Why pi-Stomp? (and other FAQ's)

When it comes to amp modeling and effect processing, we’re in a golden age for guitarists. The choices for gear are almost endless. Why do we need another?

Well, almost all of the available solutions are based on proprietary platforms. pi Stomp is open source software running on hardware that You built.

Being a more open and general purpose platform, you can hack or extend it by:

• Importing additional effect or synth plugins
• Coding your own plugins. Create that Tremo-Transmogrifier-Drive'u'luxe you’ve dreamt of!
• Adding additional audio software. If it runs on linux, it’ll likely run on pi Stomp and you could mix in the audio via the onboard alsa mixer.
• Modifying the Pi-Stomp software. It’s all on github. So clone it, fork it, and do what you will to make it work for you. Add/change what’s displayed on the LCD, have the footswitches send IOT or DMX512 message packets via USB or WiFi to control your stage lighting.
• Moding, extending or upgrading the hardware. pi-Stomp Core is meant to modded! The 8 channel ADC can accept nearly any analog signal. The audio card, LCD and ADC all use standard protocols (SPI and I2C). MIDI (USB or 5-pin DIN) can be used for most any control you can imagine.

It certainly is more hackable and upgrade-able! Performance wise, it's very close in many regards, and even has some unique features. But in general, if you're not the DIY type, or if you want a pro quality product, please buy one those. Each was designed by a large team, in a well funded company, and assembled in a production factory.

pi Stomp is a hobbyist project For hobbyists. If you’ve always wanted to build your own multi-effects unit, here’s your chance. I put my blood sweat and tears into it, so you can have the same enjoyment I have when I plug in and make awesome sounds thru something that I built. If you’ve ever built your own effect or amp, you know what I mean. Now imagine an entire do-anything pedalboard that conforms to your crazy musical ideas!

• Who is the intended audience? It's not intended for non-DIY types. It may take around 2 hours to build the hardware and install the software. Could even require some troubleshooting. So… It is intended for those who understand all that, and still say, “Heck, yeah, I wanna build one!” If you're a musician who enjoys tweaking, I'm pretty sure you'll enjoy all the possibilities pi-Stomp provides.

• What all is required to build/use pi Stomp? Just your hands and a small phillips screwdriver. To download, install and use the software, you'll need a computer with wifi and a SD card slot (or USB SD card dongle), and a wifi router connected to the internet. For troubleshooting, you may need a free SSH client (like PuTTY or Mac Terminal).

• Can I buy pi Stomp as a finished product? No. That would entail many logistical issues that Tree Fall Sound is not willing to assume. Also, it's not the mission of this project. If you want a finished product, you could buy a Mod Dwarf which runs the same MOD software.

• How long does it take for pi-Stomp to boot? It depends on the number of pedalboards you have. Mine with over a dozen pedalboards, takes less than 30 seconds to boot and start passing audio. Audio is passed even without power if the pedal is by bypassed. Boot time can be reduced somewhat by disabling certain services like wifi, midi, etc. Switching pedalboards takes about 1 second. Switching presets within the same pedalboard is near instantaneous, so great for switching between parts of a song.

• How many effects/plugins can a pedalboard include? There's no hard-coded limit. The limitation is the total amount of processing required by the set of plugins. Some plugins (eg. simulators, pitch shifters, etc.) perform rather complex or numerous computations demanding more CPU than others. The MOD Web UI shows the real-time CPU demand and the number of buffer overruns. You can use these tools to assure that your constructed pedalboards won't likely exceed the available CPU resources. Keep the CPU below 70% or so. In practice, we've created pedalboards with over 15 effects without issue. Some more demanding chains might need to be limited to a pedalboard with 8 or so plugins. NAM (Neural Amp Modeling) is a total CPU hog, but with a pi5, you should be able to run at least 2 NAM instances, maybe more if you increase the frames to 256 or use lighter weight models.

• Can I swap out the Raspberry pi with a different model? Yes. pi-Stomp v3 supports all “B” form pi's (version 3, 4 and 5). The performance is much more limited using a pi3. pi4 or a CM4 on a B form carrier works pretty well. Still pi5 is recommended especially if you run NAM (Neural Amp Modeler) or some of the more compute intensive simulation plugins.

• Will the circuit board schematic and layout be made Open Source? Maybe eventually. We have no problem sharing parts of the schematic if you need it. The layout being public would likely result in many non-functional bricks of e-waste. During development, at least a half dozen prototypes were created for each version (1, 2 and 3). To retain high quality and low waste of pi-Stomp builds, Treefallsound will keep the designs as closed source for now.

• …But I have a great project idea and don't want your full kit, can you help? Yes, we have helped many pi-Stomp variants get off the ground by leveraging what we've done. Check out the forum. Feel free to ask for design tips, how we did XYZ, etc. Just don't ask for the full schematic, because if you know enough to build a decent board, you know enough to not need the schematic. Most everything in the circuit is textbook audio electronics.

• Can I purchase just the circuit board? No. See previous two answers. We provide full support for everything we design and sell. If you choose to use different parts, we can't waste our time helping you when it doesn't work. There's not much on the board itself that could be customized, so this question makes us think that you're just being cheap.

• Why is the kit cost on tindie.com different than on treefallsound.com? tindie.com charges us an additional fee. It's well more than the difference, but we appreciate the extra exposure they provide the project.

