"ArtooSynth" is
a synthesizer can be played with one finger.

Sound like "Astromech Droid Voice" "R2-..." though.
To the point that you have touch, you can display the colored circle. enjoy! :)

Futures

- You can play with one finger by touch screen.
- You move your finger on the screen to change the pitch.
- You are 5 color buttons at the top of the screen, you can choose the color of the circle of the touch point.
- When you shake the iPhone, it changes to the mode continuously changing color or size.