Blu-Ray disc recorders

System Design for Double-Layer Blu-ray Disc Recorders

Dianyong Chen
* 1
, Dayu Chen
1
, Zhisheng Fan
1
, Anton Janssen
2

1
Philips Research East Asia, Shanghai, China
2
Philips Research, Eindhoven, the Netherlands

ABSTRACT

This paper presents the system design of double-layer Blu-ray Disc recorders built in Philips Research Laboratory. The
system architecture, new components, and the advanced signal processing are addressed. The experimental results show
that writing HDTV programs to double-layer Blu-ray Discs and reading them back have been achieved. In addition, this
paper demonstrates that discs with higher density and higher capacity are possible with the advanced technologies
developed.
1. INTRODUCTION

In June 2002, a new physical format for the third generation optical storage: the Blu-ray Disc (BD) was officially
announced. Ever since then, a number of companies have shown their BD demo system according to the released
standards. Some even launched their pilot products.

Philips is one of the companies involved in the development of relevant technologies for BD ever since 1999 [1]. Philips
Research showed its first BD recorder on CEATEC in 2002. As the system designers, we have experienced more and
more design margin to make a robust system because of the technology advancement in the disc manufacturing, the
optical components, and the signal processing. In this paper, we start with a brief introduction to the basic physical
format of Blu-ray Disc, and then we discuss the system design, key components, and advanced signal processing for
double layer Blu-ray Disc recorders. In the last part, we provide with some experimental data to show that capacity
higher than 30 Gigabytes on a single layer BD disc is possible.

2. BASIC FORMAT

The Blu-ray Disc Rewritable (BD-RE) physical format is based on a blue-laser diode (λ=405 nm), an objective lens
with a high numerical aperture (NA=0.85), a disc with thin cover layer (0.1 mm thick), and phase-change recording on a
groove-only substrate. The Blu-ray Disc has a fixed track pitch of 320 nm. The different capacities (23.3 GB, 25 GB,
and 27GB) of the recording layers are realized by changing the “in-track” or tangential density (reduction of the wobble
length and hence the channel bit length) [2].

The modulation coding (17PP) adopted by Blu-ray Disc system gives advantages in DC control (absence of merging
bits) and the wider timing window in the eye pattern [3], [4]. The read-channel in optical recording is characterized by a
modulation transfer function (MTF) with almost linear roll-off, and a cut-off frequency at (2NA)/λ [1]. The minimum
run length 2T has small MTF and relatively low signal-to-noise ratio (SNR) and hence is the most critical frequency.
Some advanced digital signal processing technologies have been developed to improve the system operation margin.

By decreasing the substrate thickness to 0.1 mm, Blu-ray Disc has a similar tilt margin as DVD. Therefore, additional
tilt servo is not needed. Since the spherical aberration scales with NA
4
, the BD system utilizing a NA=0.85 objective
lens is very sensitive to variations of the cover layer thickness. After jumping from one layer to another, spherical
aberration must be adjusted. However, according the specification, if the thickness variation of the transmission stacks is
within ±2 µm or ±3 µm, real time or dynamic spherical adjustment is not necessary. This was very difficult when BD
was introduced to the public in early 2002. Nowadays, with the advanced technologies such as spin coating or
extrusion-foil gluing with pressure-sensitive-adhesive (PSA), the cover thickness variation is well within ±1.5µm and
±1µm respectively. The biggest challenge remaining in BD system is its sensitivity to fingerprints and scratches. When
designing the error correction scheme for BD, it was taken into account by introducing the 64KB long distance code

*[email protected]; phone +86-21-63541088 ext. 5923; fax +86-21-65446864
Invited Paper
Advances in Optical Data Storage Technology, edited by Duanyi Xu,
Kees A. Schouhamer Immink, Keiji Shono, Proceedings of SPIE Vol. 5643
(SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.573226
25
(LDC) and the burst indicating subcode (BIS). However, in normal use, fingerprints and scratches will inevitably
increase with time, probably beyond the capacity of error correction. An optional cartridge was proposed in the version
1.0 BD-RE standards. Recent technology advancement in the hard coating and the anti-stick coating has leapfrogged the
optional cartridge in two aspects: normal use will not generate scratches, and fingerprints are not easy to stick to the BD
discs and can be wiped off without damaging the discs [5].

