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Since its arrival on the market, individuals, companies and public institutions have
come to rely on the powerful capabilities of CD-Recordable (CD-R) as a trusted and
valued partner. Unique in the computer and music worlds CD-R discs, once written, by
their nature have high data integrity, last a long time and can be read by over 180
million CD-ROM drives and 750 million audio and other compact disc players currently
in use as well as all new DVD-ROM drives and some DVD Video players. As a result,
CD-R is used with confidence for data storage, audio recording, archiving, prototyping,
software distribution and countless other applications.
An important reason confidence in CD-R is so well founded relates to some of the
advanced techniques that are now used by CD-Recording hardware to ensure discs
are correctly written. One of the most important of these is called Running Optimum
Power Control or Running OPC.
In general terms, CD-R discs are made up of an optical stack consisting of a
polycarbonate substrate, a sensitive dye layer, a gold or silver alloy reflector and a
protective lacquer overcoat. Some media manufacturers also coat discs with an
additional protective layer to further protect against possible damage from handling and
Data is written to the disc by a CD-Recorder focusing a high power laser on the dye
layer and precisely heating and locally irreversibly altering it to create a spiral track of
variable length marks (low reflective areas) and lands (highly reflective spaces between
the marks). The resulting pattern or combination of the modulating lengths of marks
and lands, from 3 to 11 clock cycles (3T to 11T), with the 1's in the digital data stream
being encoded in the mark/land boundaries (transition regions), physically encodes the
data. CD players or CD-ROM drives can later retrieve the data by focusing a low-
powered laser beam onto the track of marks and lands and deciphering the modulated
pattern of light reflected back to a photo diode detector. (see Figure 1)
Precise mark length is therefore critical if the data is to be represented accurately. For
example, if a CD-ROM drive reads a disc with a number of 3T marks or lands that are
written too long it might misinterpret them as 4T features. While the sophisticated error
correction used by the system can handle all but the most extreme cases, there is the
possibility that the drive will be unable to retrieve the correct information or reading
performance may suffer if too much correction is required.
What Is "Running OPC?"
Running OPC is a special technique used in newer CD-Recorders for monitoring and
maintaining the quality of the disc writing and ensuring the accuracy of all the mark and
lands lengths across the disc. The term Running OPC actually describes a general
process which is also known by several trade names including "Dynamic Power
Control (DPC)" and "Direct Read During Write (DRDW)." There may be differences in
execution which gives some of these implementations competitive advantages over
others.Running Optimum Power Control
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Initial OPC Procedure
The correct amount of laser power needed to write a CD-R disc is variable and
depends on both the individual recorder, disc and sometimes even the specific location
on the disc. Due to their physical makeup, the various types of dyes used in CD-R
discs have different sized power windows and therefore require different amounts of
laser power for proper recording. Power window refers to the range of laser energy
which will properly form the correct size marks on a disc, which not only can vary
between the type of dye used but is also dependent upon the speed at which the disc
is being recorded. For example, a CD-R disc written at double speed might have a
power window of 2 milliwatts (mw) between the range of 8 and 10 mw. If the disc is
written within this range the marks formed will be of the proper size. Too much power
will create oversized marks which can interfere with each other physically and
practically when being read. Too little power will produce undersized marks and the
reduced signal levels during playback can, in extreme instances, cause read failure.
The additional fact that the dyes have different sensitivities to laser power at different
light wavelengths is also important since recorders are allowed to use lasers which
operate within an approved range (775 to 795 nm) rather than at a single frequency.
In the case of the recorder, the size and optical quality of the laser spot it uses for
writing varies from unit to unit as does its wavelength, which can change depending
upon temperature and other environmental conditions. The emission frequency of most
lasers is temperature sensitive, and thus writing performed at the extremes of the
allowable operational temperature range can result in a significant spread of
wavelengths. Consequently, before starting, all recorders perform an initial Optimum
Power Calibration (OPC) procedure to determine the best writing laser power setting
for each disc and recorder combination.
