Jane,
The internal harmonizers recognize chords on the left hand, the right hand, the whole keyboard, when you just sing they can be set for specific intervals or when you play midi files with vocal harmony tracks they just harmonize as arranged on the song. The ability to record chord changes into a sequencer as the digitech does I don't beleive can be done on any of the keyboard harmonizers. Actually I don't think i'm right. If what you are talking about is putting manually the chord changes for a song into a memory location in the digitech, I don't see why you can't record the chord changes only for a song in the keyboards sequencer in step or real time (depending on the keyboard), then go and mute all the volumes down if you don't want to hear any music play, and just sing into a mic and hear your harmony and voice track. I'm not sure about this, I will have to try it at my store. I don't even know if this is what you are asking about!
George Kaye