DIY digital piano: a janko variant

Mike Filip

About your board; so you have 2 data lines (D1/D2) and switch between the 8 sensors via A0/A1/A2. So a series of boards are wired to share the same 5v/A0/A1/A2/gnd bus while the D1/D2/gnd connector is dedicated per-board and connects to the main controller?

That's correct.

Infrared LEDs can switch very quickly. Phototransitors on the other hand take about 500ns-1.5us to transition as the led is turned on and off (from my measurements). This depends on the load on the phototransistor output - it's pulled to ground usually via 5k-10k resistor - and the brightness of the LED. This is controlled by resistors (one for IR leds and two for each of photoresistors in pair (D1/D2 output)).

Phototransitors have capacitance (I think it's called Miller effect or Miller capacitance) and as you switch from one to another you have to charge and discharge it. Having a smaller value resistor will discharge it faster, and having a brighter led will charge it faster.

I'm aiming at 20khz sampling rate for every sensor. That gives 50us total sampling window for all sensors. And 6.25us to switch in a group of 8 sensors (1us-2us taken up for transition, with 4-5us left to read the value) .

Yes, power to the whole IRled/phototransistor pair is switched on and off, so only 1 out of 8 pairs is active at a time.
The main benefit is reducing powerconsumption (and requires way less resistors to be soldered - which only matters if handplaced) - and makes it possible to power the whole device just from USB.

It does require more effort on the programming front regarding timing. (I'm slightly obsessed with making a very affordable and simple design for a narrow key piano, and I'm willing to go through the extra effort for that).

I love janko keyboards, they are way more playable for people with small hands than traditional pianos! And I really like the idea behind isomorphic layouts and it's unfortunate that they haven't found widespread adoption despite having a better design and ergonomics!

vagfilm May I ask how silent is the action compared to a common digital piano keyboard? About the same?

Around the same, yes. The loudest sound is made when the hammer strikes the hammer-hit-felt, which I dampened by doubling the felt. Overall when I place the prototype next to my korg sp250 and try them out they sound quite similar.

DavisB (I'm slightly obsessed with making a very affordable and simple design for a narrow key piano, and I'm willing to go through the extra effort for that).

Seems like we went in completely opposite directions in terms of design priorities. You are going towards affordability while I am aiming towards a 'no expenses spared'.

Mike Filip

I must say that the more I look into your prototype the more intrigued and more impressed I become. The level of attention to detail is at a whole different level from regular DIY.

The hickman action sure looks like a promising simplified action. May I ask you a couple of questions about it, since I have some doubts when looking at your photos, because there is no scale in the images ? (in fact there is a ruler in one photo, but that is a lot of interpolation…)

1. If these were regular long keys (ie, not Janko, sorry…) the actual pivot point would be at the yellow arrow, correct? If so, the capstan looks to be only a few cms behind it. What is the actual distance from the pivot to the capstan? And what is the vertical displacement of the capstan? I assume the vertical displacement is increased because of the cantilever system, and in a regular key the capstan would need to move backwards, correct?
2. How is the backcheck raised? The cantilever bottom bar (red arrow) moves backwards?
3. In the Hickman diagrams there a regulation screw that pushes at the green felt (blue arrow). In your design, that screw was removed and the white piece ((jack???) pushes the gray piece (wippen???). You found that there was no need for regulation? Different tension in the metal springs do not affect that movement?
4. The black things close to capstan are weights (light blue arrow) ?
5. In this photo it looks like the hammers have different shapes. Is this simply tests of different designs?
6. What is the actual depth of your prototype. My estimate is about 60 cms. Is it longer than that?

Again, congratulations. This design raises a lot of ideas…

Well, the entire hickman action is a rabbit hole in and of itself, even without the extensive modifications I needed to make in order to replace the regular piano keys with the rotor based janko variants (which are of my own design - the original janko version relied on parallelogram style keys that actually pushed down on regular 'key-levers'… yet another rabbit hole).

The best place to start looking into the hickman action would likely be at mechanical music digest. It has a (rather badly) ms-paint drawn animation that at the very least gets the point across, but also provides an explanation of how the action works and links the patents (with drawings) for the action.

This video, and others from that author, offer the look from the perspective of someone rebuilding the action - he apparently got it in working order in the past couple weeks, so grats to him!

This video is the only video I found of the action in action. I tried to tune my action accordingly, but I had to make changes when I realized that the video's action is a bit badly tuned - the back check is set a bit too low. Still, it gets the point across.

