It has been a while since I last posted about the Drumulator. On the one hand, I'm working on other projects in parallel. A project that seemed fairly simple at first glance—namely, implementing the Drumulator on an FPGA—wasn't quite as simple as it seemed. And worse still, as I made progress on the subject, I realized it was becoming increasingly difficult, to the point of reaching a breaking point: a total state of blockage due to my lack of expertise in FPGA programming.
I remained in this state for a very long time, which could be estimated at several years—in fact, far too many years, since I must have been stuck for about five years. Until the end of 2025, when, thanks to the contribution of AI used as a teacher, I was able to unlock my understanding of VHDL and thus make great progress.
The side effect of this new approach to FPGA programming is that I spent the first half of this year working on finalizing the digital part of the Drumulator, and as a result, I also resumed development of the MSX cartridge that I had attempted several times to build using fast microcontrollers, but with unreliable operation.
I also undertook training in 4-layer PCBs, because double-sided boards don't allow proper use of the range of FPGAs I'm currently using, the Efinix Trion FPGAs. And to complete my training on the subject, I decided to make a 4-layer PCB using a BGA-format FPGA. I've actually received the PCB. All that's left is to embark on the adventure of hand-assembling this board.
All of this is to say that I've been quite busy during this first half of 2026.
Since the start of the 'school year', I've therefore studied (again) the analog part of the Drumulator.
Thanks to my new 'skills' in FPGA development, I was able to consider a different way of operating this analog part. Also taking into account the final cost of the board, its ease of integration not only for the Drumulator but also possibly for another machine of this type, the simplicity of assembly, the quality of the audio processing, and finally the possibility of operating with a simple 5V power supply such as a USB charger, well, I ended up with something that has nothing to do with the original Drumulator circuit, while scrupulously respecting its spirit.
I no longer use multiplexing at the output of the audio converter. Each output now has its own converter. I also no longer use per-channel volume control via an external digital-to-analog converter. The volume will be calculated directly in the digital domain by the FPGA. Furthermore, I'm using 12-bit linear audio converters, rather than 8-bit companded ones as in the original. A simple conversion table directly converts the signal, and does so very simply since 8 bits only gives 256 values to process. Finally, the output capacitors on the audio path have been removed thanks to the 0V centering servo.
Moreover, all the filters present at the output have been kept strictly identical, in order to provide the exact audio feeling of the original Drumulator. This produces a circuit with a lot of components, but one that should be fairly easy to route. For information, here is the 3D rendering of both sides of the board before placement and routing.
We're still going to have to be patient to make the best possible circuit...
