Demonstrating low-pass filter issue that occurs above nyquist rate.

Author: Kevin J Walters

pico/synthio-filter-breakdown.py

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+### synthio-filter-breadkdown v1.0
+### Exploration of synthio filter sometimes producing white noise
+
+### Tested with Pi Pico W (on EDU PICO) and 9.1.4
+
+### copy this file to Cytron Maker Pi Pico as code.py
+
+### MIT License
+
+### Copyright (c) 2024 Kevin J. Walters
+
+### Permission is hereby granted, free of charge, to any person obtaining a copy
+### of this software nd associated documentation files (the "Software"), to deal
+### in the Software without restriction, including without limitation the rights
+### to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+### copies of the Software, and to permit persons to whom the Software is
+### furnished to do so, subject to the following conditions:
+
+### The above copyright notice and this permission notice shall be included in all
+### copies or substantial portions of the Software.
+
+### THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+### IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+### FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+### AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+### LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+### OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+### SOFTWARE.
+
+### low-pass filter iscussion in https://forums.adafruit.com/viewtopic.php?p=1034152
+
+### Focus on C7 (note 84) - the Biquad filter turns to noise if low pass filter frequency
+### is above half the sample rate (the Nyquist rate)
+### Sample rate is set the same on Mixer and Sythenszier
+
+
+import os
+import time
+
+import board
+##import analogio
+##import digitalio
+##import pwmio
+
+import audiomixer
+import synthio
+import audiopwmio
+import ulab.numpy as np
+
+import usb_midi
+import adafruit_midi
+from adafruit_midi.note_on import NoteOn
+from adafruit_midi.note_off import NoteOff
+from adafruit_midi.control_change import ControlChange
+from adafruit_midi import control_change_values
+
+
+debug = 2
+
+
+SAMPLE_RATE = 64_000
+
+MIXER_BUFFER_SIZE = 2048
+WAVEFORM_PEAK = 28_000
+WAVEFORM_MAX = 2**15 - 1
+WAVEFORM_LEN = 8
+WAVEFORM_HALFLEN = WAVEFORM_LEN // 2
+MIDI_KEY_CHANNEL = 1
+### Wire protocol values
+MIDI_KEY_CHANNEL_WIRE = MIDI_KEY_CHANNEL - 1
+
+if os.uname().machine.find("EDU PICO"):
+    LEFT_AUDIO_PIN = board.GP20
+    RIGHT_AUDIO_PIN = board.GP21
+elif os.uname().sysname == "rp2040":
+    ### Cytron Maker Pi Pico
+    LEFT_AUDIO_PIN = board.GP18
+    RIGHT_AUDIO_PIN = board.GP19
+else:
+    ### Custom RP2350B board
+    LEFT_AUDIO_PIN = board.GP36
+    RIGHT_AUDIO_PIN = board.GP37
+
+
+
+
+def d_print(level, *args, **kwargs):
+    """A simple conditional print for debugging based on global debug level."""
+    if not isinstance(level, int):
+        print(level, *args, **kwargs)
+    elif debug >= level:
+        print(*args, **kwargs)
+
+
+
+waveform_saw    = np.linspace(WAVEFORM_PEAK, 0 - WAVEFORM_PEAK, num=WAVEFORM_LEN,
+                              dtype=np.int16)
+midi_usb  = adafruit_midi.MIDI(midi_in=usb_midi.ports[0],
+                               in_channel=MIDI_KEY_CHANNEL_WIRE)
+audio = audiopwmio.PWMAudioOut(LEFT_AUDIO_PIN, right_channel=RIGHT_AUDIO_PIN)
+mixer = audiomixer.Mixer(channel_count=1,
+                         sample_rate=SAMPLE_RATE,
+                         buffer_size=MIXER_BUFFER_SIZE)
+synth = synthio.Synthesizer(channel_count=1,
+                            sample_rate=SAMPLE_RATE)
+
+audio.play(mixer)
+mixer.voice[0].play(synth)
+mixer.voice[0].level = 0.75
+
+filter_freq_lo = 100   # filter lowest freq
+filter_freq_hi = 4500  # filter highest freq
+filter_note_offset_low = 58
+filter_note_offset_high = 62
+filter_res_lo = 0.1    # filter q lowest value
+filter_res_hi = 2.0    # filter q highest value
+
+filter_note_offset = 37
+filter_res = 1.0    # current setting of filter
+amp_env_attack_time = 1.0
+amp_env_decay_time = 0.5
+amp_env_sustain = 0.8
+amp_env_release_time = 1.100
+
+pressed = []
+oscs = []
+
+
+
+### Simple range mapper, like Arduino map()
+def map_range(s, a1, a2, b1, b2):
+    return  b1 + ((s - a1) * (b2 - b1) / (a2 - a1))
+
+
+### pylint: disable=consider-using-in,too-many-branches
+def note_on(notenum, vel):
+
+    new_osc = []
+    f_1 = synthio.midi_to_hz(notenum)
+    filt_f_1 = synthio.midi_to_hz(notenum + filter_note_offset)
+    filter_1 = synth.low_pass_filter(filt_f_1, filter_res)
+    d_print(2, "FILTER FREQ", filt_f_1, "AM+S rate", SAMPLE_RATE)
+    new_osc.append(synthio.Note(frequency=f_1,
+                                waveform=waveform_saw,
+                                amplitude=0.5,
+                                envelope=synthio.Envelope(attack_time=amp_env_attack_time,
+                                                          attack_level=1.0,
+                                                          decay_time=amp_env_decay_time,
+                                                          sustain_level=amp_env_sustain,
+                                                          release_time=amp_env_release_time),
+                                filter=filter_1
+                                ))
+
+    oscs.clear()
+    pressed.clear()
+
+    oscs.extend(new_osc)
+    synth.press(oscs)
+    pressed.append(notenum)
+
+
+def notes_off():
+
+    synth.release(oscs)
+    oscs.clear()
+    pressed.clear()
+
+
+last_note = None
+start_ns = time.monotonic_ns()
+while True:
+    msg = midi_usb.receive()
+
+    if msg:
+        if isinstance(msg, NoteOn) and msg.velocity != 0:
+            d_print(2, "Note:", msg.note, "vel={:d}".format(msg.velocity))
+            if last_note is not None:
+                notes_off()
+            note_on(msg.note, msg.velocity)
+            last_note = msg.note
+
+        elif (isinstance(msg, NoteOff)
+              or isinstance(msg, NoteOn) and msg.velocity == 0):
+            d_print(2, "Note:", msg.note, "vel={:d}".format(msg.velocity))
+            if msg.note in pressed:  # only release note that's sounding
+                notes_off()
+
+        elif isinstance(msg, ControlChange):
+            d_print(2, "CC:", msg.control, "=", msg.value)
+            if msg.control == control_change_values.CUTOFF_FREQUENCY:  ### 74
+                ##filter_freq = map_range( msg.value, 0,127, filter_freq_lo, filter_freq_hi)
+                filter_note_offset = map_range(msg.value,
+                                               0, 127,
+                                               filter_note_offset_low, filter_note_offset_high)
+            elif msg.control == control_change_values.FILTER_RESONANCE:  ### 71
+                filter_res = map_range(msg.value, 0, 127, filter_res_lo, filter_res_hi)
+            elif msg.control == control_change_values.RELEASE_TIME: ### 72
+                amp_env_release_time = map_range(msg.value, 0, 127, 0.05, 3)
+
+        else:
+            d_print(1, "MIDI MSG:", msg)