Chroma Feature Analysis and Synthesis: "Chroma features are an interesting and powerful representation for music audio in which the entire spectrum is projected onto 12 bins representing the 12 distinct semitones (or chroma) of the musical octave. Since, in music, notes exactly one octave apart are perceived as particularly similar, knowing the distribution of chroma even without the absolute frequency (i.e. the original octave) can give useful musical information about the audio -- and may even reveal perceived musical similarity that is not apparent in the original spectra.
We provide several routines for calculating chroma representations from audio, as well as functions to do the reverse -- to synthesize audio from a chroma representation. Although mapping spectra to chroma loses information (since all octaves are mapped onto one another), it is still interesting to convert chroma back into an audio signal to hear a representation of what information has been preserved. We use 'Shepard Tones', which consist of a mixture of all sinusoids carrying a particular chroma, for resynthesis."
