Post on 08-Feb-2021
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 ==