Asynthosc

Analog synthesizer to Open Sound Control (OSC) module

Project description

Analog synth is coming back into hype among music artists. It has a strong culture of vintage, DIY, and customisation.

On the other hand, pro systems use digical control systems, such as DMX for lighting, or OpenSoundControl (OSC) for sound.

The idea for this project is to have a hardware to interface analog synth to advanced sound control systems.

This project is also designed to leverage the possibilities of open-source show control software Chataigne.

There are a lot of solution to control an analog modular synthesizer from a computer, mainly by MIDI interfaces. But not so much options to do the opposite: control others FROM analog modulations. The features and purpose are very close to existing Befaco VCMC, but with TCP-IP for OSC instead of serial for MIDI - so we need to change the microcontroller.

Some inspiration for hardware can be found at open hardware synths from Mutable Instruments company. In its repository we can find the exact reference design, typically for the Yarns V3 MIDI digital interface.

Glossary

Musical Instrument Digital Interface (MIDI)

Communication protocol dedicated to music, used for communication between musical instruments and multimedia devices. Usually it is implemented as a serial connection, with a 5-pins DIN connector.

OpenSoundControl (OSC)

Open, message-based protocol for communication between multimedia devices. Intended to be a next-generation replacement for MIDI. OSC specification 1.0 (Stanford University) It is usually implemented on TCP-IP stack, most usually as UDP datagrams for real time applications. It can be easily transmit thru Ethernet or Wifi at higher bit rate, and a much higher resolution of 32 bits float, than MIDI wich was based on 7 bits integer.

CV - Control Voltages

The analog signals generated by the synthetisers are called CV.

Gate and triggers

Other types of signals generated can be:

• Gate: a "digital" 0 and 1 signal, but with the synthesiser reference analog voltage,
• Trigger: a short-pulse "digital" 0 and 1 signal, to trigger a particular event.

Chataigne

Chataigne is a user-friendly open-source show control software, allowing to connect several multimedia devices, hardware or software between them. We develop belongside this hardware project asynth2osc a custom module for Chataigne, so this piece of software can be use easily to control anything from a real world analog modular, from sound to light, videos and activators of any kind.

Chataigne source code on Github Artist-friendly Modular Machine for Art and Technology - Chataigne Official website

Analog synthetizer

Synthetizer that uses analog circuits and signals to generate sound electronically. See the wikipedia page

Analog modular synthetizer

Modular synthesizers are composed of separate hardware modules for different functions. The modules can be connected together by the user to create a patch. The outputs from the modules may include audio signals, analog control voltages, or digital signals for logic or timing conditions. See the wikipedia page

VCV Rack

Open-source software that emulate analog modular synthetizers. Most of hardware modules are present. Especially, Tramasoft edited a module that is a kind of emulation of our asynth2osc module : cvOSCcv. This virtual module can be use to use real world analog hardware to control the software. VCV Rack cvOSCcv module page

Eurorack

Standardized mechanical format for synthetizer, in a modular 3U rack. It usually fits in a "case" to ease musicians to build their own synthetizer, and have a unique Power Supply Unit and power busses for all the modules.

The dimensions are normalised :

• 3U = 128.5 mm height (U is professionnal standard 19" wide rack unit)
• 1HP = 5.08 mm, width is multiple of 1HP
• Depth should be <2.5mm or <4mm to be shallow

Exemple: Yarns is 12 HP and 3U, so front plate is 128.5x60.96mm. Its bottom PCB is tinier: 59x106,8mm, and has 3 screw holes to fit it to the front plate.

It uses 3.5mm jack for cabling, +/- 12V power supply on a 2x5p header connector, on the right lower corner of the board (facing the bottom). Red side = -12V. Print a | on the board to indicate side. Don’t forget the protection circuit.

Benchmark

Befaco VCMC specs (close features, MIDI only)

• Teensy 32 controller

  • NXP MK20DX256VLH7
  • 2x 16-bit ADC
  • 1x 12-bit DAC

• 8 CV muxed on 1 ADC

Yarns V3 specs (close MCU, MIDI only)

• ST STM32F103CBT6 controller
  • 2x 12-bit ADC (16 channels)
  • 2x DAC
• External DAC DAC8564
  • 1x 16-bit DAC (4 channels)

First architecture (RPI for Wifi, Bluetooth and cabled Ethernet support)

The first prototype includes:

• 8 analog inputs
• Rotary encoder and screen
• 4 digital inputs and 4 buttons for 8 interrupt inputs
• 1 analog audio output
• Pretty front panel with Eurorack format
• 5V power supply input

The BOM is:

• Raspberry Pi 3
• Pi-Plates DAQC2
• Adafruit I2C rotary encoder module, with click and LED #4991
• Adafruit quad-alphanumeric segmented display with I2C #1912
• 4 mini-arcade buttons with retro-lighting

Pi-Plates DAQC2

This RPI hat is the GPIO interface, with a very good analog front end:

• Specs here

• SoC: Microchip PIC16F1517-I/PT

• 16x 10-bit ADC

• Analog input via TI SN74LV4051A 8:1 analog multiplexer

• Open Collector outputs via TI DS2003CM high-current Darlington driver and NXP 74HC 8-bit parallel-in/serial out shift register.

