Prepare the Grand Central
We'll be using CircuitPython for this project. Are you new to using CircuitPython? No worries, there is a full getting started guide here.
Adafruit suggests using the Mu editor to edit your code and have an interactive REPL in CircuitPython. You can learn about Mu and its installation in this tutorial. Mu 1.0.2 has support for detecting the Adafruit Grand Central as a valid board - if you have an older version, please upgrade.
Follow this guide for instructions on installing the latest release version of CircuitPython for the Grand Central.
You'll also need to add the following libraries for this project. Follow this guide on adding libraries. The ones you'll need are:
- neopixel
- simpleio
- adafruit_midi
Download the latest adafruit-circuitpython-bundle .zip file as instructed in the guide linked below. Unzip the file and drag those libraries to the lib folder on your Grand Central M4 CIRCUITPY drive (create the lib directory if it does not already exist).
Code
You can now upload the code to your Grand Central so it will read the pots and send USB MIDI commands.
Here is the code we'll use. Copy it and then paste it into the Mu editor. Save it to your Grand Central M4 as code.py
# SPDX-FileCopyrightText: 2023 John Park for Adafruit Industries
# SPDX-License-Identifier: MIT
# Grand Central MIDI Knobs
# for USB MIDI and optional UART MIDI
# Reads analog inputs, sends out MIDI CC values
# with Kattni Rembor and Jan Goolsbey for range and hysteresis code
import time
import board
import busio
from simpleio import map_range
from analogio import AnalogIn
from digitalio import DigitalInOut, Direction
import usb_midi
import adafruit_midi # MIDI protocol encoder/decoder library
from adafruit_midi.control_change import ControlChange
# pick your USB MIDI out channel here, 1-16
MIDI_USB_channel = 1
# pick your classic MIDI channel for sending over UART serial TX/RX
CLASSIC_MIDI_channel = 2
midi_usb = adafruit_midi.MIDI(
midi_out=usb_midi.ports[1], out_channel=MIDI_USB_channel - 1
)
# use DIN-5 or TRS MIDI jack on TX/RX for classic MIDI
midi_uart = busio.UART(board.TX, board.RX, baudrate=31250, timeout=0.001) # initialize UART
classic_midi = adafruit_midi.MIDI(
midi_out=midi_uart, midi_in=midi_uart, out_channel=CLASSIC_MIDI_channel - 1, debug=False
)
led = DigitalInOut(board.D13) # activity indicator
led.direction = Direction.OUTPUT
knob_count = 16 # Set the total number of potentiometers used
# Create the input objects list for potentiometers
knob = []
for k in range(knob_count):
knobs = AnalogIn(
getattr(board, "A{}".format(k))
) # get pin # attribute, use string formatting
knob.append(knobs)
# assignment of knobs to cc numbers
cc_number = [
1, # knob 1, mod wheel
2, # knob 2, breath control
7, # knob 3, volume
10, # knob 4 pan
11, # knob 5, expression
53, # knob 6
54, # knob 7
73, # knob 8
74, # knob 9, Filter frequency cutoff
71, # knob 10, Filter resonance
58, # knob 11
59, # knob 12
60, # knob 13
61, # knob 14
62, # knob 15
63, # knob 16
]
# CC range list defines the characteristics of the potentiometers
# This list contains the input object, minimum value, and maximum value for each knob.
# example ranges:
# 0 min, 127 max: full range control voltage
# 36 (C2) min, 84 (B5) max: 49-note keyboard
# 21 (A0) min, 108 (C8) max: 88-note grand piano
cc_range = [
(0, 127), # knob 0: C2 to B5: 49-note keyboard
(0, 127), # knob 1
(0, 127), # knob 2
(0, 127), # knob 3
(0, 127), # knob 4
(0, 127), # knob 5
(0, 127), # knob 6
(0, 127), # knob 7
(0, 127), # knob 8
(0, 127), # knob 9
(0, 127), # knob 10
(0, 127), # knob 11
(0, 127), # knob 12
(0, 127), # knob 13
(0, 127), # knob 14
(0, 127), # knob 15
]
print("---Grand Central MIDI Knobs---")
print(" USB MIDI channel: {}".format(MIDI_USB_channel))
print(" TRS MIDI channel: {}".format(CLASSIC_MIDI_channel))
# Initialize cc_value list with current value and offset placeholders
cc_value = []
for _ in range(knob_count):
cc_value.append((0, 0))
last_cc_value = []
for _ in range(knob_count):
last_cc_value.append((0, 0))