There are also these two posts talking about the action, but keep in mind that all of their discussion is purely theoretical, such as the very heated debate on whether or not such an action would even work in real life (completely ignoring that pianos with such an action were made… though there is VERY few of them - probably single digits)

Here is a picture from the patent papers showing the action with the full key:

And here is my action with labels. The blue X marks are where the action is connected (via center pins) to the frame, and the upside-down t is where the flange is screwed to the frame.

Now to try and answer your questions:

1. I set up the convoluted rotor assembly for the keys because I wanted the entire action to fit a more compact space and thus couldnt fit a full key lever. You can refer to the hickman action picture above, but the actual pivot point would be much further towards the right. It is also because of this that I needed to add a second rotor for the backcheck, because otherwise the backcheck would move way too far (since my 'key lever' rotates much more than a regular piano, closer to 8 degrees as compared to under 3 degrees of a grand piano). The vertical displacement however is the same - 9.5mm key dip, and around 6mm capstan movement.
2. See the action image above. When the key is pressed (10mm), the front and middle rotors rotate. The middle rotor then pushes up on the jack, which pushes at the driving lever (which is connected to the flange), which in turn rotates the hammer. The backcheck is attached separately to the back rotor, since if I attached it to the middle one it would move way too far.
3. Yes, I removed the two regulation screws that adjusted the maximum hammer height and the start of letoff. I didnt have enough space (due to the keys) to fit the screws while having access to them, which was further compounded by the shrinkage of the action from ~16mm wide (grand piano) to 9mm wide (my piano) due to the 5.1 inch octave span. I ended up regretting this decision as while the start of letoff isnt as important (in my case - essentially I per-calibrated that value and just print the parts to sub 0.1mm accuracy), but the maximum hammer height is still vital and I had to add an extra screw for that, though I added it to the point in the driving lever where the hammer shank rests on. The repetition springs are regulated the typical way (press on them), but also have 5 positions on the driving lever for when larger adjustments are needed (to account for different hammer weights).
4. Yes, they are car wheel balance weights and act as key-counter weights. In the new version (top) I removed half of them and replaced them with a spring based system as that is what was originally used for hickman action and allows for adjustable key-weight by the player (turn the dial, change the action from light to heavy weight! said their advertisement papers).
5. This was my 8 key prototype, so I selected 8 keys from the full 88 key range to try and see how the action will behave throughout the entire range. So I have keys 1, 13, 23, 37, 49, 61, 73, 88.
6. The 8 key prototype is 86.4mm wide, which ends up being 129.6mm for an octave (or 5.1 inches)

Mike Filip Well, the entire hickman action is a rabbit hole in and of itself

Welcome! I'm having big troubles trying to following you. If I can, I suggest spitting this thread in at least 3 (if not more) separate ones, as I have done with my own series (the last one of which is available here )

So I suggest at the very least four threads: Janko, Hickman, 3D printing and electronics. Each one of these is a huge topics on its own, and making a single thread with all of them together makes it very difficult to discuss.

For example, I was very interested in Janko for a while, but then I realized its big shortcoming and the reason why (IMHO) it never took off and how the regular piano layout is IMHO much better. On the other hand, some of its mechanical design (a variation of the parallelogram structure) can be very interesting for regular piano layout too (and in fact @dore_m and I are exploring that). But that's a huge conversation in its own.

The Hickman action seems interesting, but I'm having troubles wrapping my head around it. In my opinion, studying patents to understand how something works is a lost cause: they are written by lawyers from lawyers. Even the animations are not illuminating me. For example, I see how the thing move in the mmdigest animation page (or YT video). Yet, I don't understand why the red jack is made of two parts and why the cyan part "breaks" it in the way it does. This is the crucial point of all this action, otherwise this would be simply a Cristofori action.

Finally

Mike Filip I have plans to release the fusion 360 files along with the electronic board designs (made in easy eda), though at the moment its all just stored locally on my computer (or at least as 'locally' as fusion 360 and easy-eda with their cloud storage can be). I think I can make them 'publicly viewable', but doing anything like that is just extra work on top of this project so it depends on when I get around to it.

As I got tired of writing over and over: the best sharing is the initial sharing since others can help you improve your design (not saying that I could do that with your project, just saying that someone could), in the same way that you can help them. That is what I did and continue doing. I am always perplexed when people say things like this which imply that you need to polish things to share as if they were a final product…. so I nudge you to do that sooner rather than later (and by the way, looking at your project it appears you are already almost done!!! Congratulation, which I should has said earlier in the message 😉 and welcome again)

Mike Filip Thanks for your detailed answers. I had already found the youtube series about the M&H restoration. I will digest your information and most probably get back with a few more questions😊.

