AVR Freaks

Cheers.

Being able to discern 128 steps would be good ;-) (MIDI tends to carry 7 bits of info!)

reading around it does seem to be 10K used in most examples. I'm guessing it's because of the impedance thing. 10K presumably means 500uA across it (Vcc=5V)? If the thing might eventually be battery powered I wonder how much of a concern this might be?

 

Thanks folks I will very likely go with 10K.

 

BTW I know the picture is not much to go on but does anyone recognize the brand of pot used here. Those are pretty standard tact switches around them to give an idea of scale so perhaps 6-8mm square ?

 

Ah ha "RV09" was, indeed, the "magic" I was missing. Adding that to search terms has opened up an Aladdin's cave !!

 

I also thought that 9mm shaft length was the shortest I could get but now find lots of 6mm which could fit the bill nicely.

 

Thanks everyone. I'm going to mark avrcandies post as the actual solution but only because I only get to mark one post. Several others were very useful indeed :-)

Jay,

 

I don't suppose you have heard of the "Pocket Operators" from Teenage Engineering (a Swedish kickstarter start-up, orginally)?

 

https://teenage.engineering/prod...

 

 

Now I collect "small synths". I tend to collect them as I'm always on the look out for the perfect "handheld synthesizer". Especially one that would bring back memories of the Casio VL-1 I owned 30 years ago. Up to now I'd seen the POs but never got one.

 

Then for Valentine's Day this year my wife got me a PO-32 (the "Tonic" (as in "Gin & Tonic"), the one just disappearing off the left in the above picture) and it's a revelation! It's actually a drum machine rather an a melody generator but it's SO addictive simply to "tinker" with. In fact I've learned more in a month or two about drumming and rythmn patterns than I ever knew in the previous N decades !

 

Unlike so many small/portable synths (even down as far as the Modal Craft 2 or the Korg NTS-1 : others that I own) these truly are the synth you can slip into your pocket. And it makes them so much more usable as you can just carry it anywhere (I take it out to the shed when I'm sitting with cat#2 and it drives him demented - it even has a small speaker in the design.

 

As you can see in the picture the design is deliberately "industrial". On the flipside they are something like:

 

 

The clip holds two AAA batteries.

 

But among the range, while there are samplers, drum machines and subtractive synths, there isn't an FM synthesiser and just recently I've been getting interested in FM synthesis so I wanted to have a crack at making a handheld. Not "copying" the POs but "inspired" by them. I've also been considering getting this recently and its design is equally inspiring:

 

It has an unusual approach to FM synthesis. Instead of the modulator/carrier relationship of the operators being fixed so you only have 20-30 fixed connection "algorithms" to choose from (think "Yamaha DX7" or Korg Volca FM), the Liven XFM is different in that every one of the four operators can be connected to any of the other four in any connection pattern you choose. To achieve this a LOT of adjustments are required. Each op has four controls for it own ADSR, envelope then there is the harmonic multiplier for each operator, also control for the amount each feeds into each of the other ops and itself (feedback). In fact it's so complex that almost all the buttons you see above have a second function (printed on an overlay you put on top while editing) just to configure all the sound paths

 

In fact there is also an online FM synth demo:

 

https://notes.ameo.design/fm.html

 

 

that has a very similar design (anything can feed to anything). Only in that case it's not just 3 or 4 operators but 8!! So imagine that every square on that grid were a rotary control to set the level. In column 1 the control is the op's frequency multiplier then across the grid it's the amount of feed from each op into each other op. Finally, in the right column it's the feed of each op's output to the output mixer.

 

If you scaled that back to just 4 ops it would be kind of what I have in mind. In fact I sketched this out (actually this is an old version - current is more "matrix"y)

This is my UI design in the PC app "Ctrlr" that would send MIDI CC and just be used for testing until there's pots/encoders/whatever in real hardware to use for control.

 

The actual core synth design would be something like:

 

On which each mixer (5 of them) need 4 controls and the envelopes each need 4 controls for their ADSR. It might look like an explosion in a spaghetti factory but that's because every sine oscillator and its envelope has to be able to feed to every other one (and its self for feedback)

 

Now I know you can do this in far less controls with encoders and a display. In some user interface you would switch to the right page (eg ADSR for op3)  and then use 4 encoders (say) to wiggle that thing's controls. But I like the idea of a physical matrix where you can see (and twiddle with) the position of absolutely everything at once on a large grid of knobs. And the position of each control is your immediate feedback to show how it is set. In fact it might not need a display at all (though I have some 128x64 OLEDs coming with a view to showing oscilloscope and/or spectrum analysis view to the user). The concern of an OLED is the battery usage. That is where the segmented LCD of the POs scores highly. The display is "always on" but the things run for months on one set of AAA batteried.

 

As I want to keep my design "handheld"/portable, hence the need for a lot of little knobs. 

 

I already ordered 30 mini encoders (for just £20!) that are on a slow boat from China already but then I was thinking that pots with knobs (that have a "notch" to show where they are set) is perhaps more "old school" and immediate than an encoder that gives no positional feedback about what value the control represents.

 

I might actually get pots with knobs:

 

just in case knobs gives a better visual "feedback" to where the control is set. But it's tempting to just remove the knobs and leave them off to one side to have more "industrial"/exposed controls like pocket operators.

If it's non-linear I guess one could use some kind of translation (maybe linear interpolation) conversion table but it would be a pain if each control has to be profiled. 

Just to report back. So the Aliexpress boat finally arrived from China and I got one hundred of two different shaft length RV09 10K pots. So I wired samples up to an Arduino.

 

I thought I wanted the shorter shaft ones as they are most like the short shaft encoders on Teenage Engineering Pocket Operators. But in reality it's the longer ones that look the part and feel easily "grabbable". 

 

Inevitable video (with curious audio track added on top) of me twiddling knobs with output on that stupidly small I2C OLED screen... 

 

https://youtu.be/-MclDUuPv78

 

Nope I recorded video on my phone then uploaded to YouTube and only then realised you could hear my swivel chair creaking in the background. So I looked for a way in both YouTube Studio online and also the Studio app on my phone but it seems you can't just delete the audio. But YouTube offer a bunch of licence free music tracks you can put "on top" so I deliberately picked one that sounds "synthy".

 

It's going to be a while before I actually have my own design (FM) synth implemented. In fact I'm going to do the design in two stages. Just make a button/knob/display controller first that outputs MIDI CCs then implement the synth engine to take MIDI as input. The two can be implemented and tested in total isolation and will actually be useful without the other one. I can drive the synth using Ctrlr app on Windows without needing the physical MIDI controller and I can use the MIDI controller to input to PC apps (as long as I make the CC numbers user configurable)