Like pretty much everyone else, I decided to get into synths during the COVID lockdown period, but seeing as I'm a cheap bastard, I wasn't too excited about dropping $300+ on each piece of gear and started looking for cheaper ways to explore the hobby. I figured there was sure to be some mostly turnkey, DIY solution based on an Arduino or Raspberry Pi that would give me a quick, low-latency, headless and flexible (and good sounding!) MIDI-controllable synthesizer that I could just flash onto an SD card and be off to the races.
It turns out that this is much more difficult to find than one would expect.
Almost everything I could find is limited and/or poor-quality (everything Arduino-based) or unusably latent and/or extremely difficult to use and set up (everything RPi-based with JACK). RPis also have pretty crummy audio quality from their built-in 3.5mm headphone jack, but that's a minor gripe vs getting it to work at all.
However, I had the astonishing stroke of luck to come across a wonderful, free, open-source project that was just maturing to the point of usability when I was looking, and it checks all of my 'required' boxes and more: mt32-pi.
EDIT (5/30/2022): For the past few months, I've been using another excellent and new (at the time of this writing) software called MiniDexed, which also fills this same niche, but is based on the legendary Yamaha DX7 FM synth. Check it out!
This project describes itself as "a baremetal kernel that turns your Raspberry Pi 3 or later into a Roland MT-32 emulator and SoundFont synthesizer based on Circle, Munt and FluidSynth." For those who aren't familiar with the MT-32, it was a sound generator for MIDI keyboards back in the '80s-'90s but also found success as an ultra-high-end solution for video game audio in the days before CD-ROM soundtracks took over. For a taste of how much better the MT-32 sounded than competing sound solutions, check out this video.
So, just using the built-in MT-32 instruments, you can get some really great, authentic retro sounds, from synth bass to horns to percussion. Mt32-pi also supports SoundFonts, though, through its FluidSynth mode, which really opens up your possibilities to basically anything you could ever want. And, since it runs baremetal, there's no OS running in the background, no ALSA adding latency, no hassle of setting up a JACK server and wondering why your sound doesn't work, etc. It boots up in just a few seconds (it takes about the same amount of time as my pedalboard) and responds with live-performance-friendly low latency.
It seems most of the buzz surrounding this project comes from its usefulness as an outboard sound processor for the (fantastic) MiSTer FPGA retrogaming platform, which doesn't have the resources to do good MT-32 emulation on its own while also emulating the original PC 486 hardware needed to the play the games in the first place. For this reason, if you just go searching around, you'll likely find that most of the information and accessories (there are a number of places selling cases and hats, etc.) are catering specifically to this purpose/market rather than music/performance, but the mt32-pi project's wiki includes plenty of information to track down or build your own, should you feel the need. (as an aside, the mt32-pi developer, d0pefish, didn't even realize there was a demand for the retrogaming aspects of the project, as he created it purely for music/performance purposes, just as it was with the original MT-32)
In my case, I felt the bare minimum was a cheap OLED display (SH1106 and SSD1306 models are both great, simple and compatible options available for $2-$8 on eBay/Amazon/AliExpress) and a couple of arcade buttons to control the mode (MT-32 vs FluidSynth) and to cycle through the available SoundFonts (loadable from a hot-swappable USB drive). I also use an I2S DAC for better sound quality (again, $2-$8 from eBay/Amazon/AliExpress), but you can get by with the built-in if you don't mind occasional crackling in quiet parts. All of this is connected directly to the RPi's GPIO pins via dupont connectors.
Many of the available hats include full-size MIDI DIN connectors, but you can also use any standard USB-MIDI interface/breakout. In fact, I control mine through an Arturia KeyStep (which is an incredible value in its own right) plugged directly into the RPi's USB ports. That is, both devices are connected via USB-MIDI, with USB on both ends; no actual MIDI ports involved, which leaves those ports free on my KeyStep to control other things, like my Roland TR-08 drum machine. I do get some occasional weird behavior (some stuck/dropped notes) with this setup, though, which I think is caused by the KeyStep pulling too much voltage.
So, between the KeyStep and the RPi plus accessories, my full kit costs somewhere between $150 and $200, with the vast majority of it coming from the KeyStep. This is in-line with Korg's Volca line of products (which are great!), but with more power/flexibility in some ways (the KeyStep has an amazing arpeggiator and powerful sequencer and it can serve as the timing heart of your entire setup, while the Volcas' sequencers are simplistic and lack arps entirely) though admittedly less in others (i.e., no knobs to sculpt your sound on the fly; however, the staggering number of SoundFonts available means the Volca FM's compatibility with DX7 patches makes it the only cheap synth with comparable breadth of sonic possibilities). If you already have a MIDI-capable keyboard/controller lying around, you can get the whole thing going for sub-$50, which is simply impossible to beat, value-wise, and if you're already an RPi enthusiast, you may have everything lying around on your workbench already.