This is a SDL2-based implementation of an audio synthesizer.

Available interaction control-keys:

 * <ESC> - quits program
 * <F1> - use plain sine-wave osc
 * <F2> - use square-wave osc
 * <F3> - use sawtooth-wave osc
 * <F4> - use some combo-wave osc
 * <F5> - use noise osc
 * <F7> - toggle time-/frequency-domain display
 * +/- - change volume in rough chunks

What does it sound/look like:

 * https://www.youtube.com/watch?v=ae0erYfJn_k

How to use an attached MIDI-keyboard:

 * use alsa-command amidi to determine MIDI-device
 * amidi -l (e.g. reports hw:2,0,0)
 * cd build
 * ./software-synthesizer hw:2,0,0

I tried it successfully with these MIDI-keyboards:

 * Arturia MiniLab MkII
 * Roland RD-88 (actually a full-blown e-piano)
 * StudioLogic SL88 Studio

If you do not have a MIDI-keyboard, you can use the regular keyboard-keys
of you computer. I roughly replicated the layout of a single octave on a
piano-keyboard like so:

    s d   g h j 
   y x c v b n m ,

   C# D#   F# G# A#
  C  D  E F  G  A  B C

This is for a regular German qwertz-layout keyboard. The y and , are probably
mapped differently, if you use other keyboard-mappings. This is meant to be
used with a proper velocity-sensitive MIDI-keyboard. The keyboard is just a
debugging feature for me. You can at least play chords that way too :)

BTW, if you happen to have an Arturia MiniLab MkII MIDI-keyboard, it will
show a small "disco"-like color-cycling effect on the drum-pads. That is
a side-effect of learning about MIDI-standard/SysEx-commands.

Bugs... yes there are some... most I am aware of. This is a playground project,
not something meant for hassle-free general consumption.

The build-dependencies are modest:

 * cmake 2.8.12
 * C++-14 compiler (e.g. g++, clang++)
 * SDL 2.0.5
 * OpenGL 3.x

It compiles and runs under recent Linux-distributions (e.g. Ubuntu 16.04). It
should run with little or no changes under MacOS X and Windows 7 (or more 
recent) too. But that has not yet been tested. Feedback and patches are
welcome.