Wobbled pregrooves are designed for Recordable and Rewritable BD discs. In the information area, pregrooves are
modulated with minimum-shift-keying (MSK), harmonic-modulated-wave (HMW) or saw-tooth-wobble (STW), and bi-
phase modulation. The wobble mark addressing system is better than the use of headers, especially for double layer
discs and multi-layer discs [6].

3. THE SYSTEM DESIGN

The system design of a double layer BD recorder is to develop the optics, mechanics and electronics to read and write
BD discs that abide by the formats as defined in the Blu-ray Disc standards books. An overview of the system design of
our double layer BD recorder is shown in figure 1. It mainly consists of a blue optical pick-up unit (OPU), a
preprocessing unit, an encoding/decoding unit, a digital servo, and back-end electronics. The blocks are controlled by an
embedded microprocessor.

Laser
Driver
Actuator,
Collimator
motor
PDIC and
Amplifiers
O
p
t
i
c
a
l

P
a
t
h
Blue OPU
Spindle, sledge, tray motor
Main Block
Backend
electronics
TV set or
computer
Preprocessor
Write clock
WS generator
Servo
Amplifiers
Microprocessor
ADIP
processor
Servo control
HF data
processor
Memories

Figure 1: An overview of the system design of a double layer BD recorder

When the system is working at the reading mode, light reflected from the disc is collected by the photodetecting
integrated circuits (PDIC). The PDIC converts the laser power into weak photo current. The photo current is amplified
and delivered to the preprocessing unit. The preprocessing unit delivers error signals for servo control and HF signals
for the data path. The HF signals are later equalized and sliced to give binary data. The binary data are demodulated,
decoded to give the user data to the backend electronics. The backend electronics deal with navigation, file system, AV
stream and copy protection system, which is beyond the scope of this paper. At the writing mode, the preprocessing unit
also delivers wobble signals to the address-in-pregroove (ADIP) decoder so that the system knows the disc information,
address and reference clock for writing. The rest is mainly the reverse process of reading. The BD recorder system
26 Proc. of SPIE Vol. 5643
architecture looks very similar to that of a DVD+R/+RW recorder. However, the OPU and algorithms implemented in
the IC chips are different.

4. THE BLUE OPU

The blue OPU is one of the most important parts of a BD double layer recorder. Research work on blue OPU is mainly
focused onto two fields: developing a blue OPU with sufficient working margin and developing a blue OPU compatible
with CD and DVD. In 2004, Sony reported its Single Optical Head for BD, DVD and CD, and Philips announced the
OPU81 that will be able to read and write CD-R/-RW, DVD+R/+RW and BD. In our BD recorder, we use a blue-only
OPU with the light path drawn in figure 2 [7].

Blue LD
Beamshaper
Foward Sense
Servo Lens
PDIC
PBS
Movable
Collimator
Folding Mirror
Objective
quarter-
waveplate


Figure 2. The schematic of our blue OPU

We use blue laser diodes with typical operating voltage of 4.34 V and threshold current of 40.7 mA at 25 °C. The slope
is about 1.31 W/A. The relative-intensity-noise (RIN) is an important parameter of a blue laser diode. Experimental
results and simulations show that laser noise less than –120 dB/Hz is necessary for readout jitter below 8% [8]. The
maximum output power of the blue laser diode is very critical for double layer recording. Assuming an overall
transmission factor of 60% for the optical pick-up, and a diode coupling efficiency of 48%, a maximum diode pulse
power of 45 mW only gives 13 mW actual writing power on the disc. For some double layer BD-RE discs, the writing
power for 1x recording is already higher than 10 mW. The maximum writing power of our blue OPU is 14 mW that
allows sufficient power margin for 2x recording for the BD-RE and BD-R discs that we have.

