Monotron Duo.

MirlitronOne · Posted: Sat Apr 28, 2012 11:09 am Post subject:

Since the ribbon resistance change is linear, the oscillator response must be exponential otherwise the note spacing would change along the ribbon.

sneakthief · Joined: Jul 24, 2006 Posts: 569 Location: Berlin

MirlitronOne - the microprocessor takes the ribbon voltage and quantizes it. So you can't make any conclusions about the relation of the ribbon's output to what the microprocessor is putting out.

Notice that a pulse from VCO1 is going back into the microprocessor. I believe that this allows the microprocessor to measure the current frequency and compensate for temperature drifts and other things that affect tuning & scaling (cpu-controlled auto-tune).

chorizu - look at pin 4 of the microprocessor to figure out what V/Octave that the vco's are using.

chorizu · Joined: Apr 28, 2012 Posts: 8 Location: Heidelberg/Germany

@sneakthief-i meassured pin 4 against ground,without activating the ribbon it's 2.96 Volt.When playing the ribbon at goes down to 1.5 Volt on high notes and above 3.16 Volt on lownotes,I'd ressume that it's some feed for the OP

sneakthief · Joined: Jul 24, 2006 Posts: 569 Location: Berlin

Well that seems close to 1V/Oct, albeit if you measured it correctly than it's simply inverted.

The ribbon spans roughly 18 notes (1.5 octaves) and you're getting a voltage swing of 1.66V.

However, please measure again using the chromatic scaling mode! Then you'll know for sure what the internal voltage scaling is.

Anyhow, if you look at the schematic, the voltage from pin 4 of the microprocessor gets added to the reference voltage (VD) then goes to a current mirror (Q1 & Q2) before being sent to the 2 VCO's.

chorizu · Joined: Apr 28, 2012 Posts: 8 Location: Heidelberg/Germany

Yeah,thank You,I'll take me some time to make better meassurements.I ordered some micro equipments on conrad.

chorizu · Joined: Apr 28, 2012 Posts: 8 Location: Heidelberg/Germany

@seakthief-You seem to right,but i first explain my more disinct meassurement: Pin 4 to GND toggles around 2,20 Volt and 1,7 Volt(i dont no why). then i hit a high key it goes to 1,54 stable,then i hit 1octave lower it goes to 2,54 Volt.normaly it must have been reverse?

sneakthief · Joined: Jul 24, 2006 Posts: 569 Location: Berlin

As I said, in this case it just means that the VCO is using inverted 1V/Oct scaling.

If you want to control it externally, then invert the CV with an inverting op-amp Smile

chorizu · Joined: Apr 28, 2012 Posts: 8 Location: Heidelberg/Germany

Yeah,alright,Thank You!

chorizu · Joined: Apr 28, 2012 Posts: 8 Location: Heidelberg/Germany

@sneakthief- i'm a little uneperienced in electronics,is this NAND gate good for my purposes
http://www.stm32circle.com/resources/Datasheets/74LX1G04.pdf

sneakthief · Joined: Jul 24, 2006 Posts: 569 Location: Berlin

Posted Image, might have been reduced in size. Click Image to view fullscreen.

http://sound.westhost.com/dwopa.htm#invert

chorizu · Joined: Apr 28, 2012 Posts: 8 Location: Heidelberg/Germany

Thank You,very interesting suggestion and Site, too.First, i have to make oprations on how disconnect Pin 4.

TakeshiAndTheKid · Joined: Feb 27, 2011 Posts: 9 Location: Cork, Ireland

Hey there, looking to get one of these things soon. Sad to hear there's no easy way to do a CV Pitch.

There's only one mod I really want to do though, I'm getting one because it seems really awesome for videogamey sounds. To me, having 2 square waves seems kind of redundant so I'd like to change the second square to a pulse, maybe.

Would this be hard to do?

vonkhades · Joined: Jun 17, 2012 Posts: 1 Location: madrid

Hey guys!

Im tryin to mod my DUO so I can control it externallly... I've read the whole thread but couldnt find info on it ... i've googled a lot but still unsuccesfull to find any info or schematics. The last thing Ive tried was dinsync.info/2010/06/how-to-modify-korg-monotron.html which is for the regular monotron.. but doesnt seem to work... as you said on the last messages the CV wont be easy...

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Anyway there a couple youtube users that made some advancements on this.:

http://www.youtube.com/watch?v=ae6t6bN8Pyg
http://www.youtube.com/watch?v=--UNbu23DSY

I've tryied to contact them too, but no answer yet Sad

do you have any suggestions? what pads you solder or a schemmatic/resitor values?? Im sequencing it with a baby10, with both CV and Gate set max outputs to 5v.

cheers!!

andeje0 · Joined: Jun 11, 2012 Posts: 5 Location: Florida

Anyone have any updates on figuring this out?

