MIDImorphosis is a fast, accurate pitch-to-MIDI app, designed to work best with stringed instruments. You can plug an ordinary guitar into your Mac through an appropriate audio interface, and set MIDImorphosis to receive audio from there. The pitch of your guitar playing will be converted to MIDI, which can then be routed to synthesizers, sequencers, or DAWs such as Logic and Ableton.
The app features the innovative Spectral Eye display; it shows the frequencies present in the incoming audio signal, arranging them in a spiral that maps to musical pitch. You can see the actual tone of your guitar, and the harmonics, in a way that is not possible with a traditional FFT display. MIDImorphosis lets you easily transpose the MIDI up or down multiple octaves. If you want to play a quick bass line, simply play the notes you want in the upper register of your guitar, and transpose them down for a synthesizer.