Why?

There were a couple of motivations and use cases for putting this together.

• Needed a better way of triggering backing track playback when performing with my band Big Time Kill
• Wanted a compact but powerful synth that could be used for performances or jamming
• Stand alone guitar pedal board/amp modeling
• Something that felt like a stand-alone piece of production hardware

But an iPad can do all of this

Yes, it can do all of this and has a very rich audio production ecosystem. However:

• An iPad is more expensive
• I like to tinker and build things
• I wanted it to look cool
• There are lots of USB ports and GPIO pins which allow me to add more MIDI interfaces or maybe even trigger some lights (via GPIO)

Did you just super glued/velcro off the shelf parts together?

Yes.

What does it do?

Here are some video examples. A lot of these were filmed before the whole unit was built so just the core and the software are featured in many of these.

Parts BOM

Note that some items are no longer available or the prices have changed. I’ve listed the prices I paid for particular items at the time of purchase.

Software

Patchbox OS was used as the operating system. It comes out of the box optimized for low-latency audio playback.

MODEP comes installed with Patchbox OS and is a community-based MOD DUO emulator.

SunVox is a modular music tracker that allows me to design synthesizers, produce full tracks, or process incoming audio signals.

In order to run SunVox properly on the Pi you will need to install SDL by running:

sudo apt install libsdl2-2.0-0 libsdl2-dev

At this point it will run but the UI will have terrible FPS. To resolve this edit ~/.config/SunVox/sunvox_config.ini and add the softrender option to the file.

Design

The form factor was inspired and derived from the aesthetics of cyberdecks. Cyberdeck.cafe and the cyberdeck subreddit served as sources of inspiration.

The first step was to order all the parts. Once they arrived everything was assembled and tested. After verifying it worked I began taking measurements of the overall footprint of the gear to determine the size base needed. It ended up being 12 x 16 inches. A couple cardboard mock ups were made and I played with using an elevated shelf design to keep the overall footprint smaller.

Once I was happy with the cardboard mockup I went to work creating a base in Tinkercad. This is an easy to use free 3d modeling tool that outputs to the .stl file format used for 3d printing. The base design was slatted to reduce the overall weight/material required and had a gap area where peripherals/batteries could be placed.

And below is a mock up of what the hardware was going to look like mounted on top of the base.

After sending the above files off to a co worker that has a 3d printer it was determined the design was going to be a pain to print due to it’s size. Additionally, it probably wasn’t going to be sturdy enough. They modified the design to be solid instead of slatted and sliced it into smaller pieces for easier printing. The print medium used was ABS plastic. All I had to do was glue the pieces together with some super glue. The actual hardware was fastened to the base using 3M Dual Lock Velcro.

Numpad mapping

SunVox has very robust touch controls. That being said I wanted some physical buttons.

• Quickly access commonly used functions that are buried in the hamburger menu
• Tactile clicky buttons feel nice

Battery life and thermals

The Pi 4 draws much more power than previous generations. I used the information in this helpful comparison of the power draw for all the different models to get a ballpark estimate of what would be required by the build. The 7 inch display has a constant power draw at max brightness so that was easy. This allowed me to get the following values:

The other peripherals (MIDI keyboard, number pad, audio interface) didn’t have their power draw published anywhere. With everything hooked up while on the official power supply it ran without issue. The results when using a cheap battery bank that could output a max of 2.1A were interesting. Not only was the lightning bolt icon displayed in the top right hand corner of the screen displayed, but the Pi CPU was throttled heavily due to the deficient amperage being received. I conducted a battery drain test and got the following results.

• Large MODEP patch loaded
• Large SunVox session playing back on a loop
• Wifi enabled, Bluetooth disabled
• Screen at full brightness

Another factor to consider is that the USB-C cable being used with the power bank must also be rated to carry 3A. Many are not. A lot of the ones that claim to handle it in their specs actually don’t. Thankfully the Charmast supply I found provides a cable that can handle the 3A output by the bank. I ran some load testing by hooking up all the peripherals and running a large MODEP/SunVox session. Everything ran fine with no lightning bolt or CPU throttling. I also lowered the overall display brightness from 100% to 70%. This not only increases the run time of the unit but also prevents me from being blinded by the display in a dark venue. This is done by running the following command where n is a value between 0-255:

echo n > /sys/class/backlight/rpi_backlight/brightness

By extrapolating from my above battery drain test I can estimate that the unit can achieve a run time of at least 16 hours, though I’ve yet to actually do a full drain.

he MODEP/SunVox stress tests were run again and the temperature checked via the command line using:

/opt/vc/bin/vcgencmd measure_temp

The above outputs the temperature as Celsius. This test was conducted indoors with an ambient temperature of around 65F/18C. In this environment when under load I am seeing an average temperature of 75C. The Pi 4B will begin throttling due to temperature at 80C.

To give the unit more thermal headroom (in case I play some outdoor summer shows or in an especially hot venue) I ordered and installed a low profile heat sink with fan. It fit within the NeeGo case with plenty of leftover headroom. The fan just about lines up with the cutout logo on the back plate which allows air to pass through freely. The only difficulty that I ran into was that the plastic power plug for the fan was quite large and wouldn’t fit comfortable next to the other connections on the GPIO pins for the screen. This was resolved by snipping off the plug and soldering directly to the appropriate positive and negative pins directly. With this unit installed the temperature was reduced to 60C, a reduction of 15C.

SunVox is running on the small 7in box and is what I sequenced all the synths/samples on for the backing track. MODEP is also running on this device. On my left monitor I’ve accessed the MODEP UI on the Pi via the network so I can build the guitar patch.

Output of the Pi is recorded into my desktop running Reaper in Manjaro(right screen).

– Raspberry Pi 4 + Official 7 in display
– Presonus Audiobox USB 96
– Patchbox OS
– SunVox
– MODEP

Details:

• Raspberry Pi 4 + Official 7 in display
• Presonus Audiobox USB 96
• Patchbox OS
• SunVox

You can listen to just the audio track on SoundCloud:

Download the patch from the video here.

Make a Sound 01 Patch

Some subjects covered in this video are:

• The basic layout of the interface panes
• How to load and connect modules
• How to enter notes into the tracker
• How to create more tracks for a sequence clip
• Renaming and coloring modules
• Saving your project