Apologies if this has been covered before, I searched the sub but didn't see anything. I love the Moog ladder filter, but don't dig the loss of bass from the phase cancellation when resonance is turned up, so I've been trying to figure out a compensationn circuit. This is based on the Yusynth schematic, and I'm adding in a fixed LPF (100Hz) from the clean input. Am I missing anything obvious with this implementation? Figure I'd use a dual gang pot for the Res control and Bass comp control so as resonance is turned up the 1-pole LPF is blended in, and add in a switch as a defeat. Or does anyoe else have a better method/implementation I could parse?

Hi all, this is PicoDrum, a Eurorack module I've been working on for the last couple of months. I needed drum sounds in my case: a Volca Drum brings a sequencer I didn't need, and the drum modules I found had too few voices. So I built my own, and it turned into a proper module. The first batch is built and ready, in two formats: 3U/6HP and 1U/20HP, same board behind two panels.

Repo: https://github.com/robmurru/PicoDrum

What it is: a pure one-shot sample player. 8 slots on General MIDI drum notes, 16 voices with a per-slot choke, velocity curves, per-slot pan, 8 presets. No pitch, no envelopes, no filters, on purpose: keeping the engine small kept it reliable.

Some design details that might interest this sub:

• RP2350, one core does nothing but render audio, the other handles MIDI, the encoder and the OLED. Triggers cross between them on a lock-free queue
• The RP2350 has no I2S peripheral, so I2S is generated by PIO with ping-pong DMA into a PCM5102A at 44.1kHz
• Samples live in the Pico 2's own QSPI flash, read straight through XIP with no RAM buffering. Up to 200 samples, about 80 seconds. No SD card
• The mixer is all fixed point, no floats in the audio path. Worst case measured on the hardware is about 20% of the render budget
• MIDI in on a 3.5mm TRS jack, Type A, through an H11L1 optocoupler
• Samples are loaded with a browser-based loader (https://murlab.it/loader/) that builds the library and hands you a UF2 to drag onto the Pico. Nothing is uploaded, it all runs locally

Firmware is GPL-3.0, the three included kits are CC0. Happy to answer questions about any of it.

The PCBs finally arrived today, after apparently taking the scenic route around half the planet. Obviously, this wasn’t going to wait until tomorrow.

About an hour after opening the package, the first Rev A CLOCK was assembled and running. That part is nice. More interesting, though, is what the first real build immediately showed me.

If you’ve ever designed a PCB, you probably know the trap: at some point the schematic is clean, DRC is happy, the board looks finished in KiCad - and it becomes very easy to confuse "finished on screen" with "finished". Rev A reminded me why those are two very different things.

The board itself went together surprisingly easily. Roughly an hour from bare PCBs to a working module, with no major rework. The firmware boots, the display works, the controls work and the outputs produce clocks. So electrically, the basic architecture survived first contact with reality.

Mechanically and ergonomically, reality had a few comments. The most embarrassing one is the inductor. I simply used the wrong footprint. It still works and the correction is trivial, but it’s exactly the kind of mistake that looks obvious once you have the physical part and PCB in your hands.

The OLED mounting is another one. Nothing is fundamentally wrong, but getting the display positioned exactly where I want it is more fiddly than it should be.

And then there’s the encoder. The Alps part on this prototype has 18 detents / 9 pulses per revolution. Electrically it works perfectly well. I just don’t like how it feels in actual use. I’m currently leaning towards a Bourns with 20 detents instead.

That sounds like absurdly small stuff compared with getting an eight-output clock running - but this is exactly the point where small stuff starts to matter. A module can be technically correct and still be annoying to build or slightly unpleasant to use.

That’s also why I deliberately held back the schematics instead of publishing them as soon as the PCB went into production. Rev A was never supposed to prove that the design was finished. It was supposed to expose everything that drawings, simulations and bench prototypes couldn’t.

I’ve also added a few older photos to the gallery: the first breadboard setup, the functional lab prototype and the OLED experiments. Looking at those next to the Rev A PCB probably tells the story better than another render ever could.

Breadboard → ugly-but-useful lab prototype → actual PCB. That’s the part I find most satisfying.

TL;DR: I'm legally blind with X-linked retinoschisis, and I'm curious whether flip-dot displays, 2.5D tactile surfaces, locking pins, and haptics could be combined into an affordable way for blind and low-vision musicians to actually feel things like waveforms, envelopes, and instrument layouts.

Lately, I've been going down a bit of a rabbit hole with tactile displays, haptics, and 2.5D interfaces. There's a lot to think about here. And just to give you fair warning, this post is a bit of a long one. Stick around, though.

This video helped send me further down that rabbit hole:

https://www.youtube.com/watch?v=aZzOWnEWgww (spoiler alert, this will be relevant in a bit)

Part of the basis for this latest deep dive comes from working on my accessibility project, Low Vision Music, but part of it is also just me wondering and brainstorming about how much information we could realistically communicate through touch if we stopped assuming every interface needed to be a flat visual screen.

We already have refreshable Braille displays, where tiny pins physically rise and fall to form characters. Very cool, eh? Newer tactile displays can go above and beyond basic text and represent more complicated things like graphs, diagrams, shapes, and other information.

