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NucleoSynth 6-Voice MIDI Virtual Analog Synthesizer

Document created by Christopher Pappas on Feb 1, 2017•Last modified by Christopher Pappas on Apr 24, 2018

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Project info:

An expandable feature 6-voice virtual analog synthesizer built using just one STM32F7 Nucleo-144 board, one stereo audio jack, and a USB-Micro-B to USB-A adapter. Play directly thru USB via any keyboard equipped with a USB-MIDI port (Host). Play directly from your PC thru USB using any Sequencer software (Device). Play directly thru Serial MIDI with any keyboard equipped with a MIDI OUT 5-pin Din jack using additional MIDI Input Interface circuitry (MIDI). Daisy-chain multiple boards together for increased polyphony/multi-timbrality (Slave). Play using a standard 8 x 8 x 2 Keyboard Matrix Keybed (Keyboard).

Specifications:

* 6 voices of polyphony.
* Dual oscillator per voice.
* VCA (with ADSR) per voice.
* VCF (4-pole lowpass with ADSR) per voice.

* Resonance
* Master tuning.
* Waveform select saw/saw, saw/square, or square/square.
* Oscillator mix.
* De-tune.
* Three separate LFOs for Modulation, PWM, and VCF.
* Scale for upper oscillators.
* PW/PWM on the square wave.
* Separate adjustable keyboard follow for ENV and VCF.
* Keyboard velocity routable to VCA and VCF.
* VCF envelope level.
* Editable parameters.
* Stereo simulation effect.

* Serial Interface allows daisy-chaining several boards together to increase polyphony or multi-timbrality

Hardware components :

1 - NUCLEO-F746ZG - STM32 Nucleo-144 development board.

or

1 - NUCLEO-F767ZI - STM32 Nucleo-144 development board.

1 - Stereo audio jack.

1 - USB-Micro-B to USB-A adapter.

Software Tools :

Keil uVision 5 (Free Evaluation Version)

Source Files:

Nucleo Synth USB Host Version 0.8.1 Project
Nucleo Synth USB Device Version 0.8.1 Project
Nucleo Synth Slave Version 0.8.1 Project
Nucleo Synth Serial MIDI Interface Version 0.8.1 Project
Nucleo Synth Keyboard Matrix Version 0.8.1 Project
Nucleo Synth Sound Demo Wave file
Nucleo Synth ReadMe text file

Version Control:

* Version 0.8.1: Added a SelfTest subroutine to play random triads. Does not requires an external MIDI USB connection.

* Version 0.8.0: Fixed a massive bug that caused the VCF to close completely, resulting in no sound!

* Version 0.7.9: Added a high-speed serial interface that allows daisy-chaining several Slave boards to a Host board for increased polyphony/multi-timbrality. Added a separate LFO for the VCF.

9 people found this helpful

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21 Comments

Mr Bluexav Feb 18, 2017 12:17 AM
Great project !
I'm looking forward to hearing some sounds !
Are you using the internal 12 bits DAC ?
Cheers
(I'm the man behind Dekrispator...)

Xavier
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- Christopher Pappas @ Mr Bluexav on Apr 19, 2018 1:37 PM
  Hey Xavier,
  
  Thanks for your reply. Yes I am using the internal 12-bit DAC simply to cut cost, and to keep latency as low as possible.
  I am satisfied with what can be accomplished with just a $23 board.
  I am hoping that other users can have a good starting point to begin, and make additions and improvements to the code.
  Your Dekrispator project was very inspirational for me.
  I just wanted to implement a minimum of 6 voices of polyphony without having to add any external hardware to the Nucleo board.
  
  Thanks again, Xavier!!
  
  Christopher
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Deven Corella Oct 10, 2017 6:12 PM
Awesome looking project! I was wondering, how difficult would it be to convert this to a mono synth? I have not yet had a chance to examine your code for this, but I do plan on getting around to it.
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- Christopher Pappas @ Deven Corella on Oct 12, 2017 2:05 PM
  Yes, it is very easy to make all voices play simultaneously in mono mode. It will just require a quick modification to the play_note routine so that it populates the synth engine's six voices all at the same time.
  
  By the way, I just added a ReadMe text file to the project which should make it much easier to get up and running quickly. Check it out!!
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Jason Michel Lambert Oct 16, 2017 6:23 PM
This looks (and sounds) like a great project, any suggestions as to how it can be ported it to the eclipse environment?
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- Christopher Pappas @ Jason Michel Lambert on Oct 17, 2017 1:30 PM
  Since I used ST's project template and STMCube_FW folder structure, it is easy to port over to SW4STM (System Workbench) which is an Eclipse-based IDE.
  
