Create cpx-basic-synth.py
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Mike Barela
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Circuit_Playground_Express_USB_MIDI/cpx-basic-synth.py
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Circuit_Playground_Express_USB_MIDI/cpx-basic-synth.py
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### cpx-basic-synth v1.4
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### CircuitPython (on CPX) synth module using internal speaker
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### Velocity sensitive monophonic synth
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### with crude amplitude modulation (cc1) and choppy pitch bend
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### Tested with CPX and CircuitPython and 4.0.0-beta.7
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### Needs recent adafruit_midi module
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### copy this file to CPX as code.py
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### MIT License
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### Copyright (c) 2019 Kevin J. Walters
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### Permission is hereby granted, free of charge, to any person obtaining a copy
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### of this software and associated documentation files (the "Software"), to deal
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### in the Software without restriction, including without limitation the rights
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### to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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### copies of the Software, and to permit persons to whom the Software is
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### furnished to do so, subject to the following conditions:
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### The above copyright notice and this permission notice shall be included in all
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### copies or substantial portions of the Software.
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### THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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### IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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### FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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### AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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### LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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### OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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### SOFTWARE.
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import array
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import time
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import math
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import digitalio
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import audioio
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import board
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import usb_midi
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import neopixel
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import adafruit_midi
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from adafruit_midi.midi_message import note_parser
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from adafruit_midi.note_on import NoteOn
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from adafruit_midi.note_off import NoteOff
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from adafruit_midi.control_change import ControlChange
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from adafruit_midi.pitch_bend import PitchBend
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# Turn the speaker on
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speaker_enable = digitalio.DigitalInOut(board.SPEAKER_ENABLE)
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speaker_enable.direction = digitalio.Direction.OUTPUT
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speaker_on = True
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speaker_enable.value = speaker_on
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dac = audioio.AudioOut(board.SPEAKER)
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# 440Hz is the standard frequency for A4 (A above middle C)
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# MIDI defines middle C as 60 and modulation wheel is cc 1 by convention
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A4refhz = const(440)
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midi_note_C4 = note_parser("C4")
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midi_note_A4 = note_parser("A4")
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midi_cc_modwheel = const(1)
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twopi = 2 * math.pi
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# A length of 12 will make the sawtooth rather steppy
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sample_len = 12
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base_sample_rate = A4refhz * sample_len
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max_sample_rate = 350000 # a CPX / M0 DAC limitation
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midpoint = 32768
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# A sawtooth function like math.sin(angle)
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# 0 returns 1.0, pi returns 0.0, 2*pi returns -1.0
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def sawtooth(angle):
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return 1.0 - angle % twopi / twopi * 2
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# make a sawtooth wave between +/- each value in volumes
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# phase shifted so it starts and ends near midpoint
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# "H" arrays for RawSample looks more memory efficient
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# see https://forums.adafruit.com/viewtopic.php?f=60&t=150894
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def waveform_sawtooth(length, waves, volumes):
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for vol in volumes:
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waveraw = array.array("H",
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[midpoint +
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round(vol * sawtooth((idx + 0.5) / length
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* twopi
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+ math.pi))
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for idx in list(range(length))])
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waves.append((audioio.RawSample(waveraw), waveraw))
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# Make some square waves of different volumes volumes, generated with
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# n=10;[round(math.sqrt(x)/n*32767*n/math.sqrt(n)) for x in range(1, n+1)]
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# square root is for mapping velocity to power rather than signal amplitude
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# n=15 throws MemoryError exceptions when a note is played :(
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waveform_by_vol = []
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waveform_sawtooth(sample_len,
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waveform_by_vol,
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[10362, 14654, 17947, 20724, 23170,
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25381, 27415, 29308, 31086, 32767])
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# brightness 1.0 saves memory by removing need for a second buffer
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# 10 is number of NeoPixels on CPX
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numpixels = const(10)
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pixels = neopixel.NeoPixel(board.NEOPIXEL, numpixels, brightness=1.0)
