Closed
Description
CircuitPython version and board name
Adafruit CircuitPython 9.2.6 on 2025-03-23; Raspberry Pi Pico 2 with rp2350a
Adafruit CircuitPython 9.2.6 on 2025-03-23; Raspberry Pi Pico with rp2040Code/REPL
"""
filter Q over 1.41 causes glitches in the filter at certain frequency
"""
import time, board, synthio, audiobusio, audiomixer
SAMPLE_RATE = 44100
BUFFER_SIZE = 2048
i2s_bck_pin = board.GP20
i2s_lck_pin = board.GP21
i2s_dat_pin = board.GP22
audio = audiobusio.I2SOut(bit_clock=i2s_bck_pin, word_select=i2s_lck_pin, data=i2s_dat_pin)
mixer = audiomixer.Mixer(sample_rate=SAMPLE_RATE, channel_count=2, buffer_size=BUFFER_SIZE)
synth = synthio.Synthesizer(sample_rate=SAMPLE_RATE, channel_count=2)
audio.play(mixer)
mixer.voice[0].play(synth)
midi_notes = [ 49, # glitch near 170 & 150 Hz
37, # glitch near 100, 70, & 60 Hz
36, # gitch near 70 & 60 Hz
42, # glitch near 100 Hz
47, # glitch near 120 Hz
]
while True:
for n in midi_notes:
midi_note_f = synthio.midi_to_hz(n)
note = synthio.Note(midi_note_f)
note.filter = synthio.BlockBiquad(synthio.FilterMode.LOW_PASS,
frequency=200, Q=1.5)
synth.press(note)
for f in range(21): # modulate the filter down by hand to find glitches
filter_freq = 200 - (10*f)
print("Q: %.2f filter freq: %3d note freq: %3.2f" %
(note.filter.Q, filter_freq, midi_note_f))
note.filter.frequency = filter_freq
time.sleep(0.3)
synth.release(note)Behavior
When synthio.BlockBiquad is set to certain frequencies with Q>1.41, strange oscillations / "glitches" occur that should not. In general it seems to be around the pitch of the playing note and lower.
Description
Here's a video showing the above code running on a Pico2 RP2350 driving an I2S DAC.
The behavior is the same on a Pico RP2040 with I2S DAC. The behavior is also the same with PWMOut.
This behavior has existed since synthio has had filters, but with BlockBiquad it's more noticeable because we can easily modulate the filter frequency.
synthio_filter_glitch.mp4
Additional information
No response