A blue laser diode has a large aspect ratio and Gaussian shape power profile that gives low rim density if high coupling
efficiency of the diode laser is required. The typical parameters of a blue laser diode are: 9 degree of full-width-half-
maximum (FWHM) horizontal beam divergence, 27 degree of vertical divergence. Because the BD standards are
defined on a circular beam and low rim intensity gives low effective NA, the beam-shaper is a key component in our
blue OPU. Philips has developed an integrated plastic beam-shaper that not only circularizes but also homogenizes the
elliptical intensity profile of a blue laser diode. The beam-shaper has a low aberration level of 20-25 mλ at an NA of
0.15.
Proc. of SPIE Vol. 5643 27
Another key component of our blue OPU is the high NA objective. Since the capacity of an optic disc is proportional to
(NA/λ)
2
, the objective with an NA of 0.85 is a key to 25GB and 27GB capacity on a single layer disc. The objective
that we use is a dual-lens objective. Although single lens objectives are now available, the dual-lens objectives have
better quality. The objectives we use have a numerical aperture of 0.85, a free working distance (FWD) of 0.15 mm, and
wavefront aberration well below 30mλ rms. In 1999 Philips invented an objective with a moveable front lens to correct
spherical aberration [10]. In order to make the system more robust, we apply a rigidly mounted objective and use a
movable collimator to correct spherical aberration.

As already discussed in the basic format part, switching between the two recording layers of a Blu-ray Disc results in a
significant amount of spherical aberration. The spacer layer is 25 µm as defined in the standards book. In designing of a
blue OPU, detection and compensation of the spherical aberration must be considered. Several methods can be used to
compensate the spherical aberrations: adjusting the position of the collimator lens, adjusting a telescope, or the using
liquid crystal elements. The movable collimator that we use is mounted on a mechanical actuator. When current is
applied to the coil, the collimator can move back and forth to correct spherical aberration. Typical methods to derive the
spherical aberrations signals reported in literatures include: using a holographic optical element called polarizing
diffraction grating (PDG) [11], [12], and using a photo-detector split into inner segments and external segments.

The normal photo-detectors used for CD and DVD have low efficiency for blue laser. Philips has designed a photo-
detector IC (PDIC) with sufficient sensitivity for blue. The PDIC has programmable gain, since the reflectivity of BD-
ROM, BD-R and BD-RE may be very different. It also has high bandwidth and low noise for amplification. The PDIC
that we use has a bandwidth larger than 180 MHz, capable of detecting signals up to 2x BD single layer and 1x BD
double layer, considering that the optical cut-off frequency for 25GB BD discs at 1x reference speed is 20.6MHz and
the signal-to-noise ratio must be high enough.

5. THE ADVANCED SIGNAL PROCESSING

Advanced signal processing is a key factor in the proper functioning of the Blu-ray Disc system. The main challenge in
signal processing is bit-detection of low SNR signals and high speed. The requirement for the HDTV contents pushes
the channel bit rate of a 1x BD system to 66 Mbit/s.
5.1. Limit equalizer and run-length push back

The 17PP modulation coding adopted by the BD system is a (1,7) RLL coding, which gives higher coding efficiency
and shorter minimum run length than those of DVD. This results in a highest frequency that is closer to the optical
cutoff frequency and therefore leads to smaller amplitude in the eye pattern. The option of a higher linear density in the
BD system makes the amplitude of the highest frequency even smaller [13]. Figure 3 shows the dominant noise source
is low-frequency media noise and from the data spectrum we observe that the most critical data frequency is the 2T
pattern.

The problem of a low signal-to-noise ratio for a specific pattern can be solved in several ways. One method is the use of
Viterbi detector. Another method is to improve the SNR by an equalizer. Although the method based on partial response
maximum likelihood (PRML) and Viterbi detection is very powerful, we are forced to measure the bit error rate instead
of jitter [14]. Therefore, an equalizer is preferable, because we can measure jitter of readout signal using an equalizer,
therefore the routines defined in the standard books for optimization can be followed. Although boosting the 2T pattern
with a linear equalizer or conventional equalizer will improve the signal-to-noise ratio, high boost gives rise to inter-
symbol-interference (ISI). This boost is limited to approximately 7 dB.

