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 A Blank is not a Blank

 

No - this is not a trick. The fact is that although a blank has nothing written on the medium, it does contain information - information which can be very important to its use.

A brief recap on the construction of a writable disc. From bottom to top, the three layers are a polycarbonate plastic; a metal; and a lacquer (plastic) top coat which may be overcoated or printed. A write-once blank has a dye in the polycarbonate; the dye changes its infrared transmissivity when written with a high-power laser. An erasable uses an alloy for the metal layer; that alloy changes state between crystalline and amorphous when written, thereby changing its reflectivity.

That polycarbonate slice of the sandwich has a spiral groove pressed into the plastic with a stamper. The laser beam is servoed to stay in the groove as it writes. The tighter the spiral, the greater the total length along the spiral and the longer the disc can record and play. Unfortunately, a tighter groove is harder to follow accurately, so both writing and reading an 80-minute blank are more demanding than the same operations on a 74.

The spiral is not plain; it is modulated with a wobble to give the laser tick marks to locate its place along the spiral. That regular modulation or jitter is further modulated with digital information in what's called the ATIP. Specifically, information in the ATIP includes

  • Manufacturer

  • Writable/Rewritable
  • Dye type
  • Spiral length in blocks
  • Rated speed
  • Audio

Since the length of the spiral is pressed by the same stamper which encodes the ATIP, that information must be correct. Everything else is true or not depending on whether the stamper is used by the manufacturer who had it made to press the intended medium. Ideally, the stamper is tuned to exactly the material of the blank - but there's no guarantee of that or of the manufacturer of the blank being the one who had the stamper made.

Only a writer can read the ATIP because only a writer cares about what's in it. If the disc is so badly corrupted that it cannot stabilize in the drive, the ATIP cannot be read, so the writer doesn't even know whether the disc is erasable. Obviously, competent software won't attempt to erase a write-once disc, so that's one way to kill a rewritable medium. Dye type is of little practical interest; whatever is encoded there is overruled by the result of power calibration when the burn begins.

Rated speed is important when specified. If there is no specification, an erasable is always written at 2x; otherwise it may be rated for 4x, so erasable discs not coded to permit 4x writing will not allow it. Similarly, only discs encoded for "audio" can be written in the standalone writers (otherwise, they are identical with that manufacturer's conventional blanks). It is not clear what happens when a standard, write-once disc without a speed specification is to be written; at least in general, it appears that whatever you try will be attempted, but that does not mean that it will work.

Finally, we come back to groove length. The minimum inner and maximum outer radii for a CD are specified in the standard. The manufacturer's rated number of blocks in the ATIP is the amount that can be written in that space, allowing for the runout (leadout) track. In practice, the spiral goes beyond the maximum radius, so there is more room than the manufacturer allows - but it is room at the expense of the design maximum of travel on the writer and the reader.

Writing beyond the rated number of blocks is called overburning. It is a somewhat risky operation for reading and for writing. If the disc is fully written with overburning, then the runout track goes beyond the maximum radius. Then the reader, which needs the runout track for operation, may not be able to read it and may be unable to sync on the disc. That's one way that a disc can work in one reader but fail in another. At least theoretically, it's possible for overburning to damage a writer by forcing it to travel farther than its design permits. In short, overburning is risky; it can pay off in some cases, but if you use it, you're on your own.

 


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