  - From the NucleoSynth project folder, open the AXIM project from the SW4STM folder.
  - Add all of the NucleoSynth source and include files to the project (from the Src and Inc directories).
  - Add all the required sources and includes for the Middlewares and Drivers folder.
  - Build the project.
  
  The reason why I switched from Eclipse to Keil is because the code is very lean (only 19k), and the free evaluation version of Keil allows up to 32k of code. I prefer the features and extra debug tools that Keil has (Logic Analyzer, realtime display of variables, etc).
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  - Jason Michel Lambert @ Christopher Pappas on Oct 17, 2017 3:59 PM
    Ah, good to hear. Right now I have Eclipse configured with the GNU-ARM
    plugin suite so I'll try the approach you are describing within my setup,
    if it does not work I'll go with SW4STM32 (I, just starting with the STM32
    so my options are still very open :-). Keil looks very nice indeed, but I'm
    on a Mac so I have to do with Eclipse...
    
    Thanks a lot!
    
    On Tue, Oct 17, 2017 at 7:30 AM, pappas.chris <
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schweizer.roger Apr 19, 2018 9:48 AM
Hi Christopher and synthesizer freaks
A great project. Thank you for publishing this. That will help many synthesizer freaks. I tried the project and basically it works. I have a headphone amplifier after the DAC's for good sound quality.
Now I have two questions:
1. Does the synthesizer really have no cutoff regulator?
2. The self-test works perfectly and I can hear chords in a very good sound quality without a USB Midi Controller. But when running MX_USB_HOST_Process, the sound is heavily distorted and unusable. Even setting via midi controll commands does not work satisfactorily. The parameters jump strongly.

Do you have an explanation there? Am I doing something wrong ?
I use a nanoKorg midi controller and a Triton Taktil also from Korg.
Both show the same behavior.

Kind regards
Roger
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- Christopher Pappas @ schweizer.roger on Apr 19, 2018 1:31 PM
  Hey Roger!
  
  I am not sure what you mean by the term "cutoff regulator". I am guessing that you mean the VCF cutoff frequency. Use the "VCF Envelope Level" parameter to regulate the depth of the VCF.
  
  If you can email me a sound file recording of the synth while running the USB Host process, I can try to figure out what is going on with the distorted, unusable sound.
  
  A possible explanation would be that the compiler is not using level 3 optimization, and the CPU is just giving up on properly processing the synth engine output, trying to handle the USB processes instead. Are you using Keil uVision 5 IDE to compile the code? Are you using either a Nucleo-F746 or Nucleo-F767 board? Is the CPU cache enabled?
  
  As for the parameters jumping around wildly, the easiest way to determine what is happening is to set a watch for the "paramvalue" in DEBUG mode, so you can actually see the values coming from your midi controller. they should be varying smoothly as you adjust each slider on your midi controller, and not jumping when no slider is being moved.
  
  Sincerely
  Christopher
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  - schweizer.roger @ Christopher Pappas on Apr 19, 2018 2:14 PM
    Hi Christopher
    Thank you for rapid answer !
    
    Yes i mean the cutoff Regulator of VCF. Normally you can control the cutoff with a separate regulator.
    As with your synth the resonance. But cuttoff I see in your synthesizer only in combination with VCF-ADSR.
    
    Yes I'm using the Level 3 optimization (One ELF Section per Function).
    I'm using Nucleo-F767 board.
    
    Is the CPU cache enabled? How can I do that ?
    
    Sincerely
    Roger
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    - schweizer.roger @ schweizer.roger on Apr 19, 2018 2:43 PM
      Hi Christopher
      Now i can see the CPU Cache enable. I'm using your last original code. CPU Cache is enable.
      /**
      * @brief Main program.
      * @param None
      * @retval None
      */
      int main(void)
      {
      /* Enable the CPU Cache */
      CPU_CACHE_Enable();
      /**
      * @brief CPU L1-Cache enable.
      * @param None
      * @retval None
      */
      .....
      void CPU_CACHE_Enable(void)
      {
      /* Enable I-Cache */
      SCB_EnableICache();
      /* Enable D-Cache */
      SCB_EnableDCache();
      }
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      - Christopher Pappas @ schweizer.roger on Apr 19, 2018 7:26 PM
        I think I understand what you mean now.
        
