S2L - Sound 2 Light

NIF and Membrane Element for turning sound samples into RGB values suited for reactivity to music.

A C compiler, make, tar, and curl or wget. There is nothing to install first — aubio is downloaded, checksummed and compiled from source as part of mix compile, then linked statically into the NIF.

If available in Hex, the package can be installed by adding s2l to your list of dependencies in mix.exs:

def deps do
  [
    {:s2l, "~> 0.1.0"}
  ]
end

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/s2l.

Feed the analyzer one hop of f32le mono at a time, in order:

{:ok, analyzer} = S2l.Aubio.create(44_100)

{:ok, analysis} = S2l.Aubio.process(analyzer, frame)
#=> %{beat: false, bpm: 120.1, confidence: 1.4, onset: true, bands: [...]}

S2l.Aubio.frame_size/1 gives the exact byte size each call expects. Pass the stream's real sample rate to create/2; analyzing 48 kHz audio as 44.1 kHz reports a tempo about 9% off rather than failing.

Two modules turn that into colour, neither of which depends on a framework, so both carry to a device unchanged:

• S2l.ColorMapper — smooths analysis into paced frames carrying level, hue, spectral centroid, dominant pitch, per-band levels, peak-hold caps and a history buffer for waterfall displays.
• S2l.Palette — position in, RGB out, with built-in palettes and support for your own.
• S2l.Strip — fits a frame onto however many LEDs you have, and turns it into a list of {r, g, b}. Band count and LED count are unrelated, so something has to interpolate going up and aggregate going down; this is that something.

S2l.Membrane.Analyzer is the Membrane sink that drives the whole thing from an audio stream. See examples/ for both wired up.

The NIF is built with elixir_make: mix compile runs the Makefile, which fetches the pinned aubio release, verifies its SHA-256, compiles it into a static archive and links that into _build/$MIX_ENV/lib/s2l/priv. Nothing is generated, so c_src/s2l_aubio_nif.c is the whole binding. The resulting .so depends only on libc and libm — there is no libaubio to install or ship.

Only aubio's DSP sources are compiled. Its io/ layer (libsndfile, ffmpeg, CoreAudio) and synth/sampler are excluded, and no external FFT backend is enabled, so aubio uses its bundled ooura implementation. aubio's own waf build is bypassed entirely — the HAVE_* macros it would write into a config.h are passed on the command line instead, which is what removes Python from the build requirements.

Because aubio is built from source with the same compiler as the NIF, a cross-build is just a matter of pointing the toolchain at it. The Makefile honours CC, AR, CFLAGS, LDFLAGS and CROSS_COMPILE, and resolves CC and AR from CROSS_COMPILE when the caller has not set them.

For builds without network access, supply the source directly:

AUBIO_TARBALL=/path/to/aubio-0.4.9.tar.bz2 mix compile   # local tarball
AUBIO_SOURCE_DIR=/path/to/aubio-0.4.9 mix compile        # extracted tree

The download is cached under _build, so it happens once. Two mirrors are tried in order, each pinned to its own checksum.

make verify builds and runs c_src/verify.c, a standalone C program that drives the same aubio call sequence over a synthesized 120 BPM click track and exits non-zero if the analysis misses its thresholds.

It shares those thresholds, and its exact input signal, with the ExUnit anchor test in test/s2l/aubio_test.exs. When both agree the binding is faithful; when make verify passes and mix test does not, the fault is in the Elixir/C glue rather than the audio analysis.

GPL-3.0-or-later, in full in LICENSE.md.

This follows from aubio, which is GPL-3.0-or-later and is compiled into the NIF as described above. Anything built on this library inherits the same terms.