Most Digital Audio Workstation (DAW) programs and many live audio products support Mackie Control (MC) protocol for controlling levels, transport controls, channel muting, etc. There are quite a few low-cost control surfaces on the market, but many lack features that more expensive controllers such as the Mackie MCU Pro or Behringer X-Touch support.

This project uses two Raspberry Pi Pico boards to implement a channel strip, meter, and time display for USB MIDI control surfaces that support Mackie Control (MC) protocol but do not have displays. You insert the pico-mc-display-bridge in line with the USB connection from the DAW to the control surface. One Pico board acts as a USB Device and connects to the DAW or other device that expects to talk with a MC control surface. The other Pico board acts as a USB host and connects to the MC compatible control surface, and it acts as a button interface for the additional buttons provided. The photo shows the pico-mc-display-bridge between a PC running the Cubase DAW and a Korg nanoKONTROL studio.

The two Pico boards together form what should be a transparent interface between the control surface and the DAW. The Host Pico reads the control surface USB device information and sends it to the Device Pico before the Device Pico initializes its USB stack. When the DAW computer asks the Device Pico for its device information, it sends the information of the connected control surface instead.

The pico-mc-display-bridge supports:

• one 64x128 (portrait mode) OLED per channel strip that shows
  • 2 lines of 7 characters of text
  • a representation of the MC VPot LEDs (above the text),
  • a representation of the MC meter LEDs (to the right of the VPot LEDs),
  • a representation of the REC, MUTE, SOLO and SEL button LEDs (above the VPot LEDs).
• one 128x64 (landscape mode) OLED for time display (either Bars/Beats/Subdivisions/ticks or SMPTE timecode), VPot mode display (the two-digit 7-segment display on the Mackie MCU Pro, for example), and the channel strip button mode. This display is shared with the MIDI Processor UI (see below)
• one button per channel strip (the black buttons in the photo)that can function as either Select, Mute, Solo, Record or VPot press depending on which of the 4 mode buttons (yellow, blue, red or white button) was last pressed.
• one button for choosing name or value display (green button)
• one button for choosing the time display mode (Bars/Beats/Subdivisions/Ticks or SMPTE Timecode, the gray button).
• 7 buttons (or a 5-way navigation switch plus 2 buttons) for navigating the MIDI processor UI (located on the blue board above the landscape display)
• a MIDI processor unit that determines which virtual MIDI cable in the MIDI USB stream is carrying MC data, and that implements useful features such as fader soft pickup (to prevent value jumps when you move faders), button remap, etc. The MIDI processor settings are stored in the Device Pico's program flash memory. Each MC device type (unique USB VID and PID) can have up to 8 preset settings that you can recall using the MIDI Processor UI.

This is what I used for the project in the photo.

• 2 Raspberry Pi Pico (not Pico W)
• 4 20-pin 0.1 header pins (2 per Pico board)
• 4 20-pin 0.1 header sockets (2 per Pico board)
• 2 3-pin right angle header pins (for Picoprobe debug, one per Pico)
• 2 2-pin right angle header pins (for Picoprobe UART0, one per Pico)
• 9 0.96 128x64 OLED modules. I chose displays that show white dots, but these are available in blue dots, yellow dots, etc. Shop carefully. You can pay a lot for these. I was able to find a 10-pack for about $2.30/display.
• 9 4 pin sockets (one for each display)
• 14 tact switches with button caps
• a 5-way navigation switch module board with 2 tact switches. You can just use 7 buttons if you prefer.
• a microUSB to USB A female adapter (so you can connect your MIDI device to the pico-mc-display-bridge's USB host port)
• a microUSB to USB A male plug (so you can connect your pico-mc-display-bridge to your DAW computer).
• prototyping board (I used one with double-sided plate through holes), standoffs, screws and nuts, 30 AWG wire for signals, 22 AWG wire for power and ground, solder.

I did not draw a wiring diagram. The pin usage is documented below and in the code. Just wire signals directly from the Pico boards to the displays and one of the pins for each button. The VBus pin of the Device Pico connects directly to the VBus pin of the Host Pico. The UART1 pins of the Device Pico wire directly to the UART1 pins of the Host Pico. The Device Pico's 3.3V output provides power to all 9 OLEDs.

Even though there are only 4 pins. the pinout of OLED modules will vary depending on the brand. Take special care to note which pin is GND, which will go to one of the Device Pico's ground pins, and which is VCC, which will go to the Device Pico's 3.3V power output. Each display draws no more that about 4 mA, so power draw should not be an issue.

