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PseudoRandomRhythmSection

Dylan Leigh - August 2018

Generates a pseudorandom chord progression and piano/bass comp to go with it; outputs a score in MusicXML with all parts and chord symbols. Ideal for practicing improvisation or ideas for composition.

Installation/Requirements

Other than Python the only hard requirement is the Music21 library http://web.mit.edu/music21/ which is used for writing the MusicXML and performing chord inversions and similar operations.

A pip requirements file is provided, and it is recommended that Virtualenv or a similar system be used for a clean environment:

virtualenv prrs
cd prrs
. bin/activate
git clone https://github.com/dylanleigh/PseudoRandomRhythmSection.git
pip install -r requirements.txt

Usage

The file to save to must be specified as an argument. It can then be opened with any MusicXML software such as Musescore:

$ python prrs.py output.xml
$ musescore output.xml

A one-liner showing progress:

$ python prrs.py output.xml --show-symbols && musescore output.xml &

Warning: The output file will be clobbered if it already exists.

For full commandline options use --help.

Algorithms

(This may change as improvements are made)

To generate the chord progression, PRRS puts a V7 and Imaj7 at the end and then works backwards, making a weighted random choice between chords that tend to resolve to the "current" chord. For example:

I                 V     I
I           ii    V     I
I     vi    ii    V     I

Then the actual notes are generated working forwards. First a duration is chosen for the current chord, then for each instrument a function is called to generate the part for that instrument until the next chord change. Chords that are more cadentially significant are more likely to have a longer duration.

The bass part is just a basic walking bassline up and down the chord notes. The piano plays the current chord using a random inversion (weighted in favour of the root inversion) and random note length and rests (weighted to rest more on the beat to syncopate off the bassline). There is also a special function for each instrument to do a random closing riff at the end of the song; this is just a random rhythm that is increasingly likely to hold the note for longer.

TODO

  • Choose piano chord inversions to fit smoothly with ones before/after
  • Swing Beat set automatically without having to add it in musescore (not sure how we can do this with music21)
  • Make duration more likely to be odd if we are already off-beat to even it up
  • Drum kit (start with hihat at least)
  • More varied bassline - occasional quavers, runs, fills etc
  • More varied piano - some passing notes and skeletal chords etc
  • Other keys (defaults to C in output but roman notation used in code)
  • Improve performance, don't do unnecessary stuff with music21 objects

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Generate a MusicXML score with a random chord progression and piano-bass comp

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