BareMetal RasperryPi Port of the PreenFM2 Synthesizer https://github.com/Ixox/preenfm2 included extrafilters from https://github.com/pvig/preenfm2

The PreenFM2-Synth looked really good but i was not happy with the STM32-uController... I really like the STM32 uControllers and did some Projects with them but i was constantly annoyed with the buggy and rather strange Software-Support of STM (HAL-Layer rather kludgy, CubeMX-Code-Generator produces buggy code, poor support etc, its a pitty because i really liked the STM32F-Disco Kits, but an SPI BUSY Errata that costed me to much time & hair....). Then i stumbled over the Circle-Framework for BareMetal Programming of the Raspberry Pi https://github.com/rsta2/minisynth and ported PreenFM2 with it to Raspi 3B, HiFiBerry DAC+ Audiocard and a MAX6957 for the connection to the encoders and buttons. (see hardware/Encoders.cpp for details) and got a latency of about < 3ms @ 48kHz

The Port is a really ugly hack, was does not work:

• Bootmode
• usb-midi doesnt work (now...), MIDI is done thru the serial-interface UART0 (GPIO 14/TX Pin8, GPIO 15/RX Pin10 )
• Oled-Timer
• Clock-Led
• probably more :-(

About the code, i wanted to get this quick up and running without changing to much, so i made some evil things...

Its compiled as 64Bit, just put the kernel8.img, bootcode.bin, cmdline.txt, start.elf & fixup.dat in the root of the sd-card of your Raspi 3B(+) and off you go!

The best way to build it is to get https://github.com/rsta2/minisynth

set in circle/Rules.mk

AARCH	 ?= 64
RASPPI	 ?= 3
PREFIX	 ?= doesent_care-
PREFIX64 ?= /home/where/your/crosscompiler/lives/gcc-arm-8.3-2019.03-x86_64-aarch64-elf/bin/aarch64-elf-

Change the Sounddevice in src/config.h

#define USE_I2S

and build it to make & test the framework (it won't sound (HiFiBerry is not supported), just to test if its compiling with the library) the addon/fatfs must also be build.

To Compile the RasPreenFM2

in the minisynth directory do

git clone https://github.com/styro2000/RasPreenFM2.git RasPreenFM2 to get the Source

i had to change line 63 in circle/Rules.mk to

ARCH ?= -DAARCH=64 -march=armv8-a+fp+simd -mtune=cortex-a53 -mlittle-endian -mcmodel=small

and the flags (line 121) to

CFLAGS += $(ARCH) -Wall -fsigned-char -ffreestanding -mstrict-align $(DEFINE) $(INCLUDE) $(OPTIMIZE) -g

i added at line 156 at the clean: section find . -type f -name '*.o' -delete to clean up all subdirs

cd in the RasPreenFM2 directory and do make

What i didn't find out was a compatible assembler instruction for

#define __USAT(ARG1,ARG2) \
({                          \
  uint32_t __RES, __ARG1 = (ARG1); \
  __ASM ("usat %0, %1, %2" : "=r" (__RES) :  "I" (ARG2), "r" (__ARG1) ); \
  __RES; \
 })

but it worked without :-) (but if somebody could give me a hint it would be great!)

The soundbuffer-calculating async in the main loop didn't work to well, gave glitches while screen writes, so i put the calculating in the DMA-Irq of the sounddevice. A strange thing happened then, it crashed in prepareMatrixForNewBlock in the LFO Section while loading a new patch (during the usb-file operations). A long and fruitless search why? didnt help so i did a really ugly hack and disabled the prepareMatrixForNewBlock() while loading patches.

Other things i changed:

• Changed BLOCK_SIZE to 16 for smaller latency, gave me first horrible aliasing in the higher notes, changing Osc.h Line 128 for (int k=0; k<32; ) to for (int k=0; k<BLOCK_SIZE; ) { seemed to help

  the Circle-framework has to be changed at lib/i2ssoundbasedevice.cpp accordingly to https://www.raspberrypi.org/forums/viewtopic.php?f=72&t=90130&sid=977ea3a6593937475847a5a61421587e&start=225#p1568424

  @@ -71,6 +71,9 @@
  #define TXC_A_CH2POS__SHIFT	4
  #define TXC_A_CH2WID__SHIFT	0
  
  +#define DREQ_A_TX__SHIFT	8
  +#define DREQ_A_TX__MASK		(0x7F << 8)
  +
  @@ -263,6 +266,8 @@ boolean CI2SSoundBaseDevice::Start (void)
  
    // enable I2S DMA operation
    PeripheralEntry ();
  +	write32 (ARM_PCM_DREQ_A,   (read32 (ARM_PCM_DREQ_A) & ~DREQ_A_TX__MASK)
  +				 | (0x18 << DREQ_A_TX__SHIFT));
    write32 (ARM_PCM_CS_A, read32 (ARM_PCM_CS_A) | CS_A_DMAEN);
    PeripheralExit ();

• the BPM/LFO/Env/ARP/etc internal clock changed to match the smaller BLOCKSIZE

• the Operators start a 0°, didn't like the Click to much at 90°

• When the same note is struck again it allocates a new voice (if voice > 1) instead cutting off the voice. On NoteOff it switches of the longer running voice first.

• The ComboName is memorized on load/save and used as default for saving

• changed the glide-times to longer

• integrated the extra filters (thanks to Toltekradiation!)

• made the Filter polyphonic (Filter per Voice) instead paraphonic (Filter per Timbre)

• some checks reading the settings values and prevent loading DX7-Patches > 31 over MIDI ProgramChange that would crash the Raspreen.

• put Polyphony to 20 and Operators to 120 (takes about 160us to calculate on full load with Filter h3o+ on Voice, ca 45% of the Sound-DMA IRQ Rate of 333uS, with Filter LP it takes about 120uS)

• the length of the Stepsequencers can be set from 2-16 steps (value 1 over F on the reset-step)

• new modmatrix sources p1u,p2u,p3u,p4u -> unipolar performance control (0.0 - 1.0) cnst -> constant (1.0)

• display is flipped 180° (for contrast reason of the used DFROBOT SKU_DFR0550 5" display (colors looks better upside down))

• clicking an encoderbutton in the performance-screen opens a volumes-screen to adjust the volumes of all 4 timbres

• midi ctrl 7 adjusts the volume of the timbre

• an 'X' is shown in the upper left corner when the audio is clipping

known Bugz:

• sometimes it ignores a note, no idea what it could be, will take probably long to figure out, till then it goes as "mandatory humanize"-function :-)

ToDo:

• usb-midi

• sort of bootmode to access the SD-Card or USB-Stick from outside, the RaspberryPi does not have a USB-Device/OTG functionality, so probably over ethernet or so, but in the meantime popping them out and putting them in the computer to transfer data is not so terrible....

• nicer GUI

  actually i find the usability very good for the limited amount of Buttons/Encoders and the editor is great, and changing this would take quite some time so this is not on the priority-list

Hope that anybody has fun using the synth and many thanks for bug reports!

Many thanks goes out to Xavier Hosxe for the great PreenFM2-Synth and R. Stange for the amazing Circle Framework!

• THIS IS ALPHA-SOFTWARE! DONT USE IT FOR LIVE PURPOSES!
• I AM NOT RESPONSIBLE FOR ANY TRAGEDIES AT GIGS :-)