Meet The Nexus. This small, hacker friendly breakout box connects your multichannel guitar to the outside world using a specialized (LEMO compatible) multi-pin connector for up to 12 channels of audio and 7 channels of analog control voltages for by-wire 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.
19 Pin Connector
1) Audio signals from the multichannel Nu pickup together with outputs from one or more monophonic pickups, 2) control voltages, and 3) power are sent through a single 19-pin LEMO compatible connector. The pinouts are as follows:
- pins 1 to 8: audio
- pins 9 to 12: audio or control voltage
- pins 13 to 15: control voltage
- pin 16: +12
- pin 17: -12
- pin 18: 48v (optional)
- pin 19 is for signal ground
- shield; main ground
There are no fast-moving digital signals. Only audio and analog signals plus clean power (using ultra-low noise voltage regulators) pass through the cable. That is intentional. The rationale is that we want to keep the signals as pristine as possible. High-frequency digital signals can and will corrupt the audio signals through crosstalk along the cable.
For even better noise immunity, signal ground is separate from power ground. We utilize signal ground to remove any remaining power supply glitches and other environmental electrostatic noise. The signal ground has the same impedance as any of the outputs. It is not directly connected to power ground, instead through an impedance balancing resistor —or what’s called asymmetrical balanced. You can use signal ground just as you would any other audio signal. For example, it can be used as common in a pseudo differential analog to digital converter.
Up to 12 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 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 12 can be configured to receive either audio or control voltage (CV) using configuration pins. The buffered outputs are available on 6 stereo jacks (Neutrik) at the back.
Channels 11 and 12 can also be configured as stereo mix of channels 1 to 8. There is a header block for assigning each of the first 8 channels to either hard left or hard right. This header block can of course be utilized for more complex mixing, for example with volume, pan and EQ per channel.
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 12 can be configured to receive either audio or control voltage (CV) using configuration pins.
Control voltages are converted to MIDI messages. 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. A 16 bit MCU (MSP430) performs the CV to MIDI conversion.
Please don’t be confused with MIDI control and MIDI conversion. We are not doing any pitch tracking and MIDI conversion of the notes.
The Nexus is designed to be hackable. Space is provided for custom electronics and expansion. There are expansion headers with all 16 channel signals available. Other signals (e.g. stereo configuration and outputs) may be extracted from any of the configuration headers. The front panel is intentionally kept bare with just the 19 pin connector. The front panel may be populated with buttons, controls and displays.
There are programming pins for the MCU. It’s basic purpose is to convert CV to MIDI, but this general purpose MCU can be utilized for other tasks as well. All I/O pins from the MSP430 MCU are available.
The design strategy for the Nexus is to keep it as simple as possible: the MVP (Minimum Viable Product). We want a system that is minimal, but nevertheless complete. It is an evolutionary step, but one that can be utilized now and incrementally evolved in the future. It is meant to be fully expandable, especially for tech-savvy users. It can evolve as your need evolves over time.