One pin of every button that is wired to the Host Pico is attached to ground; the other pin is attached to the Host Pico at the assigned pin. No pull-up resistors are required because the Host Pico enables the on-chip pull-up resistors. Buttons consume all but one of non-UART I/O pins on the Host Pico.

You don't see the Pico boards in the photo because I placed them on the other side of the board under the displays.

The following sections call out pins by GP number not Pico board pin number. It is documented this way to allow you to use any board that has the RP2040 on it as long as it has enough I/O pins available. UART pins are documented in TX, RX order. I2C pins are documented in SDA, SCL order.

The USB is configured in USB Device mode. This Pico controls 9 SSD1306-base 128x64 OLED modules over I2C. Some OLED modules have solder jumpers that allow selection of address 0x3C or 0x3D, but not all do. The code assumes all displays are on address 0x3C. There is one OLED module per channel strip. The graphics driver code for this project allows you to update all 9 displays in parallel using 8 I2C ports from 2 PIO modules plus I2C1. The USB Device Pico communicates with the USB Host Pico via UART1. UART0 is used for debug console.

• UART0 on pins GP0 and GP1 is used with the picoprobe for debug console
• I2C1 on pins GP2 and GP3 is wired to the timecode OLED
• UART1 on pins on GP4 and GP5 is wired to the USB Host Pico
• PIO0 on pins GP6 and GP7 is configured for I2C and is wired to channel CH 1 OLED
• PIO0 on pins GP8 and GP9 is configured for I2C and is wired to channel CH 2 OLED
• PIO0 on pins GP10 and GP11 is configured for I2C and is wired to channel CH 3 OLED
• PIO0 on pins GP12 and GP13 is configured for I2C and is wired to channel CH 4 OLED
• PIO0 on pins GP14 and GP15 is configured for I2C and is wired to channel CH 5 OLED
• PIO0 on pins GP16 and GP17 is configured for I2C and is wired to channel CH 6 OLED
• PIO0 on pins GP18 and GP19 is configured for I2C and is wired to channel CH 7 OLED
• PIO0 on pins GP20 and GP21 is configured for I2C and is wired to channel CH 8 OLED
• 4 Unused pins GP22, GP26-GP28

The USB is configured in USB Host mode. It connects to the control surface's USB port. It also controls 8 channel strip buttons with 4 buttons for choosing what those buttons send, 1 button for choosing time display mode, and 1 button for choosing whether the text shows value or or name.

The USB Device Pico communicates with the USB Host Pico via UART1. UART0 is used for debug console.

• UART0 on pins GP0 and GP1 is used with the picoprobe for debug console
• GP2 is used for the Beats/SMPTE button
• GP3 is used for the Name/Value button
• UART1 on pins on GP4 and GP5 is wired to the USB Host Pico
• GP6-GP12 are used for the MIDI processor UI navigation
• pins GP13-GP20 are used for the per channel strip buttons 1-8
• pins GP21, GP22, GP26, GP27 are used to select the per channel strip buttons mode
• GP28 is unused

pico-mc-display-bridge/
    device/
    ext_lib/
    host/
    lib/
    LICENSE
    README.md
    (any jpg files)

The device directory contains the source code for the Device Pico and the host directory contains the source code for the Host Pico. You must build one image for each Pico, and program each Pico with the appropriate image.

cd ${PICO_SDK_PATH}
git pull
cd lib
git submodule update tinyusb

cd lib/tinyusb
git remote add upstream https://github.com/hathach/tinyusb.git
git remote set-url origin https://github.com/rppicomidi/tinyusb.git
git fetch origin
git checkout -b pio-midihost origin/pio-midihost

cd [the root directory where you want to put this project]
git clone --recurse-submodules https://github.com/rppicomidi/pico-mc-display-bridge.git
cd pico-mc-display-bridge

Assumes you just completed the "Get the Source Code" steps above. Also assumes you have correctly set up your command line build environment per the instructions in the Getting started with Raspberry Pi Pico guide.

cd device
mkdir build
cd build
cmake ..
make

The build image is called pico-mc-display-bridge-dev.uf2. Use the one of the methods described in the Getting started with Raspberry Pi Pico guide to program the Device Pico with this image.

Assumes you just completed the "Build the Device Pico Image" steps above

cd ../../host
mkdir build
cd build
cmake ..
make

The build image is called pico-mc-display-bridge-host.uf2. Use the one of the methods described in the Getting started with Raspberry Pi Pico guide to program the Device Pico with this image.

1. Connect a MIDI device with MC support to the Host Pico's USB port.
2. Plug the Device Pico's USB port to the computer running your DAW software; you may need to plug via a powered hub if the computer's USB host port cannot provide enough power to power the attached MIDI device and the pico-mc-display-bridge hardware. You should observe the OLEDs lighting up and the computer should enumerate the pico-mc-display-bridge as if you had attached the MIDI device directly to the computer.