The limit equalizer invented by Pioneer uses a limiter to limit the amplitude of run length longer than 2T. It is able to
boost the high frequencies without introducing ISI [15]. We have developed a limit equalizer with additional features.
The diagram of the equalizer is shown in figure 4. The first feature is a notch in the low frequency part of the spectrum
for low amplitude. It greatly suppresses the low frequency media noise. The second feature is that this filter allows also
asynchronous samples. In addition, we designed a run-length-push-back (RLPB) [16] unit after the equalizer. If a
28 Proc. of SPIE Vol. 5643
shortest run length violation of the 17PP code occurs, the 1T pattern is corrected to a 2T pattern by shifting the signal
transition with the largest phase error in PLL.




Figure 3. Measurement on a double-layer BD disc Figure 4. Limit equalizer diagram


The clip level of the limiter is set to 25% of the peak amplitude of the incoming signal, and k is set to 1.25. The
comparison of a conventional (fixed) equalizer and a limit equalizer is shown in figure 5 and figure 6. The jitter
performance of a limit equalizer is better than that of a conventional equalizer.

5
7
9
11
13
15
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Focus Offset (V)
J
i
t
t
e
r

(
%
)
Limit Equalizer
Fixed Equalizer


Figure 5. Jitter versus defocus Figure 6. Jitter versus tilt
5.2. Improved wobble detection

The blank Blu-ray Disc has a continuous groove with a track pitch of 320 nm to generate a tracking signal. The groove
is wobbled in a pre-determined manner so that it can also be used for write-clock generation, for retrieving timing and
address information, and for storing disc information. The information is indispensable for the system to know how and
where to write. The typical amplitude of the wobbling is ± 10 nm [17]. To derive a stable and accurate write clock, the
wobble is predominantly a single-tone carrier. To cope with different distortion, the modulation scheme is a
combination of MSK that is strong for media noise and STW that is strong for wobble shift in a single wobble-address
format. One wobble period corresponds to exactly 69 channel bits. Each group of 56 nominal wobble lengths (NWL)
starting with a MSK mark is called an ADIP unit. There are 8 such ADIP unit patterns defined in the Blu-ray Disc
standards books. The patterns are shown in figure 7. The 4 sync units are used for synchronization purpose, the data_1 is
used to represent the bit value “1”, and the data_0 is used to represent the bit value “0” [7]. A MSK mark is 3 monotone
Limiter T T T
T T
+
-k
-k
5
7
9
11
13
15
-1.2 -0.8 -0.4 0 0.4 0.8 1.2
Tilt (degree)
J
i
t
t
e
r

(
%
)
Limit + Radial
Conv + Radial
Limit + Tangential
Conv + Tangential
Proc. of SPIE Vol. 5643 29
periods long. MSK mark and STW wave can be mathematically expressed as m(t) and s(t) in formula (1) and formula
(2) respectively.

( ) ( )
∏ ∏ ∏
|
.
|

\
|
− ⋅ ⋅ − |
.
|

\
|
− ⋅ − |
.
|

\
|
− ⋅ ⋅ =
2
5
} 5 . 1 2 cos{
2
3
} 2 cos{
2
1
} 5 . 1 2 cos{ ) ( t f t f t f t f t f t f t m
wob wob wob wob wob wob
π π π

(1)
( ) { } ( ) { } t f t f t s
wob wob
⋅ ⋅ ± = 2 2 sin 25 . 0 2 cos ) ( π π (2)
Where Π(x) is the rectangle function defined as:

∏
¹
´
¦ ≤ < −
=
elsewhere , 0
5 . 0 5 . 0 , 1
) (
x
x (3)

sawtooth wave representing a "1" sawtooth representing a "0"
monotone wobble
MSK mark
NWL number 0...3...
...55
sync_1 unit
sync_2 unit
sync_3 unit
data_0 unit
data_1 unit
monotone unit
reference unit
syn_0 unit


Figure 7. ADIP unit types

The MSK marks and STW wave can be detected using the same heterodyne circuit which consists of a carrier
multiplier, an integrator and a sample and hold element as shown in Figure 8. The wobble signal is multiplied by the
cosine carrier of the fundamental frequency for detecting MSK in the multiplier. On the other hand, it is multiplied by
the sine carrier of the second harmonic frequency for detecting STW.