# range_index converts an analog value (ctl) to an indexed integer
# Input is masked to 8 bits to reduce noise then a scaled hysteresis offset
# is applied. The helper returns new index value (idx) and input
# hysteresis offset (offset) based on the number of control slices (ctrl_max).
def range_index(ctl, ctrl_max, old_idx, offset):
if (ctl + offset > 65535) or (ctl + offset < 0):
offset = 0
idx = int(map_range((ctl + offset) & 0xFF00, 1200, 65500, 0, ctrl_max))
if idx != old_idx: # if index changed, adjust hysteresis offset
# offset is 25% of the control slice (65536/ctrl_max)
offset = int(
0.25 * sign(idx - old_idx) * (65535 / ctrl_max)
) # edit 0.25 to adjust slices
return idx, offset
def sign(x): # determine the sign of x
if x >= 0:
return 1
else:
return -1
while True:
# read all the knob values
for i in range(knob_count):
cc_value[i] = range_index(
knob[i].value,
(cc_range[i][1] - cc_range[i][0] + 1),
cc_value[i][0],
cc_value[i][1],
)
if cc_value[i] != last_cc_value[i]: # only send if it changed
# Form a MIDI CC message and send it:
midi_usb.send(ControlChange(cc_number[i], cc_value[i][0] + cc_range[i][0]))
classic_midi.send(
ControlChange(cc_number[i], cc_value[i][0] + cc_range[i][0])
)
last_cc_value[i] = cc_value[i]
led.value = True
time.sleep(0.01)
led.value = False
Before we test it out, let's have a closer look at how it works.
Using Adafruit USB MIDI
The adafruit_midi library code for CircuitPython allows you to easily send the most commonly used MIDI messages.
Create the MIDI Object
The MIDI object can be instantiated this way:
usb_midi = adafruit_midi.MIDI(midi_out=usb_midi.ports[1], out_channel=USB_MIDI_channel - 1)
This allows us to refer to it in the code with the nice, short name midi And we're also using the out_channel argument (which is zero indexed) to set the outgoing MIDI channel through which messages will be sent -- in this case, MIDI channel 1. The possible range is 0-15, which correlates to MIDI channels 1-16.
Then, to send messages, we can use four types:
- Note On
- Note Off
- Control Chage (a.k.a., Continuous Controller or CC)
- Pitch bend
Note On
note_on is used to send a MIDI Note On message. First argument is the note number, 0-127. Second argument is the velocity, 0-127. Typical 88-key piano note range is 36-108 which correlate to pitches C2 to C8.
Example:
-
usb_midi.send(NoteOn(60, 64))sends a MIDI message of Note On number 60 (C3 on the keyboard) at a velocity of 64
Note Off
note_off is used to send a MIDI Note Off message. First argument is the note number, 0-127. Second argument is the velocity, 0-127.
Example:
-
usb_midi.send(NoteOff(60, 64))sends a MIDI message of Note Off number 60, at a velocity of 64. (The velocity doesn't actually matter in most cases, but can be used for interesting effects with harpsichords and other plucked instruments.)
Pitch Bend
pitch_bend sends a MIDI Pitch Wheel message. Range is 0-16383. A value of 8192 equates to no pitch bend. A value > 8192 is an upward bend, while a value < 8192 is a negative pitch bend.
Example:
-
usb_midi.send(PitchBend(10000))sends and increasing pitch bend, in this case a value of10000
Control Change (CC)
control_change sends a MIDI CC ('control change' or 'continuous controller’) message. First argument is the controller number, 0-15. Second argument is the control value, 0-127.
Example:
-
usb_midi.send(ControlChange(4, 100))sends a MIDI control change on control number4with a value of100.
This is a good resource for greater details on the MIDI protocol.
Each MIDI channel can use up to sixteen CC controller numbers. In software, these are usually assignable to anything you like. Here's a table of typical uses, particularly on hardware synthesizers and other MIDI gear:
- 0 Bank Select
- 1 Modulation Wheel
- 2 Breath Controller
- 3 Undefined
- 4 Foot Controller
- 5 Portamento time
- 6 Data Entry Most Significant Bits
- 7 Volume
- 8 Balance
- 9 Undefined
- 10 Pan
- 11 Expression
- 12 Effect Controller 1
- 13 Effect Controller 2
- 14 Undefined
- 15 Undefined
The good news is, you can pretty much ignore these crusty old standards in your software and map any knob to any function! So, once you're inside your favorite software, you'll pick a software knob, enter MIDI learn mode, and assign one of your sixteen Grand Central knobs to do the job!
Libraries
First, we'll import the libraries:
import time
import board
import busio
from simpleio import map_range
from analogio import AnalogIn
from digitalio import DigitalInOut, Direction
import usb_midi
import adafruit_midi
from adafruit_midi.control_change import ControlChange
MIDI Instance
USB MIDI
Then, we'll define the adafruit_midi instance and tell it which MIDI channel to use. MIDI channel number 1 is a good default, unless you have something else plugged into your computer already using it.
usb_midi = adafruit_midi.MIDI(midi_out=usb_midi.ports[1], out_channel=USB_MIDI_channel - 1) (Remember, this is zero indexed, so a 1 is subtracted)
Classic MIDI
With a similar setup, we'll use the TX/RX UART serial port with a TRS jack or DIN-5 connector to send classic MIDI:
uart = busio.UART(board.TX, board.RX, baudrate=31250, timeout=0.001) # initialize UARTclassic_midi = adafruit_midi.MIDI( midi_out=uart, midi_in=uart, out_channel=CLASSIC_MIDI_channel - 1, debug=False)
Knob Setup
Next, we'll set up the knob inputs. We'll use a variable to define the number of knobs used, which makes it simple to go with fewer than the max of 16 if needed.
