- 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.
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
- Dye type
- Spiral length
- Rated speed
Since the length
of the spiral is pressed by the same stamper which encodes the ATIP,
that information must
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.