Hi, The answer to your question really depends on which way you want to convert the aspect ratio.  If you have a 16:9 video that you want to play well on a 4:3 screen, you need to use one approach.  If you have a 4:3 video that you want to play best on a 16:9 screen you use a different approach.  A little more detail would help get you the best answer to your question.