knob_count = 16
We'll then define the list of knobs with easy to use names that are actually pointing at the AnalogIn pins. We could do something like this 16 times:
knob0 = AnalogIn(board.A0)
but it's neater to wrap it up into a loop like this:
knob = []
for k in range(knob_count):
knobs = AnalogIn(getattr(board, "A{}".format(k)))
knob.append(knobs)
CC Ranges
While the MIDI CC value range runs from 0 to 127, in some cases we'll want to output only a subset of that range when we turn a knob fully. For example, 0 to 127 is great if you're controlling a mixer knob, but it's too huge of a range if you're sending out chromatic pitch values. Typical 88-key MIDI keyboards range from 21 (a very low A0) to 108 (a super high C8).
When using a knob for sequencing melodies, you'll probably want an even tighter range, such as 36 (C2) to 84 (B5) or smaller.
So, we will create a list of ranges that can be adjusted in code per knob:
cc_range = [
(36, 84), # knob 0: C2 to B5: 49-note keyboard
(36, 84), # knob 1
(36, 84), # knob 2
(36, 84), # knob 3
(36, 84), # knob 4
(36, 84), # knob 5
(36, 84), # knob 6
(36, 84), # knob 7
(0, 127), # knob 8: 0 to 127: full range MIDI CC/control voltage for VCV Rack
(0, 127), # knob 9
(0, 127), # knob 10
(0, 127), # knob 11
(0, 127), # knob 12
(0, 127), # knob 13
(0, 127), # knob 14
(0, 127) # knob 15
]
cc_value = []
for c in range(knob_count):
cc_value.append((0,0))
Hysteresis
This helper function, created by Jan Goolsbey, is used to reduce value jitter when a potentiometer is right on the edge between two values:
def range_index(ctl, ctrl_max, old_idx, offset):
if (ctl + offset > 65535) or (ctl + offset < 0):
offset = 0
idx = int(map_range((ctl + offset) & 0xFF00, 1200, 65500, 0, ctrl_max))
if idx != old_idx: # if index changed, adjust hysteresis offset and set flag
# offset is 25% of the control slice (65536/ctrl_max)
offset = int(0.25 * sign(idx - old_idx) * (65535 / ctrl_max)) # edit 0.25 to adjust slices
return idx, offset
The next function is a helper used along with the range_index function to determine the direction of the potentiometer's movement:
def sign(x): # determine the sign of x
if x >= 0:
return 1
else:
return -1
Main Loop
Now, in the main loop of the program we'll do these things:
- Read the knobs
- Adjust their values to conform to the range table and reduce jitter
- Send their values out as the properly formed USB MIDI messages
Here is the loop that checks and adjust the knob values:
while True:
# read all the knob values
for i in range(knob_count):
cc_value[i] = range_index(knob[i].value,
(cc_range[i][1] - cc_range[i][0] + 1),
cc_value[i][0], cc_value[i][1])
MIDI Message Send
And finally, the thing we've been waiting for -- sending the message, but only if it has changed since the last check.
for i in range(knob_count):
cc_value[i] = range_index(
knob[i].value,
(cc_range[i][1] - cc_range[i][0] + 1),
cc_value[i][0],
cc_value[i][1],
)
if cc_value[i] != last_cc_value[i]: # only send if it changed
# Form a MIDI CC message and send it:
usb_midi.send(ControlChange(cc_number[i], cc_value[i][0] + cc_range[i][0]))
classic_midi.send(
ControlChange(cc_number[i], cc_value[i][0] + cc_range[i][0])
)
last_cc_value[i] = cc_value[i]
led.value = True
In Use
Now, it's time to use your Grand Central USB MIDI Knob Controller! With it plugged into your computer over USB, launch a DAW, software synthesizer/sequencer, or DJ tool. Here are some examples of free, open source synths for Linux, Windows, and mac os:
This page shows more details on using a MIDI controller with Helm.
MIDI Monitor
You can use this handy Chrome browser MIDI Monitor web app to simply read the values of your Grand Central controller.
Rack Patch
This is an example of a patch made in VCV Rack, the open source modular software synthesizer:
The module in the upper left corner, MIDI-CC, is used to connect the Grand Central MIDI Knob controller to the rest of the modules. You can see 16 patch cable running from it to an 8-step pitch sequencer, as well as various other modules to control the envelope and filter of the sound. All with your real, physical knobs!
The Grand Central USB MIDI Knob Controller also works great for controlling many iOS apps that have tiny virtual knobs, for example AudioKit Synth One (iPad only).
You'll need to check if your app supports MIDI controller assignments and us an OTG cable to plug in the Grand Central.
Of course, you can also use the Grand Central with Ableton Live, Propellerhead Reason, FL Studio, Logic, Traktor, Max/MSP, and other professional apps!
Page last edited January 22, 2025
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