• I have plenty of power supplies, why do pi-Stomp kits include yet another? Indeed, it does pain me, from an ecological point of view, to ship a supply with every kit. But the consequences of not including a supply are many. The supplies most people have on-hand will likely:
  • be noisy
  • provide the wrong voltage or be badly regulated 5v
  • not provide enough current (RPi's need 2Amp) leading to flakey performance and hard to diagnose support calls

• Can I use software other than Modep/MOD on pi-Stomp? Yes! As long as it runs on a generic linux OS and can be compiled for the Raspberry pi architecture (armv71). You could even replace the patchbox OS completely, and replace it with another pi compatible OS. More on that here: Other Software running on pi-Stomp hardware

• How can I get involved in development of pi Stomp? First, build one. If you dig it, and want to make the software better (or worse), that's awesome. The next step would be to clone the software repository at: github If you would like to discuss details, please email: support@treefallsound.com

• Can you add feature XYZ? Maybe. Or maybe you can, see previous question. For now, please log software problems in github: Issues or feature requests in the pi-stomp forum

With 24-bit 96kHz I/O and a quad core 1.4 GHz processor, pi Stomp certainly has the processing power to rival the some of the best units. A vast number of current commercial recordings have been produced within computer systems with less power than pi Stomp. And remember, rendered CD quality is 16-bit 44.1kHz.

Where commercial multi-effects are likely to excel, is in the quality of the individual effect software models. Each included effect has a whole team of sound professionals committed to creating near perfect models of popular classic and novel amplifiers, cabinets and effects. The current pi Stomp software uses the LV2 open standard for coding of plugin effect models. There are over 400 available LV2 plugins out there. However, like with any community developed open software, the quality can range from awesome to unusable.

To get a sense of what LV2 plugins are capable of, check out the example pedalboards with sound samples on the Listen page.

The LV2 standard is relatively new and the audience is smaller than the similar Windows/MacOS based audio plugin formats (VST, AU, AAX, etc.). Some of those plugins may be able to be ported to LV2. But there are also some companies/organizations/individuals creating some awesome native LV2 plugins, like guitarix and CALF. As the community grows, so will the quality. You can even be part of that! Create your own plugin (Build & deploy a LV2 plugin ), add it to your pi Stomp, share it with the world!

Although pi Stomp! can replace a full pedalboard, it can also be used quite well with your other pedals chained before, after or within the fx-loop. So your personal tone can still leverage the pedals that you love and then use pi Stomp to add effects that you don't otherwise own, wish to consolidate or automate with presets, etc. One usecase, is to use it as a simple stomp box instead of a full pedalboard, providing less used effects (eg. pitch shifter, envelope follower, rotary speaker, etc.) as an alternative to cluttering your physical pedalboard with each of those.

To my knowledge, there are only three makers of HiDef Raspberry Pi audio cards that include inputs (ADC). All 3 (AudioInjector, IQAudio Codec Zero, HiFiBerry DAC+ADC) work with pi-Stomp with just a simple software change. Those, and most DACs in fact, run on 3.3volt supplies allowing a maximum headroom of about 3 volts. That still exceeds 0dBu Pro ref level (2.19v) and 0dBV consumer ref level (2.828v) but is significantly lower than 9v. If you demand the headroom offered by a 9v or even 18v analog pedals, you might not like pi-Stomp. It may feel compressed or lacking in dynamics. Just keep in mind, your favorite commercial multi-FX unit (unless it's a Fractal with 11v max) also likely uses a 3.3v ADC. Even Kemper and NeuralDSP don't specify their max input level. I wish I could afford one to determine for myself.

Now we get into some geekier details. Most commercial multi-effects units are DSP based. Their processing is purpose-built to process audio in a real-time operating system (RTOS). pi Stomp! being based around Raspberry Pi and Linux, a general-purpose operating system (GPOS), is at somewhat of a disadvantage because the CPU can be busy processing tasks that a DSP doesn't have to worry itself about. The Raspbian based OS used in pi Stomp! has been optimized to eliminate or reduce many of the potential distractions that might be encountered if the Raspberry Pi was being used as a “desktop” computer, but still, buffer overruns (aka Xrun's) can be encountered when the CPU is overwhelmed and that can result in audible anomalies (eg. glitches, pops, etc.) Given the power of the Raspberry Pi 3's quad cores, Xruns are typically infrequent unless you're running a pedalboard with many plugins, greater than 8 or 10 maybe, or if quite a few of the plugins are “simulators”. The bottom line is that there's more potential for audible glitches with a GPOS based processor. A glitch here and there is usually tolerable by most recreational musicians. If you're of the more professional variety, a commercial unit might suit you better.

Digital noise is also more likely in a GPOS based system because the CPU is constantly processing requests for its many services, passing data over its various interfaces (GPIO, Wifi, etc) and radio frequency noise is just rather prevalent on development boards like Raspberry Pi. The route of digital noise into the audio path is usually via inductive coupling. Shielding and physical distance are the typical solutions applied to reduce that coupling. Experiments with shielding were done but didn't seem to reduce coupled noise by any measurable amount. Instead, I chose to keep the audio portions of the system physically sequestered to one side of the board, away from the LCD backlight and Pi as much as possible. There will be some high frequency noise noticeable when the output is sent to an amp running at high volumes. In my opinion, it's not as annoying as the noise from many gain pedals or the common 60Hz hum, but it is there and something to consider if the highest of Fi is your grail.

Actual lab test results of pi-Stomp can be found here