There are 7 navigation buttons: Up, Down, Left, Right and Shift. In the photo of the hardware, navigation buttons are on the blue board. The 5 way switch implements the first 5 buttons. BACK is labeled SET and SHIFT is labeled RST. Holding a button causes the button press to repeat.

When you press the Select Button, the pico-mc-display-bridge enters MIDI Processor setup mode. The landscape OLED screen will show text menus. Use the Up and Down buttons to navigate to a menu item. Holding shift while pressing Up or Down buttons scroll more than one step. Menu items you can modify will show in reverse video. When you press the the Select button the pico-mc-display-bridge will either enter a sub-menu or execute a command.

When there is more than one editable field on a menu, use the Left and Right buttons to navigate among them. If you add an item to a menu and want to delete it, hold Shift and press the Left button.

To return to the previous menu, press the Back button.

To exit setup mode, either press the Back button repeatedly or hold Shift and press the Back button (this is Go Home function).

USB MIDI can have up to 16 MIDI ports per bulk USB endpoint. Dedicated control surfaces such as the Korg nanoKONTROL Studio have only one MIDI IN port and one MIDI OUT port. However, many keyboard control surfaces such as the Arturia KeyLab Essential 88 have multiple MIDI ports per USB cable; there is a dedicated USB port for MC messages, and one or more dedicated ports for the music keyboard, pads, step sequencer, etc. Before the pico-mc-display-bridge will display any MC data on the OLEDs, you have to tell it where to find MC traffic. To do that, you will use the built-in MIDI processor unit to route MIDI data to the display processor. Because the MC data is coming from the DAW MIDI OUT, you need to set up the processing on the MIDI OUT path.

1. Press the Select button on the navigation switches so that the landscape mode timecode and VPot mode display switches to the processor setup menu.
2. Use the navigation buttons to select Setup MIDI Out N... where N is the MIDI port number that your MIDI device uses for Mackie Control. For example, the Arturia KeyLab Essential 88 uses N=2. The Korg nanoKONTROL Studio uses N=1.
3. Select Add New Processor... and choose MC Display. You should see MC Display on the list before Add New Processor... back on the Setup MIDI OUT N screen.
4. Navigate to Preset 1[M] and press the Select button.
5. In the Current Preset: 1 make sure Next Preset: is also 1 and then navigate and select Save next preset. You should observe the [M] is no longer there, which means preset 1 has been successfully saved. You won't have to display configure processing again for this device because the pico-mc-display-bridge will remember this setup on power-up.
6. Press the Back button again. The setup menu should be gone and you should see the mode indication on the VPot mode and Timecode display.

You may have noticed there are other processing units you can add other than MC Display in the list of processors.

The MC Fader Pickup processor is useful if your control surface does not have motorized faders. MC Fader Pickupwill intercept the fader movement message from your MIDI controller and prevent sending it to your DAW computer until you move the fader passed the point where the DAW thinks the fader is. Because this processor primarily operates on the messages that go to the DAW's MIDI IN, insert this processor using Setup MIDI IN N...

The Channel Button Remap processor is useful if the button messages your control surface uses don't match the mapping your DAW expects. Insert this processor using Setup MIDI IN N... and then set up the MIDI mapping by navigating to the Channel Button Remap item in the processing list and pressing the Select button.

Other processors that are specific to individual control surfaces may be added to the processing library in the future.

There is one button per channel strip and 4 buttons that determine what happens when you press the channel strip button. Each of the 8 per channel strip buttons is black in the hardware photo. What MIDI message the pico-mc-display-bridge sends to the DAW is determined by the channel button mode, which is displayed on the lower right of the landscape OLED:

• SEL : a per channel button press will select the channel
• SOLO : a per channel button press will solo the channel
• MUTE : a per channel button press will mute the channel
• REC : a per channel button press will arm the channel for record
• VPOT : a per channel button press corresponds to pressing the VPOT switch for that channel.

You press the Yellow button to toggle the mode between SEL and SOLO. You press the Blue button to toggle the mode between SEL and MUTE. You press the Red button to toggle the mode between SEL and REC. You press the White button to toggle the mode between SEL and VPOT.

You can directly change mode from one to another just by pressing another button.

• Beats/SMPTE gray button press will send the Beats/SMPTE toggle message to the DAW. Normally this button will tell your DAW to toggle the time display between Bars-Beats and SMPTE Timecode. Most DAWs support this.
• Name/Value green button press will send the Name/Value toggle message to the DAW. Normally this will tell your DAW to change whether the channel strip displays show the parameter name or the value. Not all DAWs support this.

