RFC: support for non-western systems

Description

This issue popped up during poster discussions at ismir2017 (tagging @vrangasayee): librosa’s support for non-western (eg, Indian) scales is currently non-existent, but it would be easy to add.

Things that would be useful to have and trivial to implement

• Unit conversion functions: hz_to_*, note_to_*, etc
• Visualization decorations, specshow(y_axis='...')

More fundamentally, it would be nice to have support built in for chroma, and if possible, down to cqt basis filters. The latter might take some refactoring, but if it works out, we should also be able to support non-equal-temperament scales. (Consider these as stretch goals.)

[vrangasayee]

srutis.pdf Uploaded to github. This should work.

[kaustuvkanti]

I think hz_to_svara is the safest approach, as there are a considerable number of cases (not so much in the Dunya CC collection but overall) where the tuning deviates from the A-440 standard. However, note_to_svara can easily be adopted and octave is less important.

The functions …_h and …_c are indispensable, the definition of svara is very different in the two sub-genres. My idea of bringing in the histogram discussion was to learn the precise intonations by profiling the peaks. But for the case of Hindustani, assuming equal temperament is reasonable. The octave subdivisions are 12 in both, the nomenclature differs. The Carnatic system (left subfigure) uses 16 names for the 12-tet based on the raga context.

The idea of chroma display sounds fantastic, I totally second your approach.