding of “Special information” used by the recoder to optimise the writing

Wobble functions

2004/5/14 30Lecture 6

ATIP points

ATIP frame: 30 / 12 / 71Frame length: 42 bitsSpatial length: 16.0 - 18.7 mm

0 1 0 0 1 1 1 0 10

100110110000

11

01

10

01011001010110

010

11

10

10

1101100000

10

11 0

0 11 0 1

1 1 0 0 0 1 0 1 1 0 0 01

111111010010

01

10

10

100001111010011

1010

100

01

01

10000011010101

11

01 0

1 10 0

0 0 1 0 1 0 1 1 0 1 0 0 1 10

10

1010101110101

10

10

00

01

01110100111010110

1011

10

10

01

00

01010111001010

01

01 0

0 10 0

1 0 0 11 1

01101100

10

00

10

11

00

00

011

0101

1100

10101101

001011

00001

10

0 1 1 0 1 10 0

0 01 0

1 00 1

11

01001011010001

1101000

11

10

10

01

01

10

11

101000011010010001101

00

11

10

10 1 1

1 0 0 0 0 1 0 1 0 0 1 1 1 0 1 0 1 0 0 1 10 1 0

1 01 1

1 00 1

01

01

10

01

000011010010110010010

11

10

01

00

01

10

101

1010

1010

11010101001101100001010110

10

01

01

10

10

011

1010110110001010111

10

01

00

10 1 0 1 0 1 1 0 0 1 0 1 0 0 1 0 1 1 0

2004/5/14 31Lecture 6

CD-R signals: EFM signal

EFM signal is the pattern coming from the detector can be seen on an oscilloscope when a recorded CD-R is read back. The signal is build up from read-out RF signals.

time4T0 8T

dete

ctor

sig

nal

12T 16T 20T 24T 28T 32T 36T 40T

3T 7T 5T 9T 4T 3T 9T

2004/5/14 32Lecture 6

time4T0 8T

dete

ctor

sig

nal

12T 16T 20T

3T 7T 5T 9T

time4T0 8T

dete

ctor

sig

nal

12T 16T 20T

5T 11T 3T

time4T0 8T

dete

ctor

sig

nal

12T 16T 20T

time4T0 8T

dete

ctor

sig

nal

12T 16T 20T

Build-up of the EFM signal

2004/5/14 33Lecture 6

CD-R testing by manufacturers

1. To test ‘important’ parameters in the Orange Book and make sure they are within the spec.

2. To test read/write functions in ‘major’ disk drives in the market and make sure they are compatible.

3. ‘Important’ parameters (for example):• ATER < 10%• Reflectivity > 0.6• I3R (m3) ave 0.3 ~ 0.7• I11R (m11) ave >0.6• Cross talk < 0.5• BLER max 100 count/sec• Jitter < 35 ns• E32 (Cu) 0

4. ‘Major’ drives: Plextor, TEAC, Ricoh, HP, Panasonic, Acer, Yamaha, Sanyo, Lite On, TDK, Creative, Sony

2004/5/14 34Lecture 6

Signal quality: SNR vs. CNR

•SNR (ratio of signal power to noise power)–not readily measurable except in theory–can be replaced by CNR for many purposes

•CNR (carrier-to-noise ratio)–write disk with a fixed-frequency signal–measure the difference between signal peak and noise level at a nearby frequency (in dB)–by convention, the measurement BW of the spectrum analyzer is set to 30 KHz

•SNR(dB) = CNR(dB) + 10 log(30KHz / signal BW)–example: 1 Mbyte/sec data channel encoded with mark length MFM, signal BW ~ 10 MHz

CNR = 45 dB, SNR @ 10 MHz = 45 - 25 = 20dB

2004/5/14 35Lecture 6

carrier level

average noise levelaverage noise levelaverage noise levelaverage noise levelaverage noise level

CNR

RBW = 30 KHz, VBW = 300 HzFrequency span = 2 MHzWriting Frequency = 4 MHz

Readout Signal: CNR

2004/5/14 36Lecture 6

CD-R specification Wobble CNR (WCNR)

DefinitionThe Wobble Carrier to Noise Ratio indicates how clear thefrequency of the wobble is read back with respect to the background noise frequencies

Orange book specificationSpec. 16.3: WCNRb > 35 dB (BW = 1 kHz)

Spec. 17.2: WCNRa > 26 dB (BW = 1 kHz)

Remarks• A low WCNR can lead to speed locking problems.

• A low WCNR can lead to ATIP encoding problems

2004/5/14 37Lecture 6

Wobble CNR (WCNR)

Wobblecarrier

frequency

Frequency

Noiselevel

Carrierlevel

WCNR

ATIPATIP

2004/5/14 38Lecture 6

Unrecorded CD-R spec: ATIP Error Rate (ATER)

Definition: The percentage of ATIP frames which contains error(s).

