Star rating in ipod touch gets erased
I synced an mp3 music from my library which doesnt have any star rating. Now when I rate this song in my iPod touch, it is only saved in the device itself. When I connect the ipod in iTunes and look for this song in the ipod (the Music icon under the ipod touch's name, not in the iTunes library), the star rating is gone. How can I save the star rating done in the iPod so that I can easily make playlists based on star rating? Thanks for anyone's response.
iPod touch (4th generation), Windows 7, iTunes 10.6.1 iOS5.1