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Digital Signal Processing (or Pi). Adventures in making my Raspberry Pi 3 realtime, and running audio DSP.
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DSPi: An Audio Raspberry Pi Audio module/journey

This repository is me self-documenting my journeys in using a Raspberry Pi as an integral part of my music setup. I usually make alternative/electronic/indie music without a computer, but I don't want to dismiss the benefits of having a computer in my setup. I want to be able to use:

  • VST, VSTis
  • USB MIDI Controllers
  • Guitar Amp Simulators
  • Audio Recording
  • Synthesizer Programs
  • DSP
  • Pure Data Patches/Super-Collider

... without stifling my creativity through excessive use of mouse and keyboard. This necessitates a low-latency environment with tons of connectivity. My plan is to cycle through all of the aforementioned functions through a standardized MIDI CC/Program Change Schema, effectively creating a MIDI-controlled module that is a jack-of-all-trades in my music setup.

I: Installing Raspian

I1: Download Raspian Lite Lite, since you want a minimal os with no gui, really.

I2: Parddy time: Using dd to back up and install Raspian

I use dd, and a sd card reader to manage my raspi sd card. /dev/mmcblk0 is where my sdcard mounts. Correct the below to correspond! Unmount your sdcard, but leave it plugged in. Makes this happen easier.

sudo dd bs=4M if=/dev/mmcblk0 of=from-sd-card.img status=progress

Backs up your SD card to a file called from-sd-card.img

sudo dd bs=4M if=2017-04-10-raspbian-jessie-lite.img of=/dev/mmcblk0 status=progress && sync

Installs Raspian to your sd card.

Feel free to use NOOBS or whatever way you install raspbian. This is just what I do! :)

I3: Making your new pi experience better

touch /media/cheekymusic/boot/ssh enables ssh

sudo nano /media/cheekymusic/rootfs/etc/wpa_supplicant/wpa_supplicant.conf the stuff on the bottom to auto-connect to your networks.

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
country=US # Country code can differ, of course



II: Kernel Building (Cross Compiling)

If you want to process low-latency DSPi, you're going to need a pre-emptable kernel... Sadly, step one is to compile a kernel... Good luck! These instructions are for cross compilation on a linux system (Ubuntu Studio) but should definitely work for other debian-based OS's. I also happen to have an SD card port, which definitely eases Raspberry Pi setup.

II1: Cleaning your kernel dir (Or grabbing a new one it if you're starting from scratch)

cd ~
git clone
git clone
cd linux/
make clean
git clean -f
git reset --hard
git checkout raspberrypi-kernel_1.20170405-1

Make sure, by the way, to check out a commit/release where the kernel version matches (look below for patches, etc) Check this file (and similar commits) for the kernel version. This usually happens after a release and whatnot.

II2: Grab Kernel Patch

cd ~/linux
wget[*.patch.gz] # Replace with patch matching the kernel you grabbed from kernel repo
zcat [patch.file.patch.gz] | patch -p1

Make sure the kernel patch matches the kernel version in the previous step EXACTLY! I've made this mistake, and it prevents the pi from booting. YMMV. I used patch-4.14.71-rt44.patch.gz as of this commit.

II3: Grab configs from raspi

source REPO/bash/kernel.source
make bcm2709_defconfig
make olddefconfig # this sets defaults for a lot of stuff
make menuconfig # Need a large terminal

# Kernel Features > Preemption Kernel (Low Latency Desktop)
# CPU Power Management > Frequency Scaling > Performance
# CPU Power Management > Tick Frequency > 250 MHz

make menuconfig may fail if your terminal isn't maximized.


you can just mv .basicRtKernelConfig ~/linux/.config and use the preconfigured config file in the repo... this was generated with the directions above with the aforementioned kernel/patch versions

II4: One last step before building the kernel--Mis en place

Plug in your Raspian SD card (should definitely work for other distros). Note how it mounts. You really just need to find the /boot and /lib directories.

sudo apt-get install git build-essential make lzop ncurses-dev gcc-arm-linux-gnuebi fakeroot kernel-package dev-essential
# Not sure if we need all of them

sudo yum install #... may work for non-debian systems. The only things that may differ are build-essential and dev-essential.

Edit and kernel.source to match the directories for your system. Note: /media/cheekymusic is where the sd card was mounted. /home/cheekymusic === ~ chmod 755 so you can execute it.

TODO: Figure out if we really need to delete the firmware here.