        The VCF Envelope Level controls the overall level of the VCF ADSR applied to the filter. Here is one way to manually regulate/adjust the VCF cutoff;
        
        1. If you don't need VCF Key Follow, set the parameter "VCF Key Follow Level" to zero (no follow) so that the note position does not affect the overall VCF cutoff. My default is set to half-follow.
        2. My default is to have velocity controlling the level of the VCF. You can turn that off by setting the parameter "Velocity Select" to zero, since you want to regulate the VCF manually anyway.
        3. Set the VCF Sustain Level to full on, and the VCF Attack time to zero. This will open the filter all the way as soon as the note is triggered.
        4. Manually adjust the VCF cutoff by using the "VCF Envelope Level" parameter.
        
        Are you still experiencing distorted, unusable sound? If so, just email me a sound file. I will be able to tell what is going on just by listening to it.
        
        Thanks.
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        schweizer.roger @ Christopher Pappas on Apr 19, 2018 9:11 PM
        That's exactly what I meant. The most important parameters for a synthesizer are the cutoff and resonance controls.
        
        Now to my problem: the sound is distorted!
        I checked your information.
        Optimization is Level 3 (One ELF Section per Function).
        The CPU cache is enabled.
        I am using your original code version 0.8.1 without any changes !
        I compiled with ARMMDK 5.
        In the test mode, the sound is good only with USB controllers, the sound is totally distorted.
        
        Please listen to my sound example.
        MP3 is only posibel to send privat ?
        
        Thanks
        Roger
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schweizer.roger Apr 19, 2018 9:18 PM

It is only possible to upload photos or videos.
Can you give me your e-mail? Then I send you the sound as MP3.
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schweizer.roger Apr 19, 2018 9:49 PM
Now in this Video is the sound
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schweizer.roger Apr 19, 2018 9:53 PM
Hi Christopher
Herer in this e-mail it is not posible.
Only text !
I tried it in a video
upload. but that does not work.

here is my privat email adress: roger.schweizer@bluewin.ch
please send me a e-mail and i will response with the mp3 file
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- Clive One @ schweizer.roger on Apr 20, 2018 3:01 PM
  Just an FYI, but you can message people privately via their user page or mouse-over, and you can share a thread with someone, or small group, privately. See "Share" top right of page in thread view.
  1 person found this helpful
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  - Christopher Pappas @ Clive One on Apr 20, 2018 6:15 PM
    Thanks Clive. Sorry about the posts here. I thought I was directly corresponding, but just realized that the STM_Jive email was CC'd and posting everything here on the forum. It should be fixed now!!
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schweizer.roger Apr 20, 2018 6:59 AM
Hi Christopher
Did you get the mp3 file?
I'm curious what you can say about it

One more note:
The power supply of the Nucleo board comes from the ST-LINK USB and the second USB port (USB host) is the Midi controller connected.

greeting
Roger
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- Christopher Pappas @ schweizer.roger on Apr 20, 2018 11:54 AM
  Yes, I had a listen and viewed it on an oscilloscope.
  
  Without a doubt, the "Mixer" routine is missing interrupts. It seems to be runing at half the normal speed, essentially halving the 64 khz sample rate, creating a terrible sounding alias in the output waveform. The filter is also dependent on this sample rate to run properly and in tune.
  
  As a quick test to verify this for sure, change this line (line number 146) to pass the routine 4 instead of 3 and re-compile the code;
  
  /* Start Timer 9 interrupts to trigger DAC updates */
  TIM9_Config(4);
  
  This will slow down the interrupt rate and de-tune the entire synth engine. so notes will sound lower, and the filter will also be tuned down, so the output will sound a bit dull.
  
  If you notice that the output sounds without distortion, then the next step is to check why the USB is using up so much CPU resources.
  
  Let me know your results.
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  - schweizer.roger @ Christopher Pappas on Apr 20, 2018 2:50 PM
    You're right.
    There is a difference in sound quality.
    When set TIM9_Config(?) to 4, there is no audible difference.
    When set TIM9_Config(?) to 20, the distortion is much smaller, but not yet disappeared.
    The sound in this setting is very dull!
    
    I do not know, but I think the USB (CPU resources) should be about the same for all USB MIDI controllers?
    
    How do you manage the board? Directly with MIDI without USB?
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