Frequently Asked Questions About CD-R and CD-RW Discs

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Some new CD recorders offer "running OPC" that is advertised as a necessary feature. If this is true, how did older recorders work without it?

Running OPC, also known as continuous OPC, can be an advantage if it is used only to compensate for media problems. If it is used instead to compensate for deficiencies in the recorder, then such a system may be inferior. Prices of CD-R writers have dropped far and fast. The first recorders were priced in the tens of thousands of dollars range. Competition forced prices down to thousands of dollars, then to hundreds of dollars. Diminishing quality has been evident, as volume and technology supported some, but not all, of the price decreases.

Since CD-R discs from different manufacturers or various lots may each have unique recording characteristics, every disc is required to have a special 22.5 second long Power Calibration Area, or PCA, for Optimum Power Calibration, or OPC. This region is located before lead-in, and cannot be accessed by a CD-ROM drive. One hundred partitions are located in this region, and one partition is accessed by a CD-R writer before each recording session.

The CD-R drive typically records fifteen tests in one PCA partition, writing one test at nominal power, seven below that level, and seven above. All tests are then read, and the output from the photodetector is analyzed to obtain fifteen values of a property known as beta. 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. If another logical track or session is recorded at a later time, the OPC procedure is repeated in a different partition in the PCA. 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.

Continuous OPC, also called running OPC, extends OPC to the entire recording session. OPC is first performed in the PCA, and that optimum power level is utilized when recording begins. During subsequent recording, a signature of the reflected high intensity recording beam is continuously compared to the optimum signature that occurred at the optimum power level during OPC. Laser intensity is then adjusted in real time during the recording process to maintain that optimum signature. This should compensate for any variations during recording and result in uniform quality across the disc.

Continuous OPC is desirable because spin coating is used to apply the dye layer during manufacture of the CD-R disc. This can cause characteristics of the dye layer to change with radius. Dye properties can also drift with temperature if the disc heats during recording. Temperature and other effects can alter the recorder optics, causing the writer to drift during a recording. Slight shifts in laser wavelength will significantly alter recording quality. Small changes in the shape or size of the focussed laser spot also affect quality.

If all other factors remained equal, then running OPC would compensate for variations across a disc and significantly benefit quality. Unfortunately, this is not always true. Tests using certain older drives without running OPC show uniformly high quality from ID to OD when media and drive quality is high and the system is properly cooled. Tests on newer systems using running OPC indicate that quality can degrade when drive optics are of inferior quality. No single feature such as running OPC can determine the quality of a CD-R writer. Only proper testing will identify superior products that will consistently produce high quality recordings.