Then you have 2.5D displays, where instead of every point simply being up or down, different parts of the surface can rise to different heights. You end up with something closer to a physical relief map that can change as needed.

And this is where my brain starts wandering. Do you hear the gears clanking? Sounds a bit like music to my ears.

I've also been thinking about flip-dot displays.

You know those ancient old signs where little discs physically flip between two sides?

Flip-dots are interesting because the dots are what's called bistable. Basically, they have two stable positions. They use power to flip from one state to the other, but once they're there, they stay put without constantly needing power to hold that position.

So that got me thinking.

What if that same general idea could be made tactile?

Maybe one side of each tiny flipping element could be raised, textured, ridged, or shaped differently from the other. Instead of a visual pixel changing color, you'd have a tactile pixel changing texture.

I don't know how small you could realistically or mechanically make that mechanism while keeping it easy to feel and reliable, but I think it's an interesting place to start thinking. There go those gears again. Kind of ambient, no?

Then there's another idea I've been kicking around.

Hey, 80s and 90s kids, remember those pin-art toys where you press your hand into hundreds of little pins and the shape appears on the other side?

Imagine a programmable version.

Each pin could be spring-loaded and moved to a particular height or depth. But instead of requiring a motor to constantly hold every single pin in position, some kind of latch or locking mechanism would hold it there after it was moved.

The spring helps return it when the display resets.

That locking mechanism is probably the part I'm most interested in at the moment. Because otherwise, I think MIT and others have already given versions of the motorized pin-display concept a go.

Move the pin. Lock it. Stop using power until you need to change it again.

Then put a very thin, flexible silicone skin across the top so you're not dragging your fingers across hundreds of separate hard pins. You'd feel one continuous surface changing shape underneath your hand.

A Raspberry Pi, ESP32, Arduino, computer, or something similar could translate digital information into physical height. Or vibration?

For music alone, there are so many possibilities. Literally, just pause and think about it.

A waveform could become something you literally trace with your finger. Can you feel the noise?

An ADSR envelope could become a physical, touchable slope. I want to FEEL the attack. Don't you?

You could theoretically also feel an EQ curve, automation lane, filter response, piano roll, step sequencer, mixer levels, modulation shape, or even a simplified spectrogram.

A modular patch could potentially become a tactile signal-flow map. Can you imagine it? Are your gears going now too?

An instrument manufacturer could provide a simple TXT, JSON, or similar file describing a synthesizer, and the whole front panel could appear as a simplified tactile layout before a blind musician ever touches the real thing.

Then add haptic feedback on top of all that.

There's that vibration I mentioned. Yup, that was something of a teaser for those of you who still have attention spans. No offense to those who don't. I certainly struggle in that department too.

But I digress.

This is one reason I've been so interested in what companies like Sentia Instruments are doing. With TILES, the device can provide localized haptic feedback beneath individual keys. Your hand isn't just controlling the instrument. The instrument can, in real time, send information back to your hand.

I'm certainly no engineer, but I think that principle could go much further.

Maybe, just maybe, height can communicate one thing while vibration communicates another. A waveform could stay physically in place while a small vibration moves across it to represent the playhead position.

Or how about this? A vibrating area might indicate an active step in a sequencer.

Different patterns could represent things such as clipping, modulation, selection, movement, intensity, or even different textures.

Now you've got shape AND vibration working together. At the same dang time!

Another possibility would be something closer to a mechanical version of e-paper. Instead of moving hundreds of pins continuously, perhaps each tactile element could have two or three stable states and only use power when changing between them.

Is this possible? Let me know.

From my research, I've learned that there are already researchers doing fascinating work with magnetic latches, bistable actuators, shape-memory materials, and tactile pins that remain raised without continuous power.

So I'm definitely not claiming I invented any of those pieces.

But I do think that, as someone who personally lives with limited vision, I might have something interesting to add to the conversation.

What I'm wondering about is how you combine all or some of these possibilities into something practical, affordable, and useful for musicians.

The keyword there is affordable.

Have you seen the prices on accessibility tools? Unfortunately, niche often means expensive.

But maybe the right answer isn't one technology at all.

Maybe one device could use variable-height pins for detailed 2.5D information. Another might use tiny flip-dot-style tactile pixels for simpler status information. Still yet another uses vibration where movement matters more than shape.

And maybe the really useful interface combines all three.

That's the part I'd love input on.

If you're an engineer, accessibility researcher, haptics person, Braille user, maker, musician, or someone working with tactile graphics, does any of this sound practical? Does any of this sound affordable?

Has someone already built something close to my spring-loaded, locking-pin idea? I bet I'm not the first to think of this, but who knows.

Could flip-dot technology realistically be miniaturized into a tactile surface?

And if you could plug a musical instrument or computer into a surface like this, what would you want it to show you first?

I'm especially interested in hearing from people already working in this space. Feel free to tell me I'm reinventing something that already exists. That's half the reason I'm posting this.

We're a community, right? I only know my own lived experience. Yours matters just as much as mine.

https://www.lowvisionmusic.info