Sebastian Holzapfel's Eurorack PMOD Puts Your Favorite FPGA Into Your Favorite Modular Synth

Embedded systems engineer Sebastian Holzapfel's Eurorack PMOD is an open-hardware effort to blend the worlds of field-programmable gate arrays (FPGAs) and music synthesizers — offering a compact module with some impressively complex capabilities.

"I had a crazy idea for a weird high-performance granular sampling device that I wanted to make using FPGAs," Holzapfel explains of the project's origins. "I started playing in this world because I couldn't find anything that really combined the hardware synthesizer world of Eurorack and this [FPGA] PMOD interface."

The Eurorack PMOD module is precisely designed to fill that gap. Offering compatibility with Eurorack-standard modular synthesizer setups on the one hand and with PMOD connectivity to FPGA boards on the other, the device offers considerable flexibility — or, as Holzapfel would have it, "You can plug it into an FPGA development board, you can synthesize one of the example designs, [and] you can use that in your Eurorack system and start making weird music."

Designed in KiCad and released under a reciprocal open hardware license, the Eurorack PMOD itself doesn't host an FPGA. Instead, it offers a standard PMOD connector which can be connected to a wide range of third-party FPGA development boards, many open hardware themselves. Bundled gateware, based on the Icebreaker FPGA, includes support for eight DC-coupled audio channels, with analog LED indicators, four of which are inputs and four outputs.

While the hardware theoretically supports 32-bit sampling at 192kHz, Holzapfel notes that the current version of the gateware uses 16-bit sampling at 96kHz. "[There's] no reason this can't be improved," he explains, "[I] just haven't gotten around to it."

As for the why, "maybe you want to start playing with DSP [Digital Signal Processing]," Holzapfel supposes. "Maybe you want to try doing things that are difficult on an MCU [Microcontroller Unit] based platform. What do I mean by that? So, if you want to do super-low-latency operations on a microcontroller-based platform, and there are a few microcontroller-based development platforms for Eurorack, you very quickly, if you want very low latency, have to start dealing with DMA [Direct Memory Access]. There are quite a few things that are easier to do in the FPGA world, especially when it comes to DSP, than in the MCU-based world."

"But even if that wasn't the case," Holzapfel continues, "it's still kind of cool in my opinion, it's a cool learning platform to play with FPGAs. You have this world where you can just play with sound, plug arbitrary things in, make a module that implements a tiny little piece of functionality and see how it's affected by all [these] different effect modules and different oscillators they have in your system, and it makes it very easy to discover things."

The hardware design files and sample gateware for the Eurorack PMOD are available on Holzapfel's GitHub repository under the CERN Open Hardware License Version 2- Strongly Reciprocal.