It’s hard to deny how far CD recorders have come in just a few years, but there are still particular features that set superior machines apart from the rest. The most notable of these is Running Optimum Power Control (OPC).
Running OPC: The Best Thing for CD-R, but What About DVD?
EMedia Professional, September 1997
It’s hard to deny how far CD recorders have come in just a few years, but there are still particular features that set superior machines apart from the rest. The most notable of these is Running Optimum Power Control (OPC). A recommendation but not a requirement of the CD-R and CD-RW standards, Running OPC has been embraced by a growing number of manufacturers convinced of its value. There are, however, a few determined hold-outs such as Pioneer.
Standardized terminology is something not easily arrived at in a free market, and the varied nomenclature Running OPC has enjoyed makes it no exception. Some hardware manufacturers like Mitsumi call it “Dynamic Power Control” while others like Kodak in their PCD Writer 600 series of recorders refer to it as “Direct Read During Write (DRDW).” While there may be critical differences in execution and the robustness of the final result, all essentially refer to the same process.
Here’s how it works: Because the correct amount of laser power needed to write a CD-R disc is variable and depends on both the disc and the recorder, all recorders can perform an initial Optimum Power Control (OPC) procedure before starting to determine the best writing laser power for the particular recorder/ media combination.
The Running OPC process begins with the recorder retrieving an Initial Recommended Optimum Recording Power value 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 on the disc called the Program Calibration Area (PCA).
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 percent (suggested in the specification) though some units now have multiple target betas and write strategies. The recorder then determines what setting achieved the +4 percent beta target and establishes that as the recording power for the disc.
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 percent beta, the recorder retrieves the reflected signal that is associated with it and establishes a mark formation signature. 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.
While Running OPC helps detect potential data errors and improves and accelerates data integrity verification, it does not provide immediate read-back verification of the data after it is written. This capability remains the province of high-end industrial mastering systems which offer Direct Read After Write (DRAW) by using a second laser beam trailing the writing laser to determine if the correct data has been recorded on the disc. Essentially, Running OPC tells you if correct mark formation is taking place, but not necessarily if the right information is being written. Typically, CD recorders can be set up and initially checked for data integrity, and unless the configuration changes, there should not be any corruption problems, save some type of component failure.
Running OPC allows a CD recorder to compensate actively for non-uniform variations across a disc. Without this feature, for example, fingerprints, dust, and scratches accrued in handling — inevitable in multisession or packet writing — reduce a recorder’s laser’s effectiveness by obscuring its writing beam from the recording layer.
During recording with Running OPC, if the system encounters “fly specs” on the disc, the light reflected back is monitored and the recorder works to boost the amount of laser power to bring the reflected signal back up in line with the ideal mark formation signature established at the outset. This capability greatly reduces the possibility of an uncorrectable error resulting on the disc. If an issue can’t be dealt with, an Absorption Control warning is generated to alert that there may be a problem on that particular region of the disc.
OPC, OFF AND RUNNING: DESTINATIONS KNOWN AND UNKNOWN
It’s easy to see that Running OPC is good for the consumer in that it creates a more reliable recording system with superior data integrity. The feature does increase hardware costs somewhat, but in the end offers manufacturers a net savings because it helps increase product tolerances. Running OPC can compensate for changes in recording spot quality, wavelength shifts, tilt, defocus, and detracking, and proves a boon for media companies as well by helping offset sensitivity fluctuations over a disc and dye conformality, as well as defects like dye bubbles, comets, and contamination. Quite simply, costs are reduced any time a system is made more tolerant.
Given Running OPC’s attractiveness, one would naturally expect it to be a requirement for DVD. Rumor has it that the standard-setting DVD Forum has finally seen the light and recommended the use of OPC for DVD-R. Unfortunately, the same can’t be said for the specification governing DVD-RAM, which doesn’t even mention running OPC.
The DVD format family, because of its smaller mark size and track pitch, is by its very nature more susceptible to the same problems that affect CD-R. DVD-R is very similar to CD-R and Running OPC would offer similar benefits. And DVD-RAM is an even more demanding situation. For example, single sided discs are designed to be removed from their cases and later re-inserted if so desired. Whether a disc can be rewritten after re-insertion into its case is a decision left to the individual manufacturer. Without Running OPC, writing again and again to such a disc would be nothing short of insanity. But Running OPC is not even a footnote in the DVD-RAM book. It just doesn’t make sense.
In all fairness to the companies involved in the development of DVD-RAM (such as Pioneer, Hitachi, Toshiba, and Matsushita) many just don’t have experience with Running OPC. Of course, this doesn’t mean they aren’t free to implement it on their own, but if it were required in the specification, tolerances could be eased at the design stage which would reduce manufacturing costs while giving users added protection and increasing data integrity. By including a requirement for Running OPC in all the DVD books, consumers and the entire industry will be rewarded.
ABOUT THE AUTHOR
Hugh Bennett, editor-in-chief of Hugh’s News, is president of Forget Me Not Information Systems, a reseller, systems integrator and industry consultant based in London, Ontario, Canada. Hugh is author of The Authoritative Blu-ray Disc (BD) FAQ and The Authoritative HD DVD FAQ, available on Hugh’s News, as well as Understanding Recordable & Rewritable DVD and Understanding CD-R & CD-RW, published by the Optical Storage Technology Association (OSTA).
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