Second architecture

This aims to integrate LEDs, buttons, and DACs on the same board to improve manufacturing and ralaibilty. The Raspberry Pi is dropped and replaced by a custom board with WiFi/TCP-IP SoC and all the analog stuff.

It is possible to use a PCB board as front panel too, as a lot of synthesizer modules do.

For the software part, the board would only output the OSC. I would consider implementing TinyOSC, a lightweight OSC implementation, preferably on Zephyr RTOS.

Requirements

Ethernet

• Full TCP-IP stack implemented, with OSC other UDP protocol.
• DHCP, Link Local and Static IP settings.
• Wired RJ45, optionnal Wifi.

DAC

• This has to be the most crucial part for latency. Quick interface is preferable, like SPI.
• 8 channels input (audio stereo 3.5mm mini-jack).
• Minimum 16 bits, preferably 24.
• Sample rate: ideally at least 100Hz, application can be quite slow (LFO are around 2 Hz). Oversamplig is a must for smoothing and lower transmission rate.
• MCP3008 is an option.

Other IO

• MIDI input (also using stereo mini-jack). Even if a lot of MIDI interface exist, and possibilties to send them other Ethernet with rtp-MIDI interfaces and protocol, forwarding some MIDI events to OSC would be a good feature. With options to set the kind of datas (Programm Changes, Note ON, Control Changes etc) to be tranmitted.

MIDI in uses an UART port, see the Arduino tutorial, or Yarns hardware. Midi-jack pinout:

UART baud rate is 31250 kbps.

This is typa A MIDI on TRS, as explain in MIDI Standard : https://midi.org/specification-for-use-of-trs-connectors-with-midi-devices Warning : There is a type B standard also in use, see https://minimidi.world/

Power supply

Eurorack standard includes a +12/-12V power supply, but the currents are quite low, and sensitive to pollution. So we should rather use a 5V additional external power supply as an option.

Mechanical

It has to fit in Eurorack format. The smaller, the better,but no hard requirement for width. Some work has to be done to fit the connectors, leds, buttons and mechanical parts nicely.

CV inputs

VC analog inputs can wiggle between several voltage ranges:

• -5V / 5V
• -12V / 12V
• 0 / 5V
• 0 / 12V

We need an input stage able to fit those input the 0 / 3.3V ADC microcontroller input.

Attenuators are specific modules used to reshape a specific CV.

The most used input is copied from Mutable Instruments and remaps a -5V / 5V input in a 0 / 3.3V output, but needs a -10V bias to handle those negatives. In Befaco VCMC, an optional voltage gizmo converts 5V power supply to -12V / +12V for standalone behaviour. We could also use an inverting circuit but it will... invert: -5V mapped to 3.3V and 5V mapped to 0V.

Voltages are clamped to 0-3.3V using Shottky diodes, but it should maybe signal saturation to the user somehow.

Better scaling:

• Several input scalers muxed?
• A fancy IC with digital control?
• Single scaler controlled by DAC output (hard)?
• 2-steps converter: rescaler (-12/12 to -5/5), scaler (-5/5 to 0/3).

Diagram

STM32 option

• Only performance range (STM32F7 and STM32H7) have RMII
• STM32H7 has 16-bits ADC, it's perfect

Power consumptions estimates

3V3 bus

• STM32H7 absolute maximum current through VDD/VSS: 620mA

• STM32H7 running at 100deg, VOS1, all peripheral enabled: around 500mA

• ETH PHY, 10BASE-T traffic: 130mA

• OLED display logic: 300uA

• 3V3 LDO: 800 mA

• Fuse: 1.25A, trip 2.5A

• Filter ferrite rating: 3A

• Schottky rating (SS24): 2A

10V

• OLED display lighting: 25mA
• Step-up rating: 2.1A
• Step-up inductor: 2.4A
• Schottky rating (SS24): 2A

12V

• Schottky rating (1N5819W): 1A
• LDO efficiency (vout/vin): 12-5V -> 42%, 5-3V -> 66%
• 800mA @ 3V3 => 800mA @ 12V (not crazy, but linear)

5V

• USB spec: 500 mA in USB 2.0
• USB spec: 900 mA in USB 3.0
• USB 2.0: 500mA on 3V3 after LDO
• USB 3.0: 900mA on 3V3 after LDO Might be useful to have USB 3.0 supply.