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# Turn NeoPixel on to represent a note using RGB x 10
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# to represent 30 notes - doesn't do anything with pitch bend
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def noteLED(pix, pnote, pvel):
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note30 = (pnote - midi_note_C4) % (3 * numpixels)
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pos = note30 % numpixels
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r, g, b = pix[pos]
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if pvel == 0:
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brightness = 0
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else:
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# max brightness will be 32
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brightness = round(pvel / 127 * 30 + 2)
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# Pick R/G/B based on range within the 30 notes
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if note30 < 10:
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r = brightness
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elif note30 < 20:
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g = brightness
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else:
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b = brightness
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pix[pos] = (r, g, b)
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# Calculate the note frequency from the midi_note with pitch bend
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# of pb_st (float) semitones
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# Returns float
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def note_frequency(midi_note, pb_st):
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# 12 semitones in an octave
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return A4refhz * math.pow(2, (midi_note - midi_note_A4 + pb_st) / 12.0)
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midi_channel = 1
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midi = adafruit_midi.MIDI(midi_in=usb_midi.ports[0],
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in_channel=midi_channel-1)
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# pitchbendrange in semitones - often 2 or 12
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pb_midpoint = 8192
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pitch_bend_multiplier = 2 / pb_midpoint
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pitch_bend_value = pb_midpoint # mid point - no bend
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wave = [] # current or last wave played
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last_note = None
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# Amplitude modulation frequency in Hz
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am_freq = 16
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mod_wheel = 0
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# Read any incoming MIDI messages (events) over USB
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# looking for note on, note off, pitch bend change
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# or control change for control 1 (modulation wheel)
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# Apply crude amplitude modulation using speaker enable
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while True:
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msg = midi.receive()
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if isinstance(msg, NoteOn) and msg.velocity != 0:
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last_note = msg.note
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# Calculate the sample rate to give the wave form the frequency
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# which matches the midi note with any pitch bending applied
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pitch_bend = (pitch_bend_value - pb_midpoint) * pitch_bend_multiplier
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note_freq = note_frequency(msg.note, pitch_bend)
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note_sample_rate = round(base_sample_rate * note_freq / A4refhz)
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# Select the wave with volume for the note velocity
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# Value slightly above 127 together with int() maps the velocities
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# to equal intervals and avoids going out of bound
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wave_vol = int(msg.velocity / 127.01 * len(waveform_by_vol))
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wave = waveform_by_vol[wave_vol]
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if note_sample_rate > max_sample_rate:
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note_sample_rate = max_sample_rate
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wave[0].sample_rate = note_sample_rate # must be integer
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dac.play(wave[0], loop=True)
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noteLED(pixels, msg.note, msg.velocity)
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elif (isinstance(msg, NoteOff) or
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isinstance(msg, NoteOn) and msg.velocity == 0):
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# Our monophonic "synth module" needs to ignore keys that lifted on
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# overlapping presses
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if msg.note == last_note:
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dac.stop()
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last_note = None
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noteLED(pixels, msg.note, 0) # turn off NeoPixel
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elif isinstance(msg, PitchBend):
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pitch_bend_value = msg.pitch_bend # 0 to 16383
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if last_note is not None:
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pitch_bend = (pitch_bend_value - pb_midpoint) * pitch_bend_multiplier
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note_freq = note_frequency(last_note, pitch_bend)
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note_sample_rate = round(base_sample_rate * note_freq / A4refhz)
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if note_sample_rate > max_sample_rate:
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note_sample_rate = max_sample_rate
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wave[0].sample_rate = note_sample_rate # must be integer
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dac.play(wave[0], loop=True)
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elif isinstance(msg, ControlChange):
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if msg.control == midi_cc_modwheel:
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mod_wheel = msg.value # msg.value is 0 (none) to 127 (max)
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if mod_wheel > 0:
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t1 = time.monotonic() * am_freq
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# Calculate a form of duty_cycle for enabling speaker for crude
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# amplitude modulation. Empirically the divisor needs to greater
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# than 127 as can't hear much when speaker is off more than half
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# 220 works reasonably well
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new_speaker_on = (t1 - int(t1)) > (mod_wheel / 220)
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else:
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new_speaker_on = True
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if speaker_on != new_speaker_on:
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speaker_enable.value = new_speaker_on
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speaker_on = new_speaker_on
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