National Amusements, which owns most of the voting shares of the two companies, sent a letter to CBS and Viacom board members Thursday saying that a tie-up would help it better compete in a rapidly changing media industry. Viacom has been struggling with too-few hits from its movie studio Paramount Pictures and sagging ratings, while CBS has fared better in prime time with sporting events and popular shows like "The Big Bang Theory" and "The Good Wife." The move follows the settlement of a long running legal battle between longtime Viacom CEO Phillippe Dauman and Sumner Redstone's daughter Shari, now a Viacom director and president of National Amusements, over the fate of Redstone's media empire.