Jackfft is a project mainly written in C++, based on the GPL-3.0 license.
OpenGL-accelerated spectrum viewer for JACK Audio Connection Kit
jackfft
jackfft is a JACK client doing short-time spectrum analysis.
It displays colored spectrum graph. Both axes are logarithmic. The lower sound frequency, the longer color's wave length. (can be reversed by changing one line in code) More, the hue and brightness also depends on frequency energy (Y value). Assuming 44.1kHz sampling rate, X axe is from about 20Hz to 22050Hz. Y axe is from -42dB to +6dB.
Ignoring memory copying, caching, interfacing JACK, drawing it on the screen etc... jackfft grabs sample from JACK, then passes it through Hann window function. The windowed signal is processed with DCT-I (Discrete Cosine Transform - I). The frequency-domain signal is scaled logarithmically and displayed.
Seriously, as an exercise (in JACK and FFTW) before writing some dynamics procesor working in frequency domain. You know how modern records are overcompressed; dynamics procesor I want to write will probably be multi-band expander bringing dynamics back to music. See also http://en.wikipedia.org/wiki/Loudness_war
jackfft is not another music visualization toy. Unlike regular music player, it calculates high-resolution spectrum with logarithmic X axe. This allows you to see tone changes, harmonics etc. pretty accurately. (for example I was able to precisely see vocal in Soundgarden's 'Ugly Truth') Hardware-accelerated display (no FFT yet, sorry...) saves CPU even if you run jackfft on full screen.
You will need following packages in Debian (paste it after aptitude install):
build-essential libc6-dev libjack-jackd2-dev libfftw3-dev libsdl1.2-dev libgl1-mesa-dev libglu1-mesa-dev
After installing them read LICENSE (I don't want to be sued for jackfft burning your GPU etc...) and
$ make run
Connect it to some source using qjackctl or similar software and enjoy!
Happy hacking!