II5: Building kernel

source kernel.source
cd ~/linux
make zImage modules dtbs -j4 # -j#, where # is CPU cores * 1.5 (of your compiling machine)
make modules_install -j4
mkdir ~/$INSTALL_MOD_PATH/boot/
./scripts/mkknlimg ./arch/arm/boot/zImage $INSTALL_MOD_PATH/boot/$KERNEL.img

grab this firmware if you want wifi (Pi 3 Model B): and copy it into $INSTALL_MOD_PATHlib/firmware

Then run ./ with your sd card mounted.

Try append these to /boot/cmdline.txt if you run into issues:

dwc_otg.speed=1 sdhci_bcm2708.enable_llm=0 smsc95xx.turbo_mode=N

sdhci_bcm2708.enable_llm=0 disables low latency mode for sd card dwc_otg.speed=1 Forces the USB controller to use 1.1 mode (since the USB 2.0 controller on the pi may cause issues with some audio interfaces) smsc95xx.turbo_mode=N Disable the turbo mode for the ethernet controller

Then add the following lines to /etc/dbus-1/system.conf: (INSIDE <busconfig> tags!)

 <policy user="pi">
       <allow own="org.freedesktop.ReserveDevice1.Audio1"/>

This allows the dbus-compiled jack server to run without a GUI running.

II6: Installing music stuff and configuring it

Install this stuff! sudo apt-get install qjackctl jackd2 guitarix aj-snapshot git a2jmidid liblo-tools mididings

  • Jackd2 (jackd2) is audio server
  • qjackctl is a QT-based GUI to manage jackd2 server. There are others if you prefer.
  • guitarix is a guitar amp simulator
  • aj-snapshot is the automatic audio/midi auto connection daemon. May be able to replace this with --jack-autoconnect (and similar CLI flags) on guitarix and amsynth.
  • git to clone this repo
  • a2jmidid for alsa to jack midi bridging
  • liblo-tools for osc support, which is used for audio recording
  • mididings for MIDI translation and script launching on MIDI message

Allow jack server to use realtime priority (it'll ask when you're installing. Say yes.)

Run sudo raspi-config:

  • Alter Boot Options so that raspi turns on with Console with auto-login (3 Boot Options -> B1 -> B2).
  • Set the GPU Memory to 16 under "7 Advanced Options -> A3"

II7: amSynth building from source

Build amSynth on your raspi using the instructions below. It's braindead simple to do. Make sure you checkout a release. git checkout release-1.7.1 is the most recent at the time of this commit (2017/04/29)

II8: Getting Music Stuff to run on boot

  1. git clone ~/DSPi
    Cloning this repo into ~/DSPi allows me to git pull to get any updates, since jackboot points at ~/DSPi/bash/ Hopefully you can do the same!
  2. Move the jackboot script into init.d: sudo cp ~/DSPi/bash/jackboot /etc/init.d/jackboot and make sure it's executable: sudo chmod 755 /etc/init.d/jackboot
  3. Edit ~/DSPi/bash/
    Edit Line 7 of ( -dhw:CODEC ) to match your soundcard (run qjackctl to figure out the name of your sound card)
    Honestly, you might have to experiment A LOT with this line. It has the biggest effect on your audio latency, which is the core of this entire project. You can use qjackctl to help fine-tune settings without busting a blood vessel in your forehead.
    For a fact, if you're not using the UCA-222/202, you probably don't want -S (Force 16-bit, since UCA-222 is 16-bit)
  4. Make sure it's executable: sudo chmod 755 ~/DSPi/bash/ NOTE: This is because the audio stuff needs to run as the pi user, and I can't figure to a better way to do that...
  5. Register it in update-rc.d sudo update-rc.d jackboot defaults
  6. Run manually, then run qjackctl &
  7. Open up the connections menu, and make all the connections you desire. Plug in any midi controllers, and route connections from them, to MIDI-Through (in alsa tab), and from system:1 to guitarix (on MIDI tab), amsynth and jack-rack. Using the seq drivers midi-through allows for easy modification of your midi-autoconnection schema. Most of the time, I just add another connection by hand (once I know the name of the controller in alsa) in the aj-snapshot.xml.
  8. Run aj-snapshot ~/DSPi/configs/aj-snapshot.xml to generate the aj-snapshot file used in
  9. touch jackboot.log if you want to have user access to the logs. Otherwise, root creates the logs, and you'll need to sudo to access them.

If you ever want to remove the script from start-up, run the following command: sudo update-rc.d -f jackboot remove

/etc/init.d instructions paraphrased from resource #4. based on resource #5.