Orange book specificationSpec. 17.2: ATER < 10 % (over 10 seconds)

Remarks• The ATER is specified before recording only• A low WCNR is often accompanied by a high ATER• The ATER should not be measured in the Lead-in area

because (10% of the ATIP frames contain “Special Information” instead of ATIP points in that area.

2004/5/14 39Lecture 6

Recorded CD-R specification Reflection (Rtop)

Definition: The reflection Rtop measures the maximum reflection when the recorded disc is read back.

Orange book specification

Spec. 8.4: Rtop > 65 % (> 70% for CD Audio)

Spec. 8.5: Maximum variation over disc: +/- 3% (relative)

Remarks

• Rtop is almost the same as the groove reflectance before recording (Rgb). It is slightly lower because of the pit marks in the adjacent grooves.

2004/5/14 40Lecture 6

time

4T0 8T

dete

ctor

sig

nal

12T 16T 20T

Itop

Recorded CD-R specification Reflection (Rtop)

Rtop is calculated from the Itop level, which is the maximum level of the EFM-signal, corresponding to the 6T to 11T land mark signal.

2004/5/14 41Lecture 6

Recorded CD-R spec: Modulation amplitude (I3 / Itop and I11 / Itop)

Definition

• The I3 / Itop measures the contrast in reflectance level between the 3T land marks and pit marks.

• The I11 / Itop measures the contrast between the 11T land marks and pit marks.

Orange book specification

Spec. 14.1: 0.3 < I3 / Itop < 0.7

I11 / Itop < 0.6

Remarks

• These specifications ensure enough contrast between the reflectance level of the land marks and pit marks.

2004/5/14 42Lecture 6

time

4T0 8T

dete

ctor

sig

nal

12T 16T 20T

Itop

I3 I11

Recorded CD-R spec:Modulation amplitude (I3 / Itop and I11 / Itop)

2004/5/14 43Lecture 6

Definition: The asymmetry (Asym) indicates the position of the 3T band with respect to the centre of the 11T band.

Orange book specification

Spec. 14.2 or 14.8: -15% < Asym < +5%

Spec. 14.9: Maximum variation over disc: +/- 2%

Remarks

• The asymmetry (β) is used by CD-R recorders for the Optimum Power Control and is therefore determined by the recorder, not by the disc.

Recorded CD-R specification Asymmetry (Asym)

2004/5/14 44Lecture 6

time4T0 8T

dete

ctor

sig

nal

12T 16T 20T

8T 3T 3T 10T

24T 28T

Pw = Popt

Pw < Popt

Popt

< Popt

I11I3

8T 3T 3T 10T

Recorded CD-R specification Asymmetry (Asym)

Disc written with a too low writing power (Pw < Popt):• Pits are written too small• I3 band shifts up with respect to I11 band

2004/5/14 45Lecture 6

time4T0 8T

dete

ctor

sig

nal

12T 16T 20T

8T 3T 3T 10T

24T 28T

Pw = Popt

Pw > Popt

> Popt

PoptI11I3

8T 3T 3T 10T

Recorded CD-R specification Asymmetry (Asym)

Disc written with a too high writing power (Pw > Popt):• Pits are written too big• I3 band shifts down with respect to I11 band

2004/5/14 46Lecture 6

Recorded CD-R specification Cross Talk (XT)

Definition: The cross talk quantifies the amount of signal noise originating from the pit-marks in the adjacent grooves.

Orange book specificationSpec. 14.4: XT < 50%

Remarks• For an 80 minutes CD-R the track pitch is smaller,

therefore the Cross Talk will be slightly higher.• The more narrow the pit marks are written, the lower the

Cross Talk.

2004/5/14 47Lecture 6

land

groove

groove

groove

land

Recorded CD-R specification Cross Talk (XT)

2004/5/14 48Lecture 6

Definition: The Block Error Rate (BLER) measures the percentage of data blocks which contain one or more bit errors.

Orange book specificationSpec. 14.3: BLER < 3% averaged over 10 seconds(This corresponds to BLER < 220 cps (data rate 7352 blocks/sec))

Remarks• The BLER is very recorder / recording speed dependent

Recorded CD-R spec: Block Error Rate (BLER)

2004/5/14 49Lecture 6

Recorded CD-R spec: Recorded Time Errors(E11, E21, E31, E12, E22 and E32)

Definition• The recorded time errors indicate in which phase of the

Error Correction the misread errors are corrected.• An E32 is a non-correctable error.

