Informatique musicale : créer un séquenceur pas-à-pas avec Python

Pycon-Fr 2016, Rennes

Aujourd’hui, que ce soit via « Maschine » de Native Instruments, « Push » de Ableton, ou encore via des machines à l’esprit plus vintage comme le « Dark Time » de Doepfer ou des émulations de vieilles boites à rythmes, les musiciens retrouvent le goût du séquencement pas à pas ou « Step Sequencing ».
Le bidouilleur que je suis avait envie de détourner son matériel pour reproduire ce type d’outil de création…

Après une introduction au MIDI et aux principes d’un séquenceur, nous verrons comment nous pouvons, avec Python et la librairie mido, « hacker » un certain synthétiseur hardware pour en faire un séquenceur pas à pas, à l’aide d’une boucle d’événements. Nous essaierons même de l’interfacer avec Ableton Live pour contrôler non plus les sons du synthétiseur, mais des samples !

Vidéo disponible ici : https://www.pycon.fr/2016/videos/informatique-musicale-creer-un-sequenceur-pas-a-pas-avec-python.html

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Nowadays, using Native Instruments "Maschine", Ableton "Push", or machines with a vintage heritage such as Doepfer "Dark Time", musicians enjoy getting back to a special kind of sequencing which is step sequencing.
I wanted to hack with my current hardware to try and do the same as these tools.

After an introduction to MIDI and notions of sequencing, we will see how, thanks to Python and the Mido library, we can hack hardware to turn it into a step sequencer, using an event loop. We will try and interface our program with Ableton Live to control not only sounds from the synthetizer, but also trigger samples!

Video (in French) available @ https://www.pycon.fr/2016/videos/informatique-musicale-creer-un-sequenceur-pas-a-pas-avec-python.html

Transcript

1. BUILD A STEP SEQUENCER
   IN PYTHON
   Pycon-Fr, Rennes, 2016
   

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2. WHO AM I?
   Yann Gravrand (@ygravrand)
   Tech lead
   Musicos
   

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3. PART 1: BACKGROUND
   Musical instruments
   Synthetizers and samplers
   Sequencers
   Step sequencers
   

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4. MUSICAL INSTRUMENTS
   Can be played by humans
   Some can be "played" by computers:
   Synthetizers
   Samplers
   ...
   

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5. SYNTHETIZERS
   Sound generators
   Generally, a lot of parameters can be tweaked
   

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6. FAMOUS SYNTHETIZERS
   Minimoog (analog) : 70s
   DX7 (digital) : 80s
   

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7. FAMOUS SYNTHETIZERS
   Nord Lead
   (analog modeling)
   Mininova
   (analog modeling)
   

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8. VST
   VST Plugins
   

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9. SAMPLERS
   Do not generate sounds themselves
   Play samples (little chunks of sound)
   

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10. SAMPLES / NOTES:
    One sample for the whole keyboard (pitch adjusted or
    not)
    

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11. One sample for each note
    

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12. One sample for a group of notes, pitch is ajusted
    

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13. DRUM MACHINES?
    Sound generator + step sequencer
    TR 909
    Tempest
    

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14. SEQUENCERS
    Play a sequence of notes
    Several tracks, instruments...
    

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15. STEP SEQUENCER
    A 4/4 measure is divided into:
    4 quarter notes
    Each quarter note is divided into 4 steps --> A sequence
    is usually 16 steps long
    

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16. STEP SEQUENCER
    Example: on the TR 808
    

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17. STEP SEQUENCER
    For each step, we define:
    the note / pitch
    other attributes: length...
    ... and activate it or not
    

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18. EXAMPLES
    Daft punk - Aerodynamic @ 1:03
    4 * 16-step patterns
    

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19. EXAMPLES
    Daft punk - Aerodynamic @ 2:28
    4 * 16-step patterns, some notes off
    

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20. USING A STEP SEQUENCER
    "Live" mode: turn steps on and off in real time, adjust
    pitch, length...
    "Step by step" mode: for each step, define the note
    attributes. No timing, no rush
    

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21. PART 2: THE PROJECT
    Project goals
    MIDI
    Using mido
    The Dirty Part: blocking, threads, asyncio...
    