One note though… mu question 8 was not about the octave width (or the full width of tje prototype) but about how deep it was (ie, from front of the keys to the enclosure backside).

vagfilm The piano size is going to be 121cm long (950cm of which is the keybed, ~13cm per octave), 470cm deep, and 220 cm high. The size unfortunately doesnt fit into any hard (or soft) cover piano bags/cases, so in order to be able to transport it I will have to order a custom hard case… not sure where yet.

Only 47 cm deep? That is not bad at all…

Mike Filip sorry for the slightly awkward and offtopic question but seeing your name written as Mihail Filippov on the PCB and someone mentioning you’re from Moldova, are you a Bessarabian Bulgarian?

CyberGene Dont think so? I consider myself to be 'from Russia', mainly due to Russia being the country most familiar to people as opposed to Moldova, my mom being from there (or more specifically, the Rostov region) and the fact that I speak ( & read & write) Russian as my first language, though English is my 'primary' language nowadays. More specific than that I cant exactly say.

Mike Filip ahh, makes sense. There’s some overlap between Russian and Bulgarian names which is why I often think someone must be Bulgarian but he’s in fact Russian 🙂 I know we have a Bulgarian minority in Moldova and Ukraine that are called Bessarabian Bulgarians and I have many such friends which is also why I thought you might be a Bulgarian.

Apologies for the offtopic. Again, your project and attention to detail is amazing, I’ll be following your journey with great interest! Thank you for joining and I hope you find our forum a good place for sharing thoughts with like minded people 🍻

Started posting the videos going over the project, with just the introduction for now, but the next two episodes talking about the janko layout and the hickman action should be going up in the next couple days.

@RIP mentioned that it might be a good idea to split this post into a couple threads, so I think I will do that. I will keep this as the 'main thread' where I will publish links to the videos, answer questions, and provide photos as updates, but once the next episodes are up I will start new threads for the janko-layout and the hickman-action to go more in depth about them, along with narrowing the scope of the conversation.

The videos after that will cover the design followed by the electronics, so those will get their own threads too.

For now though, here is the introduction video.

@Mike Filip Thanks for the video series. Easier to digest, and great job explaining.

Posting the video about the Janko layout - going to start a new sub-thread focusing on the janko key layout (and to a certain extent isomorphic layouts in general) as compared to the classical piano layout.

Posting the video about the hickman action - going to start a new sub-thread focusing on the action, such as 'how does it work', and 'how do the centerpins function' ,etc.

Posting the video about the parallelogram linkage of the keys. Will just leave it in this thread instead of starting a new one as its not really as complex as the other topics.

In summary, regular pianos feature a simple lever for their keys - which makes playing the same note at constant volume require different amounts of force based on where you play on the key (playing along the back requires ~2x more force and has 1/2 the travel distance than playing along the front). In the Janko piano this is further worsened by the keys being longer, so either the lever needs to be longer as well or the player needs to accept 3x or more difference between the front and back rows.

To solve this a parallelogram linkage is used to ensure that the key has a consistent 10mm of dip throughout the entire key - no matter if you play on the front row or the back row.

PS: just to clarify; the difference in key dip along the length of the key on a classical piano isnt exactly a 'problem' for players - you pretty much never think about pressing 'harder' when playing along the backs of the keys - you just do it subconsciously. However as anyone who played on a cheaper synth keyboards with like 1mm key-dip along the backs of the keys can tell you there are limits here - 2x difference is kind of the sweet spot, and on a Janko keyboard with its longer keys it becomes harder and harder to stay within that sweet spot for all rows - hence the parallelogram approach.

Plus, having consistent key dip is kind of nice in and of itself. Almost like taking off a weight you've been carrying for so long you no longer notice its there.

Well, time to start covering the actual prototyping and design instead of just the theoretical stuff.

Posting the video about the initial prototype, along with going over it in Fusion 360. As you can imagine I probably printed out enough parts to make 2 prototypes (at least based on weight of plastic) before I felt satisfied with the prototype… or at least until I realized that any further changes would require nearly a complete redesign.

Time for the electronics (video here). I expect this to be a bit more interesting for those in this forum, and I will be making a new thread for it here.

Video covering the final prototype that I am going to be using in the full 88 key build is now out. Just got one more video to go talking about the design (will cover the mini PC, MIDI interface, DAC, touchscreen, Pianoteq, etc) before the assembly of the full piano.