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Chip's CD Media Resource Center:
CD-R (Recordable) 3

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Pregroove, "Wobble" & ATIP

Robert Starrett has written an excellent description of how CD-Rs are manufactured which appears on the Roxio website. The thing that most surprised me was to learn that the spiral pregroove is made with a considerable "wobble" that is used to control the speed of the disc. Mr. Starrett writes

On a pressed disc, the timing information that is necessary to control the disc's rate of spin is included in each CD sector as data. But for a blank disc, the CD recorder must have some way to guide the recording laser and control the speed of the blank disc as it is being recorded. That is where the "wobble" in wobbled pregroove comes in. The wobbled pregroove provides tracking and timing information for the recording laser. The wobble is a slight sinusoidal wave that has an "excursion" of .03mm from the center of the track path... The wobble guides the recorder and provides timing information. This information is called Absolute Time In Pregroove (ATIP). It ensures that data is recorded at a constant rate. The resulting data track obliterates the wobbled pregroove, leaving recorded data in its place.

One important note: the wobble excursion is not ".03mm" (I've seen this incorrect figure repeated in numerous other sources). That would be 30 microns: ridiculously huge considering that the track pitch is only 1.6 microns! No, it is ".03um" or .03 microns, best expressed as 30nm. I think actually it's 30.5nm.

The basic pregroove "wobble" is a 22.05kHz sine wave. It is only 60 nm peak-to-peak and so has no impact on adjacent tracks (1600nm apart). The CD recorder phase-locks to this signal in order to control the speed of the disc: the recorder continuously adjusts its recording speed so that the pregroove frequency is 22.05kHz. Thus the nominal recording speed (1.2 to 1.4m/s) is fixed by the wave encoded in the pregroove.

But that's not all! This 22.05kHz carrier wave is frequency-modulated to encode other information as well. Remember the Absolute Time that is encoded in the q-channel on stamped CDs? It's what the A-MIN, A-SEC and A_FRAC fields are used for. Since there's no q-channel on a blank disc, that same data is encoded in the pregroove.

Besides the time code, there's considerably more information about the disc encoded in the pregroove, including "disc application codes", the locations of different areas on the disc, and hints about the optimum recording power to use.

I won't go into any details of the actual FM encoding scheme used in the pregroove, mostly because I don't know what they are! But it does include CRC error correction.

Disc Application Codes

In his article Mastering CD-Rs, Tape Disc Business November 1996, Richard Whitworth writes:

The disc application codes are used to distinguish between discs used for different applications. The two main application codes used are "Discs for Unrestricted Use" and "Discs for Restricted Use." Within the "Disc for Restricted Use" code, another additional encoded identification may be used for special disc applications. One example of this would be the Photo CD.

This is why, for example, you can't use blank data CD-Rs in a a consumer audio disc recorder. You must use an audio CD-R. The audio recorder will check to ensure that the blank CD is encoded for audio applications. The audio CD-R isn't any better or different, but will cost more because of copying fees paid to the RIAA. [uh, what was that source again?]

Power Control (OPC) & the Program Calibration Area (PCA)

CD-Rs include a region called the Program Calibration Area (PCA) where the recorder can calibrate the power level of the writing laser to the characteristics of the specific disc being used. This facilitates the use of various different dyes and reflective layers in the manufacture of CD-R blanks. The following is from the same Whitworth article:

The wavelength used in the recorder can range from 775 to 800 nm. The optimum recording power is encoded as an estimated power between 4 to 8 mW for a laser at 785 nm wavelength. This value depends both on the disc and recorder. According to the Orange Book Specifications, the main parameter involved regarding the disc is the sensitivity when the laser wavelength is changed. For the recorder, the two main parameters are dimensional and optical quality of the recording spot, and the actual wavelength used when recording. The wavelength of the laser depends on the type of semiconductor laser, the spread of its wavelength, and its operational temperature. The power calculated for the recorder/disc combination is called an Optimum Power Control (OPC) procedure. This calibration procedure is performed in a designated area on the disc called the Power Calibration Area (PCA). This procedure is performed by recording EFM at different powers within discrete ATIP time intervals. The most preferred power is achieved when both the high and low frequencies share the same level of asymmetry.

Last Updated Monday October 15, 2001 17:58:22 PDT