Using the Mod Devices guitar effects interface on Raspberry Pi 4

My last post was about getting my Boss GT-1 guitar effects floorboard to work in Linux. The reason I wanted to do this was to couple it with a Raspberry Pi to create a more configurable effect chain using LV2 plugins.

Once I had audio working, I installed Guitarix, an open-source guitar effects stack for Linux, and I was quickly off to the races. Guitarix works really well, but the interface is designed for desktop use - not ideal for my needs.

I had been aware of Mod Devices for a while, with their Mod Duo multi-effects hardware pedals built on top of Linux. They have added a slick, web-based pedalboard interface running on top of LV2 plugins. It is primarily designed for their hardware devices, but the project is open source.

The folks over at blokas.io have a project called MODEP, which provides a custom Raspberry Pi image for running the Mod Devices interface. Unfortunately, it currently does not work on Raspberry Pi 4, which is what I have.

So, I went straight to the Mod Devices GitHub and built it myself. It turned out to be pretty straightforward, and I was up and running after about a half an hour of tinkering. I'm going to try to outline what I did here in case anyone else wants to give it a try:

Get Jack running with audio capture and playback - there is plenty of information out there on this

Get the Mod software from GitHub:

  git clone --depth 1 https://github.com/moddevices/mod-host
  git clone --depth 1 https://github.com/moddevices/mod-ui

Install the tools you need to build the mod software. You'll need at least:

sudo apt-get install virtualenv python3-pip python3-dev git build-essential libasound2-dev libjack-jackd2-dev liblilv-dev libjpeg-dev zlib1g-dev

Build mod-host from the mod-host directory:

make
sudo make install
cd mod-ui
make
sudo make install

Make sure Jack is running, and start mod-host:

mod-host -n -p 5555 -f 5556

In another shell, build mod-ui from the mod-ui directory:

virtualenv modui-env
source modui-env/bin/activate
pip3 install -r requirements.txt
make -C utils

Run mod-ui from the mod-ui directory (you will also need to do "source modui-env/bin/activate" first if you did not just do it in the above build step):

export MOD_DEV_HOST=0
export MOD_DEV_ENVIRONMENT=1
export MOD_APP=1
export MOD_LIVE_ISO=1
python3 ./server.py

The mod-ui web server should now be running on port 8888. To access it, find out the IP address of your Raspberry Pi, and go to:

http://YOUR-PI-IP:8888/

You should see the Mod pedalboard interface. It should load whatever LV2 plugins you have in /usr/lib/lv2 with a generic look. In my case, I had Guitarix installed and all of its plugins were there and ready to be used.

The Mod folks have custom UI overlays for most popular plugins. You can grab them here:

git clone --depth 1 https://github.com/moddevices/mod-lv2-data

Look in the "plugins" and "plugins-fixed" sub-folders for your plugins. If you copy a plugin folder over your existing plugin in /usr/lib/lv2, you will install the custom UI for the plugin.

My Pi 4 has no trouble at all running a decently complex guitar effect stack, complete with multiple pedals and pre-amp and cabinet simulation, all at very low latency.

Getting the Boss GT-1 effects processor to work in Linux

---

** Latest Update: The Boss GT-1 should be working out of the box as of Linux kernel v5.14.

---

I recently picked up a Boss GT-1 guitar digital effects floorboard to use directly, and also to play with connecting it up to the Raspberry Pi.

A month later, I have everything working well. As is often the case with niche gear and Linux, though, it was a bumpy road to get there. I'm going to document the process here, in case it is helpful for anyone with a similar issue.

I booted up my Pi, connected the GT-1 via USB, ran lsusb to list the available USB devices, and there it was. Great - that was easy!

Next, I tried playing a .wav file with aplay. No go. No audio at all. Not so great.

Time to start scouring the internet. After a while, I came across this thread on linuxmusicians.com:

https://linuxmusicians.com/viewtopic.php?f=6&t=17180

It had a fix for another Boss device. It also turned out to work for Boss Katana Amplifiers, which have a software setup that is very similar to my GT-1 device.

I grabbed the kernel source code, applied the fix (with a tweak for my device), and soon I had working audio! Well, kind of working. I was getting pops and crackles every second or so - enough to make things unusable.

To understand this fix, and why it wasn't quite working requires a bit of explanation on how USB audio works. There are a number of methods that devices use to synchronize audio timing. The one used by the Boss GT-1 (and many similar devices) is called "implicit feedback". In simple terms, what this does is, on devices that have both a capture and playback that are controlled by the same clock, the capture source is used to determine the output data rate. Essentially, the host is in a USB feedback loop - outputting the same number of samples it receives - generally after doing some processing.

The fix that got my interface working basically disables the implicit feedback mechanism. This got audio moving through the system, but because there was no timing feedback it was slightly off - resulting in the pops and crackles I was experiencing.

A while later, I came across this post in the Line 6 forums about getting the Helix (another guitar processing device) working in Linux:

https://line6.com/support/topic/4426-line-6-with-linux/?tab=comments#comment-240302

The additional fix here was to hack the sample rate slightly to try to match the real timing of the device. I gave it a try, and it turned out that if I nudged the sample rate to 44105 (instead of 44100), my pops and crackles mostly went away - happening every few minutes instead of constantly.

This fix was obviously a hack, and not perfect, but it at least made my interface usable. The core problem still existed - the device should be operating using implicit feedback, but when this was enabled it didn't work at all.

I had posted about these issues on the alsa-devel mailing list, and a while later I got a helpful email suggesting that some Roland devices waited to send capture data until they had received some playback data. The kernel implicit feedback code, on the other hand, was waiting for some capture data before it send any playback data. The result was a standoff - with nothing getting sent either way.

Simply disabling the code that caused playback to wait on capture resulted in working audio with implicit feedback enabled, and a perfectly solid audio stream with no pops or crackles!

I outlined this fix on the alsa-devel mailing list here:

https://mailman.alsa-project.org/pipermail/alsa-devel/2020-January/161951.html

I hope to get a patch for this into the kernel eventually - right now I am doing some poking around to see if I can find out what other similar devices this fix might work for.

** UPDATE **

A series of patches to implicit feedback handling made it into the kernel (5.11) a while back. I had thought that these changes were fully working on the BOSS GT-1, but it turns out that I just hadn't tested well enough.

Audio playback and capture works, but with periodic dropouts. I get the exact same behavior as I did with the quirk to completely disable implicit feedback.

Changing the GT-1 playback feedback handling from IMPLICIT_FB_SKIP_DEV(0x0582, 0x01d6)  to IMPLICIT_FB_GENERIC_DEV(0x0582, 0x01d6) and applying the endpoint.c patch outlined earlier in this post results in perfect playback and capture for me.

Here is my post on the alsa-devel mailing list about this:

https://mailman.alsa-project.org/pipermail/alsa-devel/2021-April/183274.html

** UPDATE 4/14/2021 **

A version of the above fix has now been submitted as a patch and should eventually get into the mainline kernel. The fix is also being applied to other BOSS devices, such as the rest of the GT family, and the BOSS Katana. Here is the patch:

https://github.com/tiwai/sound/commit/ebe8dc5afb3912e2d4f5c62cf7c492a13143a77a

** LATEST UPDATE **

The above fix was released with Linux kernel v5.14.