This project is a redesign of the open source FM synthesiser PreenFM2 created by Xavier Hosxe.
Improvements from the original version:
fewer and cheaper components
single PCB (cheaper)
4 separate audio channels (on 2 stereo 6.3mm audio jacks)
stereo DAC chip PCM5102 can be driven directly by the I2S processor's interfaces (faster);
sample resolution can be increased to 24 or 32 bits
headphones stereo amplifier (up to 2 × 175mW on 8ohm loads) on stereo 3.5mm jack
one extra encoder and separate encoder switches
all buttons and encoders are handled by a separate STM32F1 processor so they don't need to be constantly scanned
an interrupt (Srf_Int) is generated when a new surface event is detected and the main processor can then interrogate the slave STM32F1 for changed inputs with just a fast SPI transaction
digital volume control (range is -46dBu to 2dBu).
global volume is controller by encoder5 independently by STM32F1 which also converts rotations to a logarithmic scale STM32F1 can remember previous volume settings and restore them
The board is about 15×15mm in size and has all the necessary connections
routed to .1" pins. The SPI on one side of the board needs to be used to
program a special MIDI bootloader which after reset will listen for SysEx
messages to load and program the main code running on this AVR chip. The
microSD socket on the back of the PCB won't be used and it's better to mask it
with tape to prevent any contacts with the components inside the Volca.
The board is small enough to fit near the MIDI In socket, where to optocoupler
is located. Just four wires are needed to solder it inside: Vcc and Gnd to
ground and 3.3V, Rx to the optocoupler output and Tx to the corresponding
serial input of the ARM processor. The original trace between the optocoupler
and the ARM processor needs to be cut.
The crystal on the OpenLog board is 16MHz which divides exactly to get the
MIDI bitrate of 31250 bauds. The processor main code will intercept all the
NoteOn messages "9n kk vv" and send before another Velocity message of
"Bn 29 vv" followed by the unchanged NoteOn.
The insertion of this AVR processor will delay all the MIDI messages with
about 1ms which is imperceptible. Also to avoid changing the global velocity
before each played note first it should compare the new velocity to the previous
one and ignore it if it doesn't differ more than +/-5 (velocity ranges from 0-127).
This mod will work for all of the Volca synthesizers series and also has the
advantage of easy reprogram the code inside the AVR (bypassing it entirely
if so needed) via SysEx messages using a free program like like MIDIOx on PC
or SysEx Librarian on Mac OS.
To dump or program the Volca (or recover from a brick event) connect a J-Link
interface to the SwD 6 pin connector and issue the following commands in the cmd
line utility (usually JLinkExe):
si swd
speed auto
device MB9BF164L
connect
-- then --
savebin fw_name.bin,0x0,0x40000
--- or ---
loadfile fw_name_1.bin 0x0
loadfile fw_name_2.bin 0x20000