The OPC process begins with the recorder retrieving an initial Recommended Optimum
Recording Power estimate value (for a writing condition of 785 nm at 25 degrees
Celsius) from the Absolute Time In Pregroove (ATIP) information encoded in the Lead-
In Area of the disc. Using this setting as a starting point the recorder steps through
higher and lower laser power settings while writing test information in a special
reserved space of the disc called the Power Calibration Area (PCA), located before the
disc's Lead In Area. (see Figure 2)
In practice the OPC procedure can vary from manufacturer to manufacturer and
recorder model to model but, as an example, a recorder might obtain a beginning
recording value of 5.9 mw from a disc and write fifteen times (15 ATIP frames or a fifth
of a second) in the PCA with power ranging from 4.1 to 7.7 mw.
After writing the test marks at the different laser powers the recorder reads them back
and looks for differences (asymmetry or beta) between the lengths of marks and lands.
A negative beta means that, on average, the marks are underpowered (short) and a
positive beta means that they are overpowered (long). To be broadly compatible with
the various available types of media, recorders traditionally use a beta of +4%
(suggested in the Orange Book Part II specification), though some units now have
multiple target betas and write strategies (the latest version of the Orange BookRunning Optimum Power Control
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The read signal created from previously recorded marks and lands on a CD-R disc.
Cross sectional view of a CD-R disc showing the location of the Power Calibration Area (PCA).
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Recording Power (Step)
Power Calibration Area (PCA)
Results from an Optimum Power Calibration procedure
showing a +4% beta value achieved with a recording power of 8 mw.Running Optimum Power Control
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actually mandates the use of specific target betas and write strategies.) The recorder
then determines what setting achieved the +4% beta target and establishes that as the
recording power for the disc. (see Figure 3)
Some of the earliest recorders simply used the Recommended Optimum Recording
Power estimate value encoded in ATIP as the established writing power, a procedure
that has now been more or less abandoned since it has become clear that the optimum
recording power can be a fairly sensitive function of the precise optics used in the
recorder. In fact, the well publicized occurrences of over or under powered writing in
the early days of CD-R were often a result of this simplistic approach.
A recorder with Running OPC takes this process still further. During the initial OPC
procedure the recorder also monitors the reflected light coming back from the disc
while the marks are forming and stores that information. After determining what power
setting yields the required +4% beta the recorder retrieves the reflected signal that is
associated with it, establishes a mark formation signature, and saves it in its memory.
During recording the system monitors the marks as they form on the disc using the
reflected light and compares these signals against the signature established during the
initial OPC procedure. Laser power is then adjusted on-the-fly throughout the writing
process to maintain this optimum condition. (see Figure 4)
For example, if the recorder encounters a condition that reduces the amount of laser
light reaching the dye recording layer (dust, scratches, fingerprints, etc.), rather than
the resulting mark being too short Running OPC will detect the change in the reflected
light signal relative to the stored signature and increase the laser power to attempt to
compensate. This ability to react on-the-fly is critical since the process of writing data is
far less tolerant than is the process of reading data. When reading data, such features
as embedded error correction schemes and strategies such as lowering the disc's
rotation speed can compensate for encountered problems. By contrast, a recorder has
only one brief attempt at writing data, a situation that is compounded in high speed
recording. (see Figure 5)
Specific situations that Running OPC can deal with depend upon the speed of
response or bandwidth of the recorder. As the writing process takes place the mark
formation signal is continuously being returned to the recorder, but before the hardware
can interpret and use the information the signal's waveform must first be sampled. The
more marks that are sampled and evaluated the higher the bandwidth and the shorter
the duration of a problem for which the recorder can compensate.
For example, in a low bandwidth Running OPC scheme a recorder might only sample
the formation signal returned from the longest laser pulses (11T). Since there are
always fewer long marks than short marks encoded on a disc (approximately 2:1 as a
consequence of the coding rules), only gradual changes from the disc's inner to outer
diameter can be addressed. Compensating for more localized problems that are
confined to smaller areas or particular spots on the disc is difficult for low bandwidthRunning Optimum Power Control
A writing laser pulse and the resulting mark formation signal. The exact form of the
laser pulse and shape of the signal depends upon the specific write strategy used by the recorder.
A normalized mark formation signal for three recording power settings.
An example of a "Flyspeck" disc used for evaluating the capabilities of Running OPC systems.
