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Build the "REMI Synth" (mk3) A DIY Project by M.J. Bauer |
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REMI Synth Module complete with LCD, button panel and pot control panel |
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Overview The REMI Synth is a digital monophonic MIDI-controlled sound synthesizer designed primarily for use with electronic wind instrument (EWI) MIDI controllers, in particular the REMI 2 handset. Provision of a standard 'MIDI IN' port allows the synth to be played by any MIDI controller, for example, a keyboard or another EWI with a standard MIDI output. Using a low-cost MIDI-USB adapter, the REMI synth can also be controlled by a computer running music software such as a MIDI sequencer. Features
The REMI synthesizer is implemented almost entirely in software, requiring minimal circuitry outside of the microcontroller chip - just a low-pass filter in the audio output circuit. To generate audio tones, the synth uses a dual "wave-table oscillator" algorithm offering a variety of waveforms which can range from very simple to rich and complex sounds, some resembling acoustic instruments. A PIC32 on-chip timer module can be used to generate a PWM audio output signal. The PWM "DAC" has a resolution of 11 bits, which gives adequate sound quality. For improved sound quality, an optional 12-bit SPI DAC chip (MCP4921) may be fitted. Software DSP computations use 32-bit normalized fixed-point numbers with 20-bit fractional part, allowing the application to run on 32-bit microcontrollers without hardware floating-point capability.
Synth Software Model The synth model comprises a pair of wave-table oscillators which use independent wave-tables. The two oscillator outputs are fed into a "mixer" which scales and adds the two signals in a variable ratio. The mix ratio can be fixed, or it can be varied in time as the note progresses. The "contour" envelope shaper may be patched in to control the oscillator mix ratio. This capability is used to implement "waveform morphing", a technique used to vary the harmonic content of the sound with time. Waveform morphing can be used to realise a range of effects beyond what is possible to achieve with filtering techniques. The pitch of the secondary oscillator can be "detuned", i.e. increased or decreased relative to the primary oscillator. The "detune" factor is a patch parameter having units of "cents", so that the detune resolution is 1/100th of a semitone. If the detune factor is a fraction of a semitone, typically in the range 3 to 30 cents, the resulting effect is known as "Voix Celeste" (heavenly voice). This effect greatly enriches the soundscape possibilities of the synthesizer. In addition to the two wave-table oscillators, a low-frequency oscillator (LFO) is provided. The LFO can be used to modulate the audio oscillator frequency to implement vibrato, or the LFO can be used to modulate the oscillator mix ratio. In the latter case, the mixer envelope output level determines the modulation depth. The
synth can be configured to control the
amplitude (loudness) of the note-in-progress in one of two ways: The first option (1) is intended for MIDI keyboard controllers, while the second option (2) is intended for MIDI wind controllers (EWIs), in particular the REMI-2 handset. The firmware includes several "pre-defined" synth patches providing a good variety of instrument sounds. Any pre-defined patch may be assigned to any of the 8 Presets via the user interface (GUI or CLI). How the Synthesizer is Patched The REMI synth can be programmed (patched) by the user to create a new sound, without needing to modify and re-compile the firmware. Instead of using patch-cords, knobs and switches like a modular synthesizer, however, the REMI synth is patched by means of a set of numeric parameters... (see table below). A CLI command "patch" is provided for the purpose of setting patch parameter values. A user-created patch can be saved in non-volatile memory (EEPROM) for later recall. The stored "user patch" may be assigned to any of the (8) instrument "Presets". The optional "Pot Control Panel" allows immediate adjustment of patch parameters. Table 1: REMI Synth (mk3) Patch Parameters
The firmware also provides a utility for users to create their own wave-tables. A CLI command "wav" is provided for this purpose. REMI synth makers who are prepared to re-compile the firmware can add their own patches and wave-tables, limited only by the amount of MCU flash program memory. CLI commands "patch" and "wav" include options to dump patch parameters and wave-table data as C source code definitions. A future firmware revision is planned to support multiple "user patch" parameter sets (stored in EEPROM). Details of REMI synth functionality and operation using the console CLI and (optional) front-panel GUI are provided in the Synth User Guide. Sample sound clips made with the REMI synthesizer (To download a sound clip: Right-click on the name and choose 'Save Link As...')