The two Pico boards are connected via UART1. If the interface only had to convey a single MIDI cable's worth of MIDI data, then the connection could contain the raw MIDI stream in both directions. However, the connected control surface may support up to 16 MIDI streams. In addition, the Device Pico needs to know some USB descriptor parameters from the MIDI device connected to the Host Pico:

• Vendor ID
• Product ID
• Number of virtual MIDI cables supported in each direction
• Product Name String, if available
• Vendor Name String, if available
• Serial Number String, if available
• Name of each virtual cable, if such information is available

All messages start with an header byte and are followed by an appropriate amount of data bytes, and then a checksum, which is the XOR of the header byte and all data bytes. Data values are in hex unless otherwise stated.

All messages that are not MIDI data stream messages are formatted:

cmd length [payload] checksum

where cmd is a single unsigned byte value larger than the largest MIDI header byte value, length is the number of bytes in the payload (may be 0), payload is the array of length data bytes (will be empty if length is 0) and checksum is the XOR of all bytes from the cmd byte to the last payload byte. You can find the encoding details for these messages in common/pico-mc-display-bridge-cmds.h.

header_byte midi_stream checksum

where header_byte[7:4] encodes the number of MIDI stream bytes that follow (1, 2 or 3, or 0 for a complete SysEx message).

header_byte[3:0] is the virtual cable number 0-F.

midi_stream is a stream of 1, 2 or 3 bytes, or a complete SysEx message that starts with F0 and ends with F7.

Both Host Pico and Device Pico send this message to send bytes from the MIDI stream to the other Pico.

48 checksum

The Device Pico will send this to the Host Pico when it is ready to receive MIDI data

40 0 checksum

The Device Pico will request the 18-byte USB standard device descriptor for the connected device as soon as it is ready to receive the descriptor. The Host Pico will ignore this request until it has fully enumerated the connected device.

41 12 [18 bytes of device descriptor] checksum

The Host Pico will return the 18-byte USB standard device descriptor for the connected device after the Pico Host has fully enumerated the connected device and after the Pico Host has received the Request Device Descriptor message.

49 0 checksum

The Device Pico will request the first block of configuration descriptor bytes from the Host Pico after the Device Pico has received the Return Device Descriptor message.

4A length [wTotalLength bytes of configuration descriptor or first block of bytes] checksum

The Host Pico will send the first block of configuration descriptor bytes to the Device Pico in response to the Request Configuration Descriptor block 0 command. This block of bytes will contain the wTotalLength field, so the Device Pico will know if it needs to request another block.

4B 0 checksum

The Device Pico will request the second block of configuration descriptor bytes from the Host Pico after the Device Pico has received the Return Configuration Descriptor block 0 message only if there are more bytes in the configuration descriptor than can fit in a single block.

4C length [second block of bytes] checksum

4D 0 checksum

The Device Pico will request the third block of configuration descriptor bytes from the Host Pico after the Device Pico has received the Return Configuration Descriptor block 0 message only if there are more bytes in the configuration descriptor than can fit in a the first two blocks.

4E length [second block of bytes] checksum

42 0 checksum

The Device Pico will send this message to the Host Pico in order to get a list of all string indices in the connected device. The Device Pico will send this after it receives the Core Device Info message.

43 length [a list of length single byte string indices] checksum

The Host Pico sends this message. It contains a list of every string index the device supports.

44 0 checksum

45 length_in_bytes [list of 16-bit LangIDs] checksum

The Device Pico sends this to the Host Pico to convey the list of LANGIDs the connected control surface device.

46 3 string_index langid_lsb langid_msb checksum

The Device Pico sends this message to the Host Pico to request a particular string in a particular language.

47 length string_index langid_lsb langid_msb num_bytes utf16_le_string checksum

The Host Pico sends this message to the Device Pico in response to a Request String message.

50 Nav_button_status_byte checksum

The host sends to the device the status of the navigation buttons; bit set is pressed.

51 Cable_num_byte checksum

The device sends to the host the cable number where the MC Display processor is installed

52 Channel_button_mode checksum

The host sends the channel button mode to the device in response to button presses. Modes are

This site documents the way MC uses MIDI. The Logic Control guide documents most of what you need. MC protocol uses 0x14 instead of 0x10 and 0x15 instead of 0x11 for the product IDs. There are some messages that are not documented here. I found some of these out by web search engine in forums, a look at the Cakewalk Control Surface SDK, and when all else failed, by using my DAW and MidiView.