OGFX

The Open (Guitar) Audio Effects Processor

Preliminaries

Install the alarm version for raspberrypi4 on your Rpi4 and then follow the instructions below.

You will need to install the following packages:

• base-devel
• git
• sudo
• htop
• jack2
• rtirq
• realtime-privileges
• stress
• tmux

Add the alarm user to the wheel group. Also change the root user's password to something of your liking.

Kernel Setup

We need to apply two patches to the stock linux-raspberrypi4 kernel:

• RT-PREEMPT (full)
• Low-Latency USB

We have prepared a PKGBUILD that has the required patches. It's found in the src/linux-raspberrypi4 folder. It should be fairly easy to build this kernel with makepkg. Read up on the makepkg documentation if you run into trouble, but in principle it should be a three step process to install and boot into this kernel:

• makepkg -s
• sudo pacman -U linux-raspberrypi4-4.19.69-2-armv7h.pkg.tar.xz
• sudo reboot

While the makepkg command is running it will ask you for newly added configuration settings. Make sure to set the kernel's preemption model to RT-PREEMPT (full).

Once rebooted checkout the output of e.g. htop (install it if necessary). Hit F6, select PRIORITY and you should now see some threads called irq/53-PCIe PME, and similar. These are the kernel IRQ handler threads which we want from our RT-PREEMPT system and which we will tune a little in the next step. But before that create a file calle /etc/modprobe.d/00-snd-usb.conf with the following content:

options snd-usb-audio max_packs=4 max_packs_hs=4 max_urbs=12 sync_urbs=4 max_queue=18

This tunes the parameters introduced by the Low-Latency USB patch. Either unload and reload the snd-usb-audio module (using rmmod and modprobe or just reboot once more..

Setup RTIRQ

Edit /etc/rtirq.conf to look like this:

RTIRQ_NAME_LIST="usb i8042"
RTIRQ_PRIO_HIGH=90
RTIRQ_PRIO_DECR=5
RTIRQ_PRIO_LOW=51
RTIRQ_RESET_ALL=0
RTIRQ_NON_THREADED="rtc snd"

And enable the service with

sudo systemctl enable rtirq

and either reboot or start the service now (with systemctl start). Check with htop whether the irq/54-xhci_hcd kernel thread has priority -91. If so rtirq is doing its thing correctly.

Setup the CPU Frequency Scaling

We want to disable all CPU frequency scaling to make the system behave more predictably. Edit the file /etc/rc.local and add into it:

for n in 0 1 2 3; do echo performance > /sys/devices/system/cpu/cpu"$n"/cpufreq/scaling_governor; done

Make sure it has a #!/bin/bash shebang at the top of the file and that it is executable. Then enable the rc-local service:

sudo systemctl enable rc-local

and either reboot or start it with systemctl start....

Setup RT Permissions

Add the alarm user to the realtime group using

sudo gpasswd -a alarm realtime

And relogin that user.

Setup JACK2

Now it's time to plugin your USB audio class 2.0 device (for example the Focusrite 2i2 2nd gen, or Tascam 2x2, etc). It should appear in /proc/asound/cards

$ cat /proc/asound/cards 
 0 [USB            ]: USB-Audio - Scarlett 2i2 USB
                      Focusrite Scarlett 2i2 USB at usb-0000:01:00.0-1.2, high speed

Run

jackd -R -P 80 -S -d alsa -d hw:USB -p 64 -n 2

It should startup fine (some warnings about libFFADO are to be ignored, they do not matter). Now open another terminal on the Rpi4 and run

stress -c 4

Open yet another terminal and run

sudo watch /opt/vc/bin/vcgencmd measure_temp

Now leave this running for a couple of hours. Make sure that in the jackd terminal no XRUNs are reported. Also check the terminal running vcgencmd from time to time to check whether your Rpi4 has adequate cooling. Ideally you don't want the temperature maxing out around 60 degrees to have some headroom for really hot summer days (at 80 degrees the cpu gets throttled ruining our RT-performance).

Once you are sufficiently satisfied that the system is stable, kill all three processes.

Setup SYSTEMD Lingering User Session

We want our effects processor to start up once the device is powered. And we also want to run everything as the alarm user. To start long running services and processes we can make use of systemd's user service handling. To enable user services started independent of logging in manually we have to enable lingering:

sudo loginctl enable-linger alarm

Some versions of systemd shipped with Alarm are buggy and you have to run the following command manually:

sudo mkdir -p /var/lib/systemd/linger
sudo touch /var/lib/systemd/linger/alarm

TODO

• Start jackd as lingering user service once the USB soundcard connects
• Start mod-host
• Start jack-led-peak instances
• Configure wifi0 in AP/managed combined mode so one can use smartphone to connec to...
• Start web service to control mod-host