The Nexus is a small, flexible, and hacker friendly breakout box that connects your multichannel guitar to the outside world using a specialized (LEMO compatible) multi-pin connector for up to 15 channels of audio and 7 channels of analog control voltages for by-wire (remote) control of volume, tone, patch or effects. Audio may come from standard mono-pickups or from the Nu multichannel pickup. Analog control voltages are converted to MIDI control messages.
Nexus is modular with 3 basic building blocks and ample space for user expansion. All input and output pins are provided for user customization. The diagram at the right shows the arrangement and signal flow of the modules:
- Main Board: Where all signals from the multichannel input as well as control voltages pass through, plus power regulation and conditioning.
- MIDI Module: Converts control voltages (CV) from the guitar to MIDI. The basic firmware supports up to 6 analog and 7 digital inputs.
- I/O Module: 12 input and output channels where each channel can be configured as an input or output. Typically, these are used to send multichannel audio to your effects, amp, mixer or multichannel audio interface. If you do not need all 12 channels, they can also be configured as inputs (e.g. Foot switches and Expression pedals). We use only high quality Neutrik connectors.
We have the Basic Kit plus the optional MIDI module:
- Basic Kit: Includes the Main Board and the I/O Module inside an elegant all-aluminum enclosure.
- MIDI Module:Control voltage to MIDI converter.
Note: 12 Volt power supply not included. Use a well regulated power supply suitable for audio. The Main Module includes reverse polarity protection and LC noise-filters, low-noise regulators plus capacitance multipliers to maintain noise free audio.
The Basic Kit includes the 1) Main Board where all signals from the multichannel input as well as control voltages pass through, plus where power regulation and conditioning are handled, 2) I/O Module: 12 channels where each channel can be configured as an input or output. Typically, these are used to send multichannel audio to your effects, amp, mixer or multichannel audio interface. But, if you do not need all 12 channels, they can also be configured as inputs (e.g. Foot switches and Expression pedals). The Basic Kit includes the blue boxes in the Nexus block diagram at the above.
Multichannel Guitar Input
|Pins 1 to 8||Multichannel Audio [Channels 1 to 8]|
|Pins 9 to 15||Multichannel Audio or Control Voltage [Channels 9 to 15]|
|Pin 16 to 17||12v Power.|
|Pin 18 to 19||Main Ground|
Up to 15 channels of audio are available. Audio sources may come from either the Nu multichannel pickup, or other monophonic pickups. Ideally, it is best to avoid direct switching using the traditional 5-way or 3-way toggle switches. Instead, make all outputs from all pickups available and rely on remote (by-wire) switching using MIDI patch changes with a special 5-way switch that sends control voltages.
Channels 1 to 8 are audio only. Channels 9 to 15 can be configured to receive either audio or control voltage (CV) using configuration pins (more on that later).
CV (Control Voltages) and MIDI
Control voltages coming from specialized potentiometers and switches in the guitar can be connected to pins 9 to 15. As mentioned, channels 9 to 15 can be configured to receive either audio or control voltage (CV) using configuration pins.
Control voltages are converted to MIDI messages by the MIDI module (more on that later). Standard in-guitar controls include a 5-way switch for patch settings and potentiometers for volume control, EQ, as well as generic parameter control. Other cool ideas such as a ribbon controller for pitch bend may be provided in the future.
The Nexus is designed to be hackable. Space is provided for custom electronics and expansion. There are expansion headers with all 15 audio and CV channels available. The front panel is intentionally kept bare with just the 19 pin connector. The front panel may be populated with buttons, controls and displays.
The I/O module includes six high-quality stereo Neutrik jacks which can be utilized as input or output. By default, the connectors are tied directly to the Main Module Audio Output for a total of 12 output channels. In many cases, however, you do not need all 12 channels so you can repurpose unused back panel jacks as you see fit.
Example 6-string setup: One jack is connected to a sustain switch while another jack is connected to a volume pedal. Yet another jack is connected to a dual foot-switch to provide additional patch/bank select control in addition to that provided by the in-guitar 5-way patch select switch.
