Blank CD-R discs contain special areas that are not present on a pressed (factory-made) CD. Among these are the ATIP (Absolute Time in Pre-Groove), containing such things as the block count (absolute recordable length), dye layer composition, manufacturer model number, supported recording speeds, and the manufacturer's recommendation for optimum laser power setting; the Program Memory Area (PMA); and the Power Calibration Area (PCA). The ATIP is located in the inner-most area of the disc, near the hub. The other areas are located nearby, in a reserved area before the lead-in. The beginning of the lead-in is addressed as time zero (00:00) -- the first accessible address on a disc. The PCA and PMA, because they're located before the lead-in area, have negative time addresses.
The PCA starts at -35 (-00:35) seconds and is used as a test area to determine the proper laser power for each recording session. Recording a CD-R disc is an involved process. In order for the writing to be done properly, so the final product can be interchangeable on any CD-ROM, CD-R or CD-RW multi-play drive, the microscopic marks (pits) 'burned' into the dye by the recording laser must be both the correct length and the correct distance from each other. Due to the variables involved, this can be a challenging job for drive mechanics. The characteristics of the dye on a disc may change with variations in ambient temperature and humidity, or there can be changes in dye thickness or variations in different dye batches, even from the same disc manufacturer. The deposition of the reflective coating may not be completely uniform. The sensitivity of the recording laser will vary over the surface of the disc and will also change as the disc and drive age. The surface of the disc may be microscopically uneven or the disc may be unbalanced and wobble, or the hub may be off center causing the disc to be a bit eccentric. The laser power needed to record a session last year may not be the same needed to add a session to the disc this year. Though rare, your disc may have been initially written on a completely different recorder, so the power needed for a previous session may not be sufficient for the current session.
Additional factors can affect the recording layer. The operating temperature of the drive, the selected recording speed in the software compared to the manufacturer's rated speed, and the stability of the recording laser can all require changes in the laser power needed to make a good disc. Because of this, the laser power and the wavelength are defined as a range of values, rather than a fixed number. Laser power can range between 3.6 and 8.8 milliwatts. Laser wavelength can vary between 775 and 795 nanometers.
A primary reason for the success of CD recording technology is Optimum Power Control, or OPC. You may see OPC also defined as Optimal Power Calibration - neither is wrong; the Orange Book actually uses both terms. This operation is performed prior to each writing session on the disc.
Optimum Power Control (OPC) is implemented in almost all recorders to adjust the power of the recording laser to the optimum level for the existing conditions when a write is about to begin. For a starting point, each "blank" disc manufactured has a reference value in the ATIP for the appropriate laser power range recommended by the vendor. The recorder reads the value off the disc and then uses it to calculate the best laser power range for the recording session it is about to perform.
Here's how it works: The PCA's Test area has 100 numbered partitions, each 15 frames long. These frames can be recorded with uniform samples of equal numbers of ones and zeroes, in fixed-time intervals with each frame recorded at a different laser power. If the laser reads the optimum power value from the disc as 6 milliwatts, it will then record in the Test area using a power range between 4 milliwatts and 8 milliwatts.
The drive uses the 15 frames of each Test area partition to write 15 'blocks' of data at 15 different laser power settings, seven stages above the optimum value, and seven stages below. For a recommended optimum power of 5.9mW (read from the blank disc) for example, the 15 test recordings would be at 4.1, 4.4, 4.6, 4.9, 5.1, 5.4, 5.6, 5.9, 6.2, 6.4, 6.7, 6.9, 7.2, 7.4, and 7.7mW. The sample recordings are read back by the lower-power reading laser, at about 0.5milliwatts, and are then compared for reflectivity. If the laser power is too low, the marks on the disc will be too small, and the reflected light will be of a high intensity. If the laser power is too high, the marks on the disc will be too big, and the reflected light will be of a low intensity. If the laser power is just right, the recorded marks and the areas between them will be of equal length; the light intensity will be correct.
The PCA's Count area also has 100 (tiny) numbered partitions. Each partition is one frame long and corresponds to a partition in the Test area. After the power calibration is performed in the Test area, one of these Count area frames is recorded with random data at the optimum laser power level. The recorder keeps count of the number of frames that are not written so it knows where to perform the next test operation. CD recorders can "memorize" the signature of an optimally written mark written to, and read back from, the Count area. Plextor drives incorporate a "write strategy" to enhance OPC. This write strategy utilizes a firmware-based catalog of factory tested and certified media. Random lots of media from a variety of vendors are tested under rigorous conditions to determine manufacturing quality. Those vendors who exhibit consistent quality are added to the catalog. The response of the tested media is measured, and the optimum starting value for the power range for best performance is added to the firmware settings. When the disc to be recorded is inserted, the vendor information in the ATIP is read and compared against the vendor information in the database. If the disc is on the supported list of media, the laser power is 'dialed' in using the established factory value, and then the drive performs OPC. By using the 'factory setting' as the starting point, and a narrower range of values to write the 15 frames, the laser is fine-tuned to ensure the best possible recording quality. Of course, if the disc vendor information is not listed in the catalog, the drive still performs OPC on the disc in the normal way.
The recorder begins the write using the initial optimum recording power, while constantly monitoring the written data, or signature, as it writes to the disc. The drive adjusts the write power to maintain the optimum signature. This feature is referred to as "Running OPC" or "Dynamic OPC". By using this "real-time" read back and on-going calibration, ROPC can correct for minute variations in dye thickness, fingerprints, dust, scratches, etc. When reading a disc in a player, error detection and correction, interleaving, or other methods of making the data correct are used to offset minor data problems, but in writing, you only have one opportunity to get it right.