XIAO ESP32-C3 MIDI Synthesizer – Part 3

By Kevin on June 27, 2025

So, now that the basics are out of the way I'm going to dive into the specifics of the Dream SAM2695 chip itself.

Warning! I strongly recommend using old or second hand equipment for your experiments. I am not responsible for any damage to expensive instruments!

These are the key tutorials for the main concepts used in this project:

If you are new to microcontrollers, see the Getting Started pages.

The Dream SAM2695

I first encountered this device on a breakout board several years ago, called a "GM Mini Module". Various tutorials at the time showed how to hook this up to an Arduino. For example, my specific module had the following suggested application:

The breakout was essentially just the SAM2695 with an additional MIDI IN circuit and DIN socket.

At the time it was a component of the "fluxamasynth" - an Arduino shield containing the chip. Whilst the shield is now discontinued, a number of resources still remain online about it: https://github.com/fluxly/Fluxamasynth

Since then there have been a number of modules released that use this neat synth chip, probably the most recognisable of which is perhaps the M5 MDI Synth: https://shop.m5stack.com/products/midi-synthesizer-unit-sam2695

There is also a M5 module with built-in MIDI DIN socket too.

This latest XIAO device follows in the footsteps of all these other variations but adds the microcontroller, and ESP32-C3, to make a complete "system".

I happen to have three of these devices - the original Dream "GM mini module", M5 synth and the XIAO, so I'll do a post about them together at some point, but the all work in essentially the same way.

The datasheet for the SAM2695 describes its capabilities:

MIDI control processor, serial and parallel interface
Synthesis, General MIDI wavetable implementation
General MIDI compatible effects: reverb + chorus
Spatial Effect
4-band stereo equalizer
Stereo DAC. DR: 86dB typ, THD+N: -75dB typ
Mike input. DR: 86dB typ, THD+N: -75dB typ
Mike echo
64-voice polyphony (without effects)
38-voice polyphony + effects
On-chip CleanWave™ wavetable data, firmware, RAM delay lines
Audio stereo line output

Interestingly it supports a parallel or serial MIDI access. But using serial is by far the easiest as it can link up directly to another microcontroller's UART or an opto-isolator of a standard MIDI IN circuit.

The GM Sound Engine

The SAM2695 has two complete instrument banks and a drum set. The banks are as follows:

CH1-9,11-16: BANKSEL 0: Full General MIDI Sound set
CH1-9,11-16: BANKSEL 127: MT-32 Compatible Sound set
CH10: General MIDI Drum Sound set:
- Program 1: Normal kit, notes 35-81
- Program 17: "Power Set", Normal set + note 38 only
- Program 41: "Brush", Normal set + notes 35-40 changed
- Program 49: "Orchestra", Normal set + notes 27-30, 38-53, 88 changed
- Program 128: "CM 64/32 Partial", range of effects and things - MT-32 like

The device also supports effects and an equaliser. All additional controls are configured using either MIDI NRPN or SysEx message. And there are a lot of messages defined in the datasheet!

There are also a number of control messages which apparently have to be sent over the parallel data interface.

But what I am most interested in is the MIDI implementation for the common range of MIDI messages, the most useful of which (to me) are listed below.

MIDI Message	HEX	Notes	Compatibility
Note On	9n kk vv		MIDI
Note Off	8n kk vv		MIDI
Pitch Bend	En ll hh	14-bit pitch bend data hhll	GM
Program Change	Cn pp		GM/GS
Channel Aftertouch	Dn vv		MIDI
Control Change	Bn cc dd	See table below for specific control change messages
RPN	Bn 65 ...	Registered parameter numbers (see datasheet)	MIDI/GM
NRPN	Bn 63 ...	Non-registered parameter numbers (see datasheet)	GS/DREAM
SysEx	F0 7E 7F 09 01 F7	MIDI Reset	GM
SysEx	F0 7F 7F 04 01 00 vv F7	Master volume	GM
SysEx	F0 41 00 42 12 ... F7	Range of GS specific SysEx messages (see datasheet)	GS
SysEx	F0 00 20 00 00 ... F7	Dream specific "port write" command	DREAM

MIDI Control Change Messages

Command	HEX	Defaults / Notes
Bank Select	Bn 00 cc	0
Modulation	Bn 01 cc	0 (Rate/Depth set via SysEx)
Portamento Time	Bn 05 cc
Channel Volume	Bn 07 cc	100
Pan	Bn 0A cc	64
Expression	Bn 0B cc	127
Sustain Pedal	Bn 40 cc	0 (>63 ON)
Portamento	Bn 41 cc	0 (>63 ON)
Sostenuto Pedal	Bn 42 cc	0 (>63 ON)
Soft Pedal	Bn 43 cc	0 (>63 ON)
Reverb	Bn 50 vv	4 (0..7 = reverb effects)
Chorus	Bn 51 vv	2 (0..7 = chorus effects)
Reverb Send	Bn 5B vv
Chorus Send	Bn 5D vv
All Sound Off	Bn 78 00
Reset All Controllers	Bn 79 00
All Notes Off	Bn 7B 00
Mono On	Bn 7E 00
Poly On	Bn 7F 00	Defaults to poly on power up
CC1	Bn cc vv	cc=00..5F, Fn set by SysEx
CC2	Bn cc vv	cc=00..5F, Fn set by SysEx

To be honest, I'm not sure I quite understand those last two, but that seems to be what it is saying in the datasheet...

There are a /lot/ of parameters accessible over NRPN or SysEx relating to the routing of signals in the device, the effects and the equaliser. At this point I'm just experimenting with the basics above.

Any of the MIDI interfaces allows me to test it out, but using a XIAO SAMD21 in USB MIDI device mode is the easiest as I can just plug it into a computer and fire up MIDIOx and start messing around with some of the above.

Closing Thoughts

There is a lot to the SAM2695, so I can see why it has captured interest once again after a few years seeming almost forgotten.

It would be nice to build some kind of knob-based interface that supports many of the parameters in the synth, but I guess the major application is meant to be for when some kind of "compatible" sound card is required "off the shelf". For that, it would appear that the SAM2695 can be a General MIDI Synth, a Roland "General Sound" (GS) Synth or even a Roland MT-32.

Not bad for a small, relatively easily integrated component!

Kevin