Orange book specificationSpec. 14.5: No uncorrectables (E32 = 0)(E11, E21, E31, E12 and E22 are not specified)

Remarks• The Block Error rate is measured as the sum of the En1’s:

BLER = E11 + E21 + E31

2004/5/14 50Lecture 6

Recorded CD-R specification Jitter

Definition: The jitter is the standard deviation of a mark length. It measures the fluctuation in length of the data marks.

Orange book specificationSpec. 14.7: Land jitter < 35 ns

Pit jitter < 35 nsThese limits are valid for all mark lengths (3T to 11T)

Remarks• The jitter is very recorder / recording speed dependent

2004/5/14 51Lecture 6

3T pit mark100,000 samples

0

500

1000

1500

2000

2500

500 550 600 650 700 750 800 850Mark length [ns]

Num

ber o

f sam

ples

= 20 ns = 30 ns = 45 ns

σσσ

3T pit mark100,000 samples

0

500

1000

1500

2000

2500

500 550 600 650 700 750 800 850Mark length [ns]

Num

ber o

f sam

ples

dev = 0 nsdev = -20 nsdev = 30 ns

-20 ns

30 ns

Recorded CD-R specification Jitter peak

Statistical distribution of a mark length: normal distribution• Jitter indicates peak width (standard deviation)• Mark length deviation indicates peak center (average

length)

2004/5/14 52Lecture 6

5T

5T

4T 6T

Recorded CD-R specification Jitter

Small fluctuations influence the length of a pit mark

• Small variations in the groove geometry

• Local non-uniformity in the substrate, dye layer and/or silver layer thickness

• Microscopic defects

• Fluctuations in dye sensitivity

• Small fluctuations in the laser power

• Fluctuations in the laser pulse length• ….

2004/5/14 53Lecture 6

Pit mark histogram 1,000,000 samples

0

1000

2000

3000

4000

5000

6000

500 1000 1500 2000 2500Mark length [ns]

Num

ber o

f sam

ples

3T

4T5T

6T7T 8T 9T 10T 11T

Recorded CD-R spec: Jitter histograms

The mark length distribution of a CD / recorded CD-R measured by a time interval analyzer (TIA) consists of a land and pit mark histogram, with 9 peaks each.

3T

4T5T

6T7T 8T 9T 10T 11T

Land mark histogram 1,000,000 samples

0

1000

2000

3000

4000

5000

6000

500 1000 1500 2000 2500Mark length [ns]

Num

ber o

f sam

ples

3T 4T 5T 6T 7T 8T 9T10T 11T

Land mark histogram 1,000,000 samples

1

10

100

1000

10000

Num

ber o

f sam

ples

500 1000 1500 2000 2500Mark length [ns]

3T 4T 5T 6T 7T 8T 9T10T 11T

Pit mark histogram 1,000,000 samples

1

10

100

1000

10000N

umbe

r of s

ampl

es

500 1000 1500 2000 2500Mark length [ns]

2004/5/14 54Lecture 6

Jitter

BLE

R

Land mark histogram1,000,000 samples

0

1000

2000

3000

4000

5000

6000

500 1000 1500 2000 2500Mark length [ns]

Num

ber o

f sam

ples

Land mark histogram1,000,000 samples

1

10

100

1000

10000

500 1000 1500 2000 2500Mark length [ns]

Num

ber o

f sam

ples

Recorded CD-R spec: Jitter histogram at high jitter

2004/5/14 55Lecture 6

Jitter

Pit Land

High

Low

2004/5/14 56Lecture 6

5T

5T3T

5T 8T

4T 6T

3T

8T

Jitter and intersymbol interference

Land jitter is mostly higher than pit jitter due to intersymbol interference:

- Land marks have an additional length variation (jitter) due to deflection of the pit mark lengths (deviations) by which it is formed.

2004/5/14 57Lecture 6

SNR vs CNR• SNR

- not readily measurable

• CNR(Carrier-to-Noise Ratio)- Use spectrum analyzer- On a fixed-freq signal- Difference betw’ signal peak and noiselevel at a nearby freq(in dB)

• SNR(dB) = CNR(dB) + 10log(30KHz/f)- CNR = 45 dB, SNR@ 10 MHz

= 45 - 25 = 20dB 0 MHz 8

10dB/divref=0 dbm

fundamental secondharmonic

2004/5/14 58Lecture 6

SNR in Optical Recording

Using differential detectors，shot noise limited case

•••• η= 0.5 A/W，P = 2mW，R = 0.2，θk =

1°， e = 1.6 × 10-19• SNRBW=30KHz = 74 dB

SNRBW=20MHz = 46 dB• Best reported SNR = 67 dB in MO

PReBI N η2=PRI kS θη 2sin=( ) ( )eBPRIISNR kNS θη 21010 sin2log10log10 ==