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systems since by the time a problem is detected it may be too late to take corrective
action. The types of gradual variations which reduce the amount of laser power
delivered to the CD-R disc's dye recording layer that can be accommodated by low
bandwidth systems include media manufacturing process issues such as variations in
the thickness of the dye recording layer (conformality), substrate birefringence (optical
distortion) and hardware manufacturing issues including changes in the quality of the
recording spot due to wavelength shift of the writing laser, tilt, defocus and detracking
as well as changes in disc performance due to wavelength shift, temperature, tilt and
On the other hand, a recorder with high bandwidth Running OPC might sample the
formation signal returned from all the laser pulses (3T to 11T) and as a result has very
current information to use to make adjustments for not only gradual, but also rapid
variations which diminish the amount of laser power available on the disc for writing,
including dust, scratches and fingerprints on the laser incident surface of the disc. The
additional ability to compensate for such possible recording hazards is especially
important in multisession or packet writing situations, where a disc may be handled
many times over and be subjected to physical abuse between recordings.
Many recorder manufacturers evaluate and quantify the capabilities of their Running
OPC systems by conducting tests using specially prepared "Flyspeck" discs which
have more or less standard simulated fingerprint patterns and dust spots of various
sizes printed onto them. (see Figure 6)
Absorption Control Warning
In the event that an issue can't be dealt with, some recorders generate an "Absorption
Control" warning to let the user know that there may be a problem in that particular
region of the disc. This can be done in several ways.
The first technique involves monitoring the amount of laser power the recorder is using
to write marks on the disc and comparing it against the optimum value established
before writing began. If the power exceeds a predetermined threshold for a certain
amount of time, for example 20% higher than nominal, a flag goes up and the warning
message is issued.
Rather than using the writing power level, the second method monitors the mark
formation signal during recording and compares it against the optimum formation
signature established at the outset. When the signal falls below or wanders from
optimum for a predetermined amount of time, a flag is raised and the absorption control
warning message is issued. Since compact discs employ various levels of error
correction that can compensate for some errors in the recorded data, the precise
amount of time that the signal can deviate from the optimum is based on the number of
consecutive errors (burst errors) the CD player hardware can compensate for.
Running OPC helps detect potential data errors and provides a degree of data integrity
verification. It can be used to indicate areas where possible problems have beenRunning Optimum Power Control
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detected and have the system go back and verify only those areas later, thereby
increasing verification speed significantly. In fact this has been implement in many
professional authoring systems where data integrity is a key requirement.
What Running OPC doesn't do is provide instantaneous read-back verification of the
data after it is written. This is the domain of Direct Read After Write (DRAW) systems
which use a second laser beam trailing the writing laser to determine if the correct data
has been recorded on the disc. Because DRAW systems require extra hardware to
provide and control the additional laser beam, they are expensive to commercialize and
are therefore generally reserved for industrial mastering systems and not for
professional or consumer CD-Recorders. Alternatively, the user obviously has the
option to make a verification pass after writing as is done in Magneto-Optical (MO) and
most other storage devices.
While Running OPC is very powerful it has its limits in terms of data verification. It will
indicate if correct mark formation is taking place on the disc, but not if the correct data
is being written. For example, if there is an issue that causes incorrect data to be
transferred to the recorder, such as a SCSI termination problem, the data corruption
cannot be identified by a Running OPC technique â€” only read back verification
comparing the source data to the written disc after the fact will reveal that. Typically
CD-Recorders can be set up and initially checked for data integrity and unless the
configuration changes there should not be any data corruption problems, except in the
unlikely event of some type of component failure.
How much integrity checking and data verification a user decides to do is really a
question of acceptable risks for the application. This includes not only Running OPC
and data comparison using the recorder but also the use of low-level analyzers and
interchange testing on a variety of commercially available CD-ROM drives.
What About DVD?
Following its successful implementation in CD-Recorders, it is important to note that
version 1.0 of the DVD-Recordable (DVD-R) specification also recommends the use of
Running OPC. Apart from the welcome data integrity enhancement, Running OPC
provides hardware and media manufacturers another tool for dealing with the much
tighter tolerances that are, for example, a result of using high numerical aperture
optical systems (0.6 NA for DVD-R vs. 0.47 NA for CD-R) for high density DVD
Running OPC In Perspective
Ensuring that consumers can write high quality CD-R discs is the responsibility of both
the media and recorder manufacturers. Running OPC is an important tool used to help
fulfill that obligation. However, what makes Running OPC so compelling is the fact that
it is in everyone's best interest.