The Main Board
The I/O Module
|Audio/CV Selector||These configuration pins set the functions of Channels 9 to 15. For each channel, place a shunt to select either Audio (shunt at the upper two pins) or CV (shunt at the lower two pins).|
|Audio Output||Multichannel Output Channels 1 to 15. |
|Control Voltage||Control Voltage Output Channels 9 to 15.|
|AGND||Analog Ground. Ground path for noise sensitive analog electronics.|
|AVCC||Analog Power. Power supply for noise sensitive analog electronics. LC-filtered with Capacitance multiplier.|
|DGND||Separate ground path for typically noisy digital electronics. LC-filtered.|
|DVCC||Digital Power. Power supply for noisy digital electronics.|
|Power||Main 12 volts power input (1A Max). Reverse polarity protected.|
The MIDI Module converts control voltages (CV) from the guitar to MIDI. The basic firmware supports up to 6 analog and 7 digital inputs. The Multichannel Guitar Input (see Basic Kit) can carry up to seven control voltages or switches. Some of the back panel 1/4″ jacks may also be repurposed to carry additional control voltages or switches. The MIDI output is sent through a standard 5-pin DIN connector at the back panel.
Note: Please do not be confused with MIDI control and MIDI conversion. The MIDI module does not do any pitch tracking and MIDI conversion of the notes.
Nexus is configured to support these MIDI messages:
- Sustain (digital, channel 9)
- Modulation (analog, channel 10)
- Pitch Bend (analog, channel 11)
- Effect 2 (analog, channel 12)
- Effect 1 (analog, channel 13)
- Program Change (analog 5-way switch, channel 14)
- Channel Volume (analog, channel 15)
- Program Change +5 (momentary switch)
- Program Change -5 (momentary switch)
- Program Change +1 (momentary switch)
- Program Change -1 (momentary switch)
- Bank Select +1 (momentary switch)
- Bank Select -1 (momentary switch)
The Main inputs typically come from in-guitar controls. There is really no distinction between switches (digital) and analog. Each expect a control voltage from 0v to 5v. For switches, the threshold is 2.5V. Anything less than the threshold corresponds to logic 0, anything above to logic 1.
Aux inputs are for additional switches only. Unlike the Main inputs, these are expected to be connected to simple momentary switches that shunt the input to digital ground. Aux inputs should connect directly to the MCU board digital ports (see below).
Note: The types of MIDI messages the MIDI module sends are not user configurable. Nexus itself is intentionally kept as simple as possible and does not have a user interface. Typically, your gear or DAW can be configured or “taught” about the type of control sent by a MIDI device and routes that to whatever parameter you need to control.
Pitch Bend is 14 bits, but the on-board analog to digital converter (ADC) is currently limited to 10 bits. MIDI CC messages send both the MSB (most significant byte; 7 bits) and LSB (least significant byte; 7 bits), hence 14 bits too. But again, the ADC is currently limited to 10 bits.
Here are the actual MIDI messages sent:
- Pitch Bend: 0xE0, LSB, MSB
- Program Change: 0xC0, VAL
- Control Change: 0xB0, CC(LSB), LSB, 0xB0, CC(MSB), MSB
- channel_volume: CC(MSB): 0x07, CC(LSB): 0x27
- modulation: CC(MSB): 0x01, CC(LSB): 0x21
- effect_1: CC(MSB): 0x0C, CC(LSB): 0x2C
- effect_2: CC(MSB): 0x0D, CC(LSB): 0x2D
- Sustain: 0xB0, 0x40, VAL
- Bank Select: 0xB0, 0x00, VAL
Program Change is a special form of control voltage that divides the 0-5v range into 5 voltage zones corresponding to a relative value from 0 to 4. This value (0 to 4) is then added to a base number which defaults to 0 but can be incremented and decremented using Aux switches. For example, if the base is zero (default from the factory), then the 5-way switch generates program change 0 to 4. If the base is 10, then the 5-way switch generates program change 10 to 14.
Tip: If you use the in-guitar 5-way switch, use foot switches to increment and decrement the base program change setting so you can access all 127 program change settings that MIDI allows. Use bank up/down switches to give you even more control.
For the tech savvy hacker, it is possible to re-program the firmware using the supplied Spy-Bi-Wire JTAG programming pins. The code is available as Open Source. The code is intentionally kept as simple as possible using Arduino (Energia) “sketch” and C++ to program the on-board MSP430 MCU.