Notes about JLCPCB files

Generate files

Some modification are required from Kicad output files (BOM csv and placement files) to be imported to JLCPCB for assembly.

• Change field names in BOM and placement CSV headers
• Fix component rotation

There is a way to automate both with a Python script called jlc-kicad-tools.

This script uses a rotation look-up-table called cpl_rotations_db.csv. Update this file with the one in this repository.

cp [PRJ]/kicad/tools/cpl_rotations_db.csv [INSTALLFOLDER]/JLCKicadTools/jlc_kicad_tools

Then install your version from the local folder:

pip3 install [INSTALLFOLDER]/JLCKicadTools

You will need 2 files in the project folder:

• Netlist file: project.xml
• Placement file: project-all-pos.csv

In order to get the project.xml file, in Eeschema, go to "Legacy BOM", create a custom XML exporter with the following command:

/bin/echo "%I"

Don’t forget to create a "JLCPCB" column with the JLCPCB references, for advanced components. For standard ones, keep their value and package and use the "--include-all-component-groups", otherwise the script will erase them.

Exemple use:

jlc-kicad-tools myproject -o ./assembly --include-all-component-groups

Pricing

Here is an approximate price breakdown for manufacturing at JLCPCB. Some components, such as though-hole or big components, will be ordered separately on LCSC and soldered by hand. The "component" line includes both PCBa and by-hand BOMs. Here, we manufacture 5 boards.

This is a total of 60€ / board.

Bringup

V0.1 known issues

• PHY: Error in LDO routing: VDDCR pin 6 is an internally regulated LDO 1.2V output, not to be connected to 3V3 supply. Add 1 μF and 470 pF decoupling capacitors to ground.
• PHY: nINTSEL should be 0, change LED2 connection to pull-down.
  • REF_CLK_OUT is used to generate the 50MHz clock for RMII
  • This special feature allows to use a 25 MHz crystal with the LAN8742
  • As a consequence, the INT/CLKO pin must be set to clock out, not int
  • This means nINTSEL/LED2 pulled down
• Cannot setup different interrupts for GPIO (EXTI register) having the same numbers but on different ports.
  • See this issue.
  • Issue with A8 rotative encoder pushbutton VS C8 rotary input B
  • Issue with D14 rotary input A and E14 trig input
• Shield REFCLK0: it is a 50MHz line!
• Missing pin 1 dot on diode D2 for manufacturing
• Dot around ethernet PHY: crystal dot too close
• Put poka-yoke or marking to mount USB connector on right side
• Put poka-yoke or marking for buttons
• Reduce jack connectors holes to mount more precisely
• Add testpoints for MDIO?
• Maybe disconnect PHY reset from main reset Reset is pulled low by STLink probe, it has 1V2 value when probe is connected even if not plugged in.

Bringup todo list

• Debug LEDs: OK
• Buttons: OK
• Buttons LEDs: OK, make markings for equipment
• UART2: OK
• Enable 10V: OK
• USB data: OK, make markings for equipment
• QSPI flash: OK
• MDIO bus / RMII / Ethernet: OK after rework, ldo disconnect and led2 pull-down
• Display: OK, verify grey lines, maybe voltage bias issue
• DAC: TBD
• Analog: OK, CVin ripple at +max value (+5V or +12V)
• Trigger: OK, 1.95ms rise time (max 500 Hz trig)
• MIDI optocoupler: OK at 32kHz, bit of an hysteresis at play. Collapses after 150kHz.
• MIDI: OK, tested with Roland HP201 keyboard output
• Supply (12V alternative input, current consumption measurements): TBD
  • 12V input: 5V LDO works, 214mA consumption with ADC app running, no display
  • 5V input: 208mA consumption with ADC app running, no display
  • 5V filtered output is around 4.5V

Meca panel known issues

• First CNC run:

  • Rotary hole exact diameter, add a small margin
  • Button hole too small, about 1mm missing, add margin too
  • Jack holes too wide, wiggling
  • Display TBD, might be too small or not

• Reverse-mount LED options:

  • XZDGK45WT-9 (Green)
  • XZCWD45S-9 (White)
  • XZFBB45S-9 (Blue)
  • XZMDK45WT-9 (Red)