Running OPC's ability to compensate for fluctuations encountered in both the recorder
and media helps manufacturers greatly by allowing increased product tolerances and
thereby reduce costs. The additional hardware and software programming needed forRunning Optimum Power Control
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Running OPC obviously costs a little more, but that increase is more than offset by the
opportunity to use less expensive components, materials or processes.
The powerful ability of Running OPC to compensate for substantial degradations in
recording conditions is readily apparent in Figures 7-10 where, despite large variations
in parameters such as tilt and tracking and focus offsets, Running OPC maintains
optimally recorded marks. (see Figures 7-10)
In addition to lower CD-R media and recorder prices, consumers most importantly
benefit from a more reliable recording system with superior data integrity. Confidence
comes from knowing that the recorder is not only correctly writing the disc, but
optimally writing it for the best possible result. Assurance also comes from knowing that
even in the event of a problem the recorder is intelligent enough to detect and advise of
the situation.Running Optimum Power Control
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Figure 7: Figure 8:
Beta values from a disc written with and Beta values from a disc written with and
without Running OPC under defocus conditions. without Running OPC under in-track tilt conditions.
Figure 9: Figure 10:
Beta values from a disc written with and Beta values from a disc written with and
without Running OPC under track offset conditions. without Running OPC under cross-tilt conditions.Running Optimum Power Control
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"Data integrity on CDs." Eastman Kodak Company, 1995.
"Direct Read During Write (DRDW) in the Kodak PCD Writer 600." Eastman Kodak
Recordable Compact Disc System Part II, Philips and Sony Corp., Attachment B14
Bakx J., Philips Electronics. "Running Optimum Power Control method in recordable
CD systems." US Patent 5,255,007, Jan. 1990.
Bakx J., Philips Electronics. "Calibration procedure for determining the optimum power
setting for running optimum power control in recordable CD systems." US Patent
5,303,217, June 1989.
Bennett, Hugh. "Running OPC: the best thing for CD-R, but what about DVD?" EMedia
Professional, Sept. 1997.
Hajjar, Roger. "Running Optimum Power Control." Eastman Kodak Company
presentation to OSTA CD-R Physical Compatibility Subcommittee, Oct. 1, 1996.
Iimura, Shinichiro. Sony Corporation. "Method of optimally controlling the power of a
recording laser beam." US Patent 5,216,660, June 1, 1993.
Miyauchi, Toshimitsu and Yoshito Tsunoda. Hitachi Corporation. "Optical information
recording apparatus including error checking circuit." US Patent 4,308,612, Dec. 29,
Ogawa, Hiroshi. Sony Corporation. "Optical recording apparatus recording beam
controlled in response to reduced light level reflected between successively formed
pits." US Patent 5,309,424, May 1994.
MDC. European DVD Conference, Oct. 17-19, 1997, Berlin Germany.Running Optimum Power Control
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Absolute Time In Pregroove (ATIP)
The reflected light returned from the pregroove of a CD-R disc generates a carrier
signal providing tracking, motor control and focus signals. Additional information
including the Recommended Optimum Recording Power value is also encoded in a
frequency modulation of the carrier signal.
Absorption Control Warning
A warning message issued by a recorder when writing power exceeds a predetermined
safety threshold or if the mark formation signal returned from the disc being recorded
significantly deviates from the mark formation signature.
The difference between the lengths of marks and lands. Also known as Beta.
Mark formation signal
The signal created from light reflected back to the recorder's photodetector during the
Mark formation signature
The waveform of the optimum mark formation signal captured during the Optimum
Power Calibration procedure.
Optimum Power Calibration (OPC)
A process by which a recorder determines the best writing laser power setting for each
disc and recorder combination.
A slightly wobbled spiral groove molded into the substrate of a CD-R disc which
provides a guide for the writing laser as well as timing and other information critical to
the writing operation.
Power Calibration Area (PCA)
A special area of a CD-R disc reserved for use by a recorder in Optimum Power
Recommended Optimum Recording Power
An initial write power setting encoded in the Absolute Time In Pregroove information of
a CD-R disc which acts as a starting point for the Optimum Power Calibration
Running Optimum Power Control (Running OPC)
A technique used by recorders for monitoring and maintaining the accuracy of all the
mark and land lengths across a CD-R disc.
Running OPC Bandwidth
The speed at which a recorder can respond to changing writing conditions.