The MIDI Board
|MCU Module||MCU board piggybacks using these pins (see MCU Board below).|
|Digital Ground||Separate ground path for typically noisy digital electronics.|
|5v||Regulated 5 volts supply (150mA max) from the board’s voltage regulators.|
|3.3v||Regulated 3.3 volts supply (150mA max) from the board’s voltage regulators.|
|12v||Regulated 12 volts supply from main board.|
|CV Input Pins||Control voltage inputs (0v-5v). These are converted to 10bits digital using the MCU’s on-board ADCs. For switches, the threshold is 2.5V. Anything less than the threshold corresponds to logic 0, otherwise, logic 1.|
|NC||Unused. Reserved for future expansion.|
The MCU Board
The MIDI module includes a small, general purpose MCU dev-board based on Texas Instrument’s MSP430. The dev-board is compatible with Arduino Software & Libraries. All MCU pins are provided. The MCU’s main task is to convert control voltages and switch inputs to MIDI as defined in the specification above. But being general purpose, you can use it to perform other tasks. The MCU is programmed using the supplied Spy-Bi-Wire JTAG programming pins.
|Digital I/O||P1.0 – P3.0|
|Analog In||P1.0 – P1.7 (10 bits SAR ADC)|
|UART||Tx: P1_1, Rx: P1_2|
|I2C||SCL: P1.6, SDA: P1.7|
|SPI||MOSI: P1.7, MISO: P1.6, SCK: P1.5, CS: P2.0|
|VCC||4.3V – 16V|
|Programming Pins||3.3V, GND, RST, TEST|
|Microcontroller||MSP430G2553 – Low Power 16-bit MCU|
|Input Supply Voltage||4.3V – 16V|
|GPIO Pins||24 pins (3.3V, 2mA max)|
|Analog Input Pins||8 channels (10-bit ADC, 0 to 3.3v)|
|Communication Peripherals||UART, SPI, I2C, IrDA|
|Programming and Debugging||Spy-Bi-Wire JTAG|
|Board Dimensions||38 mm x 25 mm|
For cabling, we have a LEMO compatible multichannel cable.
These are metal, self-latching multipole connectors with an alignment key and gold plated contacts. These are robust yet easy to use with its circular profile and push-pull self-latching technology. The connector itself is a mere 15 mm (0.6 inch) in diameter.
- Push-Pull self-latching
- Color coded alignment key
- Brass (chrome plated) shell and collet nut
- Nickel plated brass latch sleeve and mid pieces
- Brass (gold plated) Inserts (contacts)
- 15 mm diameter (20 mm nut)
- Multipole 19 pin contacts
In-Guitar Control and Connectivity
With up to 15 channels of Audio or up to 7 channels of control voltages, the Nexus offers a lot of versatility. There are many ways to set up your guitar with a Nu multichannel pickup alongside other standard monophonic pickups.
But how do you control an N-channel system? Take the master volume for example. Adding a master volume to a traditional monophonic guitar is easy. A simple potentiometer will do. For an N-channel instrument, best of luck if you can find an N-ganged potentiometer. Or how about switching? How do you switch N-channels? The traditional Strat-style 5-way switch and 3-way Les Paul toggle switch will no longer be adequate.
The most versatile solution of course is MIDI. Use your external gear (e.g. digital mixer or DAW) to control master volume or patch settings or just about any parameter you wish.
Control voltages may be sent along with audio through any of the 15 channels via a LEMO compatible connector. Control voltages come from specialized potentiometers and switches. This will be converted to MIDI messages in the Nexus by the MIDI module.
19-Pin Output Jack
The Guitar connects to the Nexus via LEMO compatible multi-pin connectors. This connector is designed to plug directly to the Nu Internal Breakout. This multi-pin connector replaces the standard ¼” mono Jack found in traditional guitars.
The CV Potentiometer is a simple voltage divider from a 10V or 5V source. You may configure the source voltage (default 10V, shorted 5V) using solder pads (default: 10V, shunted: 5V). This default is chosen for compatibility with the Nu Internal Breakout which has a readily available 10V supply for standard active pickups.
Use one or more Multi-purpose CV control potentiometer to send control voltages for volume, pan, effects, pitch-bend, modulation, etc.
CV Potentiometer Pinout
|VCC||10V or 5V (depending on solder pad configuration)|
|Out||Voltage Divider Out (Potentiometer Wiper)|
This Project is entirely Open Source. The designs (schematics, PCB layout, software, bill of materials, CAD drawings) are freely shared and 100% free, under the Creative Commons Attribution-ShareAlike 4.0 International License. Source code is distributed under the MIT License.
All hardware design files and source code are available in the Nexus Github Repository: https://github.com/cycfi/nexus
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