II9: Mididings - starts a python script which assists in switching which dsp is currently running on the pi. The Python script responds to values on MIDI:CH16 PC. Depending on the value, a specific program (or "DSPi") will be ran, and others will be killed:

  • 0: Runs guitarix (Turns off wireless chip)
  • 64: Runs amsynth (Turns off wireless chip)
  • 127: Turns on wireless chip

II10: Run amSynth using native nogui option

Note: As of this commit, amsynth on the raspian repos do not support this option. You must compile amSynth on the raspi to get these capabilities. REALLY easy to do, actually. VERY well documented.

amsynth -x -mjack -aalsa -r48000 -c9 -p4

  • -x is no gui
  • -mjack forces jack midi
  • -alsa forces alsa audio
  • -c9 makes amSynth respond to midi channel 9
  • -p4 is max of 4 notes of polyphony
  • -r48000 runs amSynth at 48000 sample rate

Also included amsynthSettings, contents can go right into ~/ for midi mapping (Line Number + 1 = Midi CC).

A: Jenkins can do up to II5 for you :)

  1. Set up jenkins to use /bin/bash for shell scripts. (Check in settings...)

  2. Get git plugin, and pipeline plugin.

  3. Make new project based on this repo, pointing at Jenkinsfile.

  4. Do a sudo visudo and add jenkins ALL=(ALL:ALL) NOPASSWD:ALL to your sudoers.

TODO: Should probably not give ALL these permission to jenkins...or use docker?

WARNING: Jenkins will not backup your SD Card. Consider doing it yourself. :)

B: Ansible can do the rest for you!

Install ansible on your system.

sudo nano /etc/ansible/hosts on your system, and add the following:

192.168.x.y #ip address or hostname of your pi

Go onto your pi, and add your jenkins/ansible machines ssh key to your pi Might have to do:

eval `ssh-agent -s`
sudo reboot

But I'm not sure

run ansible-playbooks raspi-playbook.yml -k, supply pi password, and ansible will handle pushing your key from ~/.ssh/

If you don't have one, use ssh-keygen to generate one, and accept all defaults (No password, etc)

Now Jenkins (via ansible) should be able to enforce all configs on your pi for you :)

Test connection by running the following command, and seeing the following response.

cheekymusic@cheekymusic-Q550LF:~$ ansible all -m ping -u pi --private-key ~/.ssh/ --become | SUCCESS => {
    "changed": false, 
    "ping": "pong"

C: MIDI Reference

  • Channel 1
    • MFTT CC 0-63 MIDI Out and Updates
  • Channel 2
    • MFTT CC 0-127 Switch Out and Indicator Light In
  • Channel 3
    • MFTT CC 0-63 Switch Animations & Brightness
  • Channel 4
    • MFTT CC 0-3, 8-31 Banks and Side Buttons
  • Channel 5
    • MFTT CC 0-63 Shift Out and Updates
    • Guitarix CC 64-103 Guitar Amp and EFX Params
  • Channel 6
    • MFTT CC 0-127 Ring Animations and Brightness
  • Channel 7
    • Octa Notes, CC 46, 47, 49 Track 7: Mutes, Volumes, and Cues
  • Channel 8
    • MFTT CC ?? Sequencer
    • Octa Notes, CC 46, 47, 49 Track 8: Mutes, Volumes, and Cues
  • Channel 9
    • amSynth Notes, CC 0-80, PC Synth Params
  • Channel 10
    • Octa Auto Channel
  • Channel 11
    • Octa Notes, CC 46, 47, 49, 112-119 Track 1: Mutes, Volumes, and Cues
  • Channel 12
    • Octa Notes, CC 46, 47, 49 Track 2: Mutes, Volumes, and Cues
  • Channel 13
    • Octa Notes, CC 46, 47, 49 Track 3: Mutes, Volumes, and Cues
  • Channel 14
    • Octa Notes, CC 46, 47, 49 Track 4: Mutes, Volumes, and Cues
  • Channel 15
    • Octa Notes, CC 46, 47, 49 Track 5: Mutes, Volumes, and Cues
  • Channel 16
    • Meta PC DSPi Switch
    • Octa Notes, CC 46, 47, 49 Track 6: Mutes, Volumes, and Cues

i Resources

  1. If I would choose one source, it'd be this one:
  2. Realtime kernel patching. Most of this is based on this article:
  3. Some general things you have to understand about kernel patching for raspi:
  4. Running stuff on boot for RasPi:
  5. Script for running jack gui-less, and disabling other unneccesary services:
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