The value that is closest to 0.04 is best, and the power level that produced that result in the PCA is designated to be the optimum recording power. This procedure is known as Optimum Power Calibration, or OPC, and the optimum power level is then used to record the disc. This is necessary because a subsequent recording could take place in a different drive, or the recording properties of the original writer might have drifted. OPC for the first writer was relatively simple. Recording was always conducted at 1X. Discs used a Taiyo Yuden cyanine dye for which the nominal recording power was 6.5 mW, and only a long write strategy was required.
Today there are many different dyes and recording speeds.
Each has a different nominal recording power upon which OPC is centered, and these vary nonlinearly with recording speed. Various dyes may require different write strategies that predistort the write waveform in order to achieve the correct read waveform. This diversity would present a difficult matrix for OPC if special information were not embedded into the ATIP bytes located in the lead-in area of each unrecorded disc. It is the responsibility of the disc manufacturer to embed a write power value upon which OPC can be centered, usually between 4 mW and 8 mW. Lowest and highest usable recording speeds, usually 1X and 4X, for the media are also embedded. Wider beta tolerances may be allowed at 4X, for which the manufacturer optionally specifies one of three alternative write strategies and either a low or a high beta category.