There are times when using a 3rd or a static interval will not sound good. For example a piece of music that has only the 1 4 and 5 chords, and without minor keys may be very good.

Harmony devices that use Midi such as the stuff from TC-Helicon will give you a broader better result. That is because you set Band in a Box to send it a chord, which it uses to make the harmony, thus if the chord is a minor the result will be a minor harmony. Imagine if you would trying to use a harmony system with 3rds but when you sing a G the harmony would be 2 below in 3rds you'd get a E and a C. But if you were singing a G and the chord was really a C minor you should be getting a G Eb and C. With band in a Box firing off chords to the harmonizer you get much better results.

That said most hardware harmonizers (like the one I have) come with settings for singing in each key, and different parameters. I find that it more or less works, but some odd results can happen on complex songs.

John Conley
Musica est vita