Combine sugar and water in a saucepan and bring to a simmer over medium-high heat. Cook until sugar is completely dissolved, about 5 minutes. Remove from heat and cool. In a bowl combine the strained raspberry puree, lemon juice, and warm sugar syrup and mix thoroughly. Mix in the Chambord or creme de cassis, and refrigerate at least one hour or overnight. Stir the fruit mixture well, then freeze in an ice cream freezer according to the manufacturer's directions.