Music Synthesizer for Android is a synth based on a very high quality emulation of the Yamaha DX7, plus a 4-pole resonant filter. It's an open source project as well, so please check out the project link for code and more.
There is also experimental USB MIDI keyboard support, for devices that support USB host mode.
This project was featured in the Google I/O 2013 talk, "High Performance Audio for Android", as an example of how to accomplish low latency, reliable audio playback.
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