Patent ID: 7616135

Claim:
An apparatus for modulating source words to codewords, the apparatus comprising: an optical pickup to emit a laser beam onto an optical storage medium to reproduce data recorded on the optical storage medium; and a modulation unit to modulate source words to codewords; wherein: a preceding codeword “A” and a following codeword “B” form a code stream “Y”; if the code stream “Y” does not satisfy a predetermined boundary condition, the modulation unit replaces the codeword “A” or the codeword “B” with a codeword “C 1 ” or a codeword “C 2 ”; if a number of “1s” in the codeword “C 1 ” is even, a number of “1s” in the codeword “C 2 ” is odd; and if a number of “1s” in the codeword “C 1 ” is odd, a number of “1s” in the codeword “C 2 ” is even.