Background music for menu - MP3 properties

[T_Scheen]

Well, I am not really an mp3 expert but maybe someone knows the answer. I use various programs to rip the music from DVD-VOBs into wav-files. Then I use another program to convert that into mp3. Before encoding, that program asks me whether I want "Stereo" or "Joint Stereo". I don't know the difference.
When adding a background music in Ulead MF3.5 (mp3-format), I can see the properties of that file as well by clicking "Info". Some of my mp3s are "Stereo" and others are "Joint Stereo". Both work of course but does anybody know the difference ?

Thomas