1. A modulation system, comprising: an encoder, transferring data words to tentative code words; a DSV control bit generator, determining the value of a DSV control bit according to the data words or the tentative code words to optimize the cumulative DSVs corresponding tentative code words, wherein the DSV control bit generator determines the value of a current DSV control bit when at least a subsequent DSV control bit is detected; and a final code word generator, generating final code words according to the determined DSV control bit and the tentative code words.