More Polyphony, slightly better sound.
I ported DEXED to both the ESP32 processor and TEENSY4.0 MCU + Audio Shield. Pictured is the Teensy4.0 + audio shield and a 128×32 OLED display to display the patch name.
With the Teensy4.0 I have been able to confirm at least 10 voice polyphony! However, there is a slight “glitchiness” to the sound which is more pronounced depending on the patch. You can really hear it in the YouTube video, especially the E.Organ1 patch! I think it is mostly software based because it is virtually non-existent on some of the other patches.
What about the ESP32？
I couldn’t get the ESP32 board to quite work but the problems can be overcome. One issue is that the ESP32 software development environment is built around a RTOS. It does not appear that one can develop an application in a “bare metal” or near “bare metal” mode like the Raspberry Pi Pico. As a side note, the Raspberry Pi Pico supports both a “bare metal” environment as well as a Arduino centric library. The ESP32 RTOS assumes your process will yield to a watchdog process and this yielding and task switching will take approximately 10 milliseconds. Well, 10 milliseconds in real-time audio is forever and causes big glitches in the sound generation. I was able to get around that by using a suggestion in some forums to set the task priority to the taskIDLE priority
xTaskCreate(someFunction, “HumanReadableNameofTask”, 4096, NULL, tskIDLE_PRIORITY, NULL)
The ESP32 restriction of working in the confines of an RTOS was an unexpected hurdle in porting DEXED. But ultimately I couldn’t get DEXED to run on the ESP32 because the DAC I have doesn’t work well with the DMA of the ESP32. I’ll have to look into a different DAC to fix that. That will be a separate project.
As an aside, the Teensy audio library only works in the Arduino environment. The Pi Pico can work either in a “bare metal” environment mode or Arduino environment. The ESP32 only works within its RTOS environment.
Cost
When I first worked on getting DEXED to run on the Pico Pi, my objective was to try and get a really cheap DX7 clone running. Since the Pico Pi is only a $5 MCU, just adding a cheap DAC seemed like I could get a DX7 clone for under $10. But I could ultimately on get 2 voice polyphony out of it. I pursued the ESP32 MCU as it is only about a $8 MCU but, as I mentioned, I couldn’t quite get it to work. Which brings me to the TEENSY4.0 board. This board list for $20 USD but since I don’t live in the US, it ends up costing me more around $25 locally. But for that amount of money, getting 10 voice polyphony out of the Teensy makes sense. The processor is board is 5 times cost of the Pico Pi which could only generate 2 voice polyphony so getting 5x polyphony would be expected to make sense from a cost perspective. The Teensy audio daughter board lists for about $14 and is more expensive than my home brew DAC I built for the Pico Pi but given that it just “worked out of the box” and I didn’t have to fiddle with any clocks or software to get the precise sampling rates was well worth the cost. The Pico Pi prototype had some noise issues and given that I am convinced that the noise issues on the Teensy are likely all software based, I think it’s well worth the cost.
But given the cost of the Teensy and daughter board, it might make more sense to just run DEXED on a raspberry pi. But then there is no fun in that as it would just likely work. There would be no porting work involved and that’s a lot of the fun!
Smutax says:
Hi JK. Really impressed with your work to emulate DX7 on MCUs!
Just wanted to bring to your attention few projects out there, in case you haven’t seen them. First, is this port of microdexed to ESP32 which apparently achieves 8 note polyphony: https://github.com/nyh-workshop/esp32-microdexed This same person also working on their version of Raspberry pico fm synth here: https://github.com/nyh-workshop/pico-fmSynth
There is also this ESP32 project which in part based on MSFA like dexed and achieves big polyphony and features: https://github.com/bwhitman/alles#synthesizer-specs
And last but not least there’s this amazing guy Martin who is doing his fm synth on ESP32 through Arduino environment with good results: https://github.com/marcel-licence/esp32_fm_synth
Thanks for sharing your work with us and all the very best!
Smutax says:
Apologies, the person’s name is Marcel, not Martin.
Smutax says:
Hi again. Discovered this incredible project today by chance! It achieves tx816 features and runs on bare metal Raspberry Pi. Btw, Mr Holger of microdexed fame being one of the contributors! Keep up a good work and all the very best!!
https://github.com/probonopd/MiniDexed
jk says:
Hi… Thanks for your comments and information about the Raspberry Pi. I saw Floyd Steinberg’s YouTube video on the MiniDexed the other day… looks really interesting and worth checking out. I am currently working on a TX816 using the Teensy 4.0
Stay tuned for more!