[1] To be replaced with a better clip demonstrating the synth's response to breath pressure (CC02), this sample at least gives you an idea of the sound produced with the simple "Recorder" patch. Using "additive synthesis", the wave-table was populated with harmonics present in the spectrum of an acoustic recorder. [2] In this sample, each of the two parts was played by a different REMI synth controlled by a MIDI sequencer. The bass part was produced by mixing two filtered sawtooth waves, one slightly detuned, to create a sound resembling that of a vintage analogue synthesizer. (The patch is called "Vinyl Cello".) [3] This clip was produced using PreSonus 'Studio One' DAW (Prime - free version). The lead voice is a REMI synth with the "Reeded Woodwind" patch, modified to use the Contour Envelope instead of Expression (CC02) to control wave-form "morphing". All other instrument voices were played with "Presence" - the software synthesizer incorporated in Studio One. [4] Same setup as clip #2, except the melody part comprises various "Hammond-ish" organ sounds changing during the sequence. Hammond organ sounds are easily synthesized (approximately) using wave-tables, because the organ tone-wheel mechanism is based on "additive synthesis". Accurate emulation of the Hammond tone-wheel mechanism, electronically, is not so simple. Construction Construction of the PIC32 synth (mk3) is based on the PIC32-PINGUINO-MICRO module (PIC32MX440 MCU) from Olimex (pictured below), priced at €9.95 (AU$17 approx). This module is available from major suppliers, e.g. Digikey, Mouser, etc, and from Olimex directly. (Check freight charges before commiting to order!)
The Olimex MCU module is mounted on a base-board (PCB) named "MIDI Audio Module" (MAM), designed by REMI project collaborator Jean-Pierre Meyer (in France). The MAM board has provision for MIDI IN and MIDI OUT interface circuits, audio output circuits (PWM or SPI DAC), I2C EEPROM, etc. The board accepts connectors to interface various on-board or external sub-assemblies including: USB/UART adapter (console CLI port), headphone amplifier, graphics LCD module (128 x 64 pixels) and a control panel with 6 potentiometers (to set patch parameters in real time). The board is powered by a 5V USB plug-pack (e.g. phone charger). It can also be powered via the USB/UART bridge module.
MAM board assembled The complete synth build incorporates a front-panel user interface (GUI) consisting of a low-cost monochrome graphic LCD module and 6 push-buttons. The MAM board is designed so that the LCD module and push-buttons can be mounted on piggy-back PCB's which plug into connectors on the back (underside) of the main board. View/Download detailed Construction Notes (PDF)___
REMI Synth Module complete with LCD, button panel and Pot Control Panel How to obtain PCB's to build a REMI synth Gerber files for PCB fabrication are available to download... see links at end of page. As well as the MAM base board, there is a piggy-back board to carry the LCD module and 6 push-buttons. To reduce the parts cost and to simplify construction, the LCD module may be wired directly to the base board and the buttons mounted on a piece of prototyping board. This approach allows more flexibility for location of the LCD module and button panel in the enclosure. If
there is sufficient interest from prospective makers, blank boards may be made available for sale. Firmware REMI firmware is built using Microchip PIC development tools - MPLAB.X IDE with XC32 compiler, free to download from Microchip's website. If you intend to modify or extend the firmware, you will need these tools. Otherwise, you just need to install the PIC programmer application (IPE, included with MPLAB.X download) on your computer. Refer to the synth Construction Notes for more details on firmware installation. Programming Tool A PIC programming
tool, Microchip PICkit-3, is required to install the synth firmware.
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If you are interested in building a REMI synth module and/or REMI 2 handset, or if you have enjoyed following the project here, kindly send me an email. Support is offerred to readers who wish to build a REMI or similar electronic music device. [MJB]
Last update: 12-July-2023 MJB Resources for Embedded Firmware Development ___ |
