This is a proof of concept demo of using an iPhone to directly control a modular synthesizer, using the Expert Sleepers Silent Way AC Encoder technique.

In a nutshell, Expert Sleepers has come up with a way to let use any audio device to generate control voltages that can be used by analog synths.

via expertsleepers:

The original idea of this was to allow non-DC-coupled audio interfaces to be used with CV generation software such as Silent Way. Taken to its logical conclusion though, it provides a method to get CVs out of any audio device.Silent Way is a suite of VST & AU plug-ins for controlling analogue synths directly from a DAW. See http://www.expert-sleepers.co.uk/silentway.html for more details.