Running Optimum Power Control
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.
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.
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, the PCA is where the OPC test is performed to find the optimum laser power setting for the writing laser and write strategy.
“Running OPC” is another technique to monitor and maintain write quality throughout the recording session.
After writing the test marks at the different laser powers the recorder reads them back and looks for differences between the lengths of marks and lands.
Asymmetry - the difference between the lengths of marks and lands. Also known as Beta
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 Book 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
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.