Integral
S&H
S&H
wobble signal
from push-pull
Carrier
for MSK: f1=1/69T
for STW: f2=2/69T
Reset
0/1 by MSK
0/1 by MSK+STW
0/1 by STW


Figure 8. The diagram of heterodyne detection for MSK and STW

Since the amplitude of wobbling is about 10 nm, the modulation is weak. On the other hand, the channel is constantly
distorted by different types of noise. The first type of distortion is media noise, the groove noise, and the crosstalk from
the main data. The second type of distortion arises when the wobble phase-locked-loop (PLL) is in lock but there is an
uncertainty in the precise wobble position with respect to the start of an address in ADIP. This is the so-called wobble
shift. The third type of distortion is the crosstalk from the wobble signals in adjacent tracks. The fourth type of
distortion comes from local defects including dust and scratches on the disc surface. Considering the abovementioned
30 Proc. of SPIE Vol. 5643
distortions, we have designed a method based on the concept of hypothetical maximum-likelihood receiver to improve
the wobble detection [18].
∫
∞
∞ −
− − ≈ dt t kT h t r y
k
)) ( ( ) ( (4)
Where Y
k
is a precomputed sequence for maximum likelihood receiver. It can be recognized as the sampled output of a
filter with impulse response h(-t) operating on the received signal r(t). The output of the filter is then passed to a pattern
comparator to find the maximum or the best matched pattern. In practice, this design has greatly improved the system
robustness against various types of distortion to the wobble signal.

6. THE EXPERIMENTAL RESULTS

We have recorded 23.3 GB, 25GB, 46.6GB, and 50GB BD-RE and BD-R discs with the recorders that we developed. In
addition, we can also read 23.3GB and 25GB BD-ROM discs with this recorder. These BD discs are tentative discs and
may not fully comply with the BD-R and BD-ROM standards that will be released soon. Figure 9 shows the jitter
measured on a double layer BD-R disc and figure 10 shows the HDTV contents that we recorded on a BD-RE disc.

10
11
12
13
14
15
16
9 10 11 12 13
Power (mW)
J
i
t
t
e
r

(
%
)


Figure 9. The 1x jitter on L0 of a double layer BD-R disc Figure 10. The HDTV contents played back

7. HIGHER DENSITY BD

It is defined in the BD standards books three capacities for single layer discs: 23.3GB, 25GB and 27GB. Nevertheless,
Philips Research has demonstrated that higher capacity above 30GB of a single layer BD disc [19]. Figure 11 and
Figure 12 show the performance of Philips BD-ROM of 25GB and 31GB capacity.

-5
-4
-3
-2
-1
0
-1.2 -0.6 0 0.6 1.2
Angle (Degree)
L
o
g
(
S
E
R
)
25 GB
31GB


Figure 11. Symbol-error-rare versus radial tilt Figure 12. Symbol-error-rate versus tangential tilt

-5
-4
-3
-2
-1
0
-1.2 -0.6 0 0.6 1.2
Angle (Degree)
L
o
g
(
S
E
R
)
25 GB
31 GB
Proc. of SPIE Vol. 5643 31
The main technology challenges are in three aspects: writing BD-RE and BD-R discs with shorter bit length, bit-
detection for low SNR signals, and mastering BD-ROM discs with shorter bit length. The first one was not very difficult
to prove with a flexible experimental platform in Philips Research. To make such discs, the wobble period is to be
shortened from 5.14 µm of a 25GB disc to 4.28 µm of a 30GB disc. For the second challenge, advanced signal
processing based on partial response maximum likelihood (PRML) and Viterbi detection has been extensively reported
[1], [20], [21]. For the last challenge, Philips invented the liquid-immersion-mastering solution based on slight
modification to the existent DVD mastering equipment to cope with it. Figure 11 and figure 12 shows acceptable
symbol error rate performance of the 31GB discs.

8. CONCLUSION

We explained some aspects of the system design of the double layer BD recorders developed in Philips Research and in
Philips Optical Drive Division. The experimental results show that writing HDTV programs to double-layer Blu-ray
Discs and reading them back have been achieved.

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Proc. of SPIE Vol. 5643 33