Cynosure · Posted: Tue Jun 26, 2012 5:03 pm Post subject:

I saw someone posted this - http://www.youtube.com/watch?v=1-E7qPsI4Cw

andeje0 · Joined: Jun 11, 2012 Posts: 5 Location: Florida

Hard to tell if he's really sequencing the pitch there or just the gate and the fiddling with the pitch knobs...will give it a go in the next few days and see...

andeje0 · Joined: Jun 11, 2012 Posts: 5 Location: Florida

Chorizu have you had any luck testing the inverted op-amp to pin 4 of the micro-controller to control the pitch?

TakeshiAndTheKid · Joined: Feb 27, 2011 Posts: 9 Location: Cork, Ireland

Is there any way to stop that annoying popping noise you get when you disengage the ribbon controller on the Duo? It kills my ears when using headphones. It's not as noticeable with the filter up but with it down it's unbearable.

andeje0 · Joined: Jun 11, 2012 Posts: 5 Location: Florida

Try reading some of the links of this thread on Muffwiggler -

http://www.muffwiggler.com/forum/topic-47195.html&highlight=monotron+noise

TakeshiAndTheKid · Joined: Feb 27, 2011 Posts: 9 Location: Cork, Ireland

TakeshiAndTheKid · Joined: Feb 27, 2011 Posts: 9 Location: Cork, Ireland

Ah got it now. I think this is talking more about the background hiss though, and the solution looks very awkward. I get a "tapping" noise when the ribbon controller is disengaged.

Bozzy · Joined: Dec 18, 2012 Posts: 1 Location: Italy

alfa66 · Posted: Mon Mar 03, 2014 8:41 pm Post subject: Making sense of the Monotron DUo

I would like to share some thoughts, after beginning modding my monotron DUO.

I just want to capture these thoughts, so they don't get lost and I can share more with the community as we go.
In fact, I have read this thread and proved very inspiring to me, so hopefully somebody else will find my comments/findings useful.
Also, please be kind if I make any mistake Smile

Ultimately I want to MIDI-fy my Monotron DUO: so far I replaced the VCF knobs with a digital potentiometer controlled (via MIDI) by a teensy 3.1 - this works great.

I wanted to use the DAC of the teensy 3.1 to control the pitch, here is where things "seem" complicated, but I now have a plan (the plan consists ultimately in desoldering completely the TI uP and substitute a few of its pins with teensy 3.1 ports.... but let's take one step at the time).

According to my calculations, V bias is 1.35V. VD is 3.36V.
So Vbias seem more like a mid point, a sort of ground for signals that vary between 0V and 3.36V.

In this post i want to start pointing out that IC3B is a non-inverting low pass filter, its topology is well described here:
http://www.interfacebus.com/opamp-low-pass-active-filter-design.html

The purpose of this filter is to turn the frequency generated by the MSP430 (pin 4, aka "PITCH", which is likely PWM) into a steadier value (CUTOFF), which is used to provide the pitch (linear voltage) of the main VCO.
The configuration of Q1 and Q2 is, in fact, another well known topology that uses logarithmic voltage (controlled with the MASTER PITCH KNOB, I think of this more as an octave selection) and linear voltage (our CUTOFF point) to control the frequency of the VCO.

All i am saying (and I have yet to try it) is that we don't need to tap on the PITCH node to change the pitch. I bet that if we put an oscilloscope probe there we will find a PWM signal (the depth of the PWM signal controlled by SW2A, which changes our scale from major to minor, etc..... again, speculation here, haven't made any measurement - I just studied briefly the MSP430).

What I am suggesting is that to control the pitch of the main VCO we should just tap onto CUTOFF.
CUTOFF is the same voltage that goes to the VCF. I thought about this, it makes sense as my old Poly 61 had a "keyboard tracking" filter, so the cutoff frequency of the filter was the same frequency of the key pressed.

Please let me know what you think of the above, I won't have time to try that myself for a least a couple of weekends.
If I have time I will start writing some equations for this filter, before digging on how the VCO topology (Q1 and Q2) work....

Have a great week !

jingkon · Joined: Aug 21, 2009 Posts: 4 Location: Michigan

@alfa66

This is exciting news, though I can't really comment on any of your theories. I hope to hear that you've cracked the code on controlling the duo's pitch. Between here and Muffwiggler I keep thinking someone will figurd it out.

John

KalleWhoever · Joined: Mar 15, 2014 Posts: 1 Location: Bremen

Hi guys,
I discovered that the MSP430 within the Monotron Duo is quite easy to program.

You know the free space on the right hand of the PCB?
Well, the five holes over there form a programming header, which will allow you to programm the MCU with your Launchpad.

If you take a look at the schematics you'll find these header next to the MCU. I soldered these pins right to the J4 Header on my TI Launchpad, but you'll find some tutorials on the web on How-To-Use-the-ti-launchpad-as-isp, so I won't explain any futher.