a hacky way to do portamento in synthio in CircuitPython

synthio_portamento_hack.py

# synthio_portamento_hack.py -- a hacky way to do portamento in synthio
#
import board, time, random
import audiobusio, audiomixer, synthio
import ulab.numpy as np

lck_pin, bck_pin, dat_pin = board.MISO, board.MOSI, board.SCK

audio = audiobusio.I2SOut(bit_clock=bck_pin, word_select=lck_pin, data=dat_pin)
mixer = audiomixer.Mixer(voice_count=1, sample_rate=28000, channel_count=1,
                         bits_per_sample=16, samples_signed=True, buffer_size=2048 )
audio.play(mixer)
synth = synthio.Synthesizer(sample_rate=28000)
mixer.voice[0].level = 0.5 # turn down the volume a bit since this can get loud
mixer.voice[0].play(synth)

wave_saw = np.linspace(30000,-30000, num=512, dtype=np.int16)  # default squ is too clippy
amp_env = synthio.Envelope(sustain_level=1.0, release_time=0.2)
synth.envelope = amp_env

# standard linear interpolate
def lerp(a, b, t):  return (1-t)*a + t*b

# hacky portamento class to slide pitches around
# it owns the synthio.Note object but you still need to synth.press() it
class PortamentoNote:
    def __init__(self, end_f, start_f=0, duration=0):
        self.set_freq(end_f, start_f, duration)
        self.note = synthio.Note(frequency=start_f, waveform=wave_saw)

    def set_freq(self, end_f, start_f=0, duration=0):
        self.start_f = start_f if start_f > 0 else end_f
        self.end_f = end_f
        self.duration = duration
        self.start_time = time.monotonic()

    def update(self):
        dt = time.monotonic() - self.start_time
        if dt < self.duration:  # we're still sliding
            self.note.frequency = lerp(self.start_f, self.end_f, dt / self.duration)


midi_notes = (44, 48, 60)
n = 0
last_note_time = time.monotonic()

porta_time = 0.5 # how long our portamentos should be
porta_note = PortamentoNote( synthio.midi_to_hz(midi_notes[n]) )
synth.press(porta_note.note)  

while True:
    porta_note.update()

    if time.monotonic() - last_note_time > 2: # play notes every 2 secs:
        last_note_time = time.monotonic()
        n = (n+1) % len(midi_notes)  # go to next note, with looping

        new_f = synthio.midi_to_hz(midi_notes[n])  # get new note's frequency
        # make old end frequency new start frequency
        porta_note.set_freq( start_f=porta_note.end_f, end_f=new_f, duration=porta_time )