Patent attributes
An optical disk read only memory having an information recording surface with a concave-convex pattern formed on the basis of a first signal and a reflective film covering the surface. The disc medium includes: an additional information recording section formed on a recording track of a first data string; and a second data string including a mark of removing or reducing the reflective film. The modulation methods of the first and the second data strings are identical, and Lh(n)>Lp1(n) and Wh(n)>(Wp1(n)+Wp2(n))/2 are satisfied, where a reference clock cycle is T, a data length is nT, a length and a width of the mark are Lh(n) and Wh(n), a length and a width of a base of the pit are Lp1(n), and Wp1(n), a length and a width of a top face of the pit is Lp2(n) exceeding Lp1(n), and Wp2(n) exceeding Wp1(n).
