Hurrah for LADSPA.
I started (yet another) project, this time it's a sort of audio synthesisizer and sequencer library. I've always been fascinated with digital audio, since there are so many cool things we can do with it! Like e.g. create and apply filters, warp the sound waves any way we want, add cool effects, and generally do things that I can't otherwise do. Not to mention that I like music... but who doesn't? :-)
My program isn't revolutionary in any way; in fact, the short story is that the program tries to "simulate" the way real, analogue audio components can be put together, as e.g. an effect rack (the guitar connects into a distortion module, which connects into a reverb module, which connects to the amplifier, which connects to a mixer board, which connects to the speakers, etc.). I stole most of the concepts from an existing non-free (but very nice) program called Reason [http://www.propellerheads.se/]. I'm sure there are also free programs that do what I'm trying to do (but part of the fun is doing it myself).
One really nice thing is that there exist a lot of free plugins that do most of the work when it comes to synthesizing instruments and applying effects. These plugins are implemented using a free library, called LADSPA (Linux Audio Developer's Simple Plugin API) [http://www.ladspa.org/], which is the interface between host programs (such as the one I'm making, but also others, like Audacity) and the plugins.
So the plugins form a directed, acyclic graph, where the nodes with no outgoing edges are the speakers (or, well, sound card), and the leaf nodes are typically synthesizers (instruments). Arranging the plugins in this way is a really nice thing, because it ensures that there are no vicious feedback cycles (if we really want some kind of feedback, like reverb, we can implement it as a plugin!) It also means that we can do a topological sort to find the order in which the plugins are to be "run".
Anyway, I've implemented these plugins for my program: A LADSPA plugin wrapper, so we can load any LADSPA plugin as a plugin for my program. An ALSA playback plugin for playing audio in real time. A WAV playback plugin for streaming to disk using libsndfile, which supports a few more formats beside WAV, by the way. A MIDI input sequencer, which reads MIDI files and presents the notes as "control output" to the synthesizer plugins. A mixer plugin, which simply combines several inputs into one output.
It would probably be possible to create an ALSA capture plugin for capturing audio in real time as well. That would be pretty cool, and we'd have a software guitar amp in no time. There seems to be a problem with using blocking ALSA streams, though, because the application sleeps while the sound card is playing the data, and we're left with almost no CPU-time to actually synthesize the sound. This can probably be solved using threads, though.
Anyway, I made something cool (I think, anyway), by hooking the CMT organ [http://www.ladspa.org/cmt/] to a plate reverb [http://plugin.org.uk/], and feeding some classical pieces into the MIDI sequencer (which now supports polyphonic songs!):
Will probably work some more on this, try to write some plugins (I have a few ideas), actually implement error handling (oops), and try to add the missing fundamental features (like reading the correct MIDI tempo, stop output at the end of the MIDI file, etc.).
One of the goals of the library is to be small and efficient, and not accept inferior solutions because they're easier to implement (some trade-offs are allowed). The next major feature will probably be to add multi-core support using pthreads, so that all CPU cores can be utilized.
The code itself can be found at github (as usual): http://github.com/vegard/trick2