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22. I HAD
    A cool synth
    Colorful (and empty) pads
    

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23. AND
    A snake
    

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24. PROJECT GOALS
    Make the synthetizer play notes using Python
    Modify and turn notes on / off to create a sequence
    Implement "step by step" and "live" modes
    Change tempo in real time
    Make interactions possible with any controller...
    ... Starting with mine, of course :)
    

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25. MIDI: MUSICAL INSTRUMENT DIGITAL INTERFACE
    Extremely old standard: 1983!
    Still largely in use today
    To synchronize and communicate between devices
    Message types:
    Notes (NOTE ON, NOTE OFF)
    Control Change (Ex: Filter resonance, Hold pedal...)
    Program Change (Change instrument)
    Sys ex
    ...
    

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26. WE WILL NEED TO SPEAK MIDI WITH DEVICES
    Midi input: pads pressed, keys pressed, knobs turned...
    Midi output: play a note, turn a LED on...
    

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27. MIDI INPUT: RECEIVING MESSAGES
    Message reception blocks
    So if we want to do something else in parallel, we have to
    handle this in a thread or coroutine or...?
    inport = mido.open_input()
    msg = inport.receive() # Blocking call
    

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28. MIDI OUTPUT: PLAYING NOTES
    --> BEEEEEEEEEEEEEEEEEEEE...
    --> ... EEEP.
    To play notes, we need a timer between NOTE_ON and
    NOTE_OFF (note duration). time.sleep?
    import mido
    outport = mido.open_output()
    msg = mido.Message('note_on', note=100, velocity=3)
    outport.send(msg)
    outport.send(mido.Message('note_off', note=100))
    

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29. ALIGNING NOTES (STEPS) WITH TEMPO
    Naive implementation:
    Two problems:
    time.sleep also blocks, so we have to handle it in a thread
    or coroutine or...
    Waking up, sleeping for X seconds, waking up...: the
    tempo slowly drifts
    while True:
    outport.send(mido.Message(...))
    time.sleep(tempo.step_duration)
    

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30. A STORY ABOUT BLOCKING AND TIMING
    So how can we have:
    1. independent yet connected blocking loops?
    2. timing accuracy to keep a stable tempo?
    

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31. SOLUTIONS
    Threads
    Many queues to avoid shared state
    Coroutines with asyncio
    Everything in a single thread, less concurrency issues
    Ok since our app is I/O bound
    ...But we have to modify mido to insert
    yield from or await...
    Greenlets with gevent
    Monkey patches time.sleep
    so we can use mido as is and have greenlets
    Drifting? calculate absolute times
    

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32. PROPOSED DESIGN
    Main process is I/O bound
    Console process is CPU bound!
    

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33. PART 3: IMPLEMENTATION
    & DEMO
    System overview
    Implementing a controller
    Action!
    

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34. SYSTEM OVERVIEW
    

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35. IMPLEMENTING A CONTROLLER
    Map messages from controller (pad pressed) to
    sequencer actions (toggle step)
    Send messages to controller for feedback (LEDs...)
    

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36. INTERPRETING EVENTS
    Some events are represented by a single message
    Others are the result of a sequence of messages (ex:
    NPRN LSB, MSB)
    Solution: a RulesChain
    Each Rule matches a message
    A state automaton keeps track of the matched rules
    Flexible rules evaluation engine
    self.register('FILTER',
    self.on_cc,
    RulesChain(Rule(type_='control_change', control='74'),
    Rule(type_='control_change', control='27',
    value='0'))
    )
    

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37. REACTING TO SEQUENCER STATES
    Event system:
    self.sequencer.on(SequencerEvents.STEP_BEGIN, self, self.on_step_begin)
    ...
    def on_step_begin(self, step):
    # Turn on current step LED
    self.sequencer.output(self, *msb_lsb_output(60, 0, 32 + step.pos))
    

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38. IN ACTION!
    

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39. IN ACTION!
    Bass pattern
    Drum pattern 1
    Drum pattern 2
    Mozart pattern (32-step sequence)
    Daft punk - da funk
    

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40. FUTURE
    Plans:
    Release in Open Source
    Chords (especially important for a drum machine...)
    Multi track
    Load / save to midi
    External tempo sync
    Reactive Web interface (iPad, ...)
    Conquer the world!
    

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41. THANK YOU!
    @ygravrand
    Soon:
    github.com/ygravrand/steppy
    

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