Hi,

I'm fairly new to designing synthetizer with Pure Data and I was wondering if you could help with this undertaking of reproducing the basic fonctionalities of a Monotron with Pure Data, which is a small analog synthetizer that reproduces vintage synth sounds, as described here : http://www.korg.com/monotron. The page also contains the basic schematics, the full schematics are available here.

I've began to grasp what are oscillators, VCF and LFOs, but I still don't understand quite well how to plug them all together to emulate the analog circuitry of the synth. Understand that I don't want to absolutely reproduce the Monotron: a simple, vintage sounding synth would be a good start.

Thank you for your time.

Imho the real interest in creating a synth in Pd inspired on an analog synth would be ... learning things, but it's very hard to get a 'vintage sounding synth'.
Did you find so 'many good vintage sounding plug-ins' over the net ?

Nau

That's right, it's to learn things that I'm doing it. There're examples of Pd synths over the net, but it's never quite the sounds I want, and I'm more interested in the building, to understand how it works rather than using a final product.

Basically you want to create an oscillator (bandlimited) then feed that through a resonant filter, then to your speakers.
Bear in mind lop~ isn't resonant, other people have created externals that are, or there is the moog~, although this will sound nothing like the monotron filter, from what I remember Korgs might be 12dB/octave (might be wrong!). Good luck in designing your own Analogue modeling filter, I tried and it was solid!

To replicate the monotron ribbon controller, it's probably simplest to control the frequency of the oscillator
create another oscillator (a simple sine wave), this will be your LFO. connect that up to either filter cutoff or amplitude
Amplitude would require you to multiply the output of your main oscillator by the output of LFO
Cutoff would mean multiplying the LFO output by The value you are inputting into your filter cutoff input.
You could incorporate a button switch so you could flick between them like the monotron

It is a very simple synth to emulate, I recommend reading this site to explain the basics of subtractive synthesis in PD , it also mentions bandlimited oscillators, which will stop them aliasing THIS IS VERY IMPORTANT!

http://en.flossmanuals.net/pure-data

A guy from this forum also wrote this site
http://obiwannabe.co.uk/html/music/6SS/six-simple-synthesisers.html

You will soon grow bored of replicating the monotron, and move into doing minimoog replications, and maybe even modular stuff.
I saw a video of someone who did a Swarmatron replication, very impressive. Wish I had that patch...

Ok, so here's what I come so far (see attached file). The pd 16x is an object that produce an antialiased oscillator that I found in the documentation. I have my LFO which is linked for the moment to a VCF filter. Notice the radio button that I put to flick it to oscillator (unfortunately, I don't know how to do that yet).

I came across nice sounds when I modulate the sliders, but I think something missing, perhaps due to the nature of the filter. Also the first parameter of the vcf~ object appears to have a huge influence on the sound, but I don't know what is it nor how to change it.

Some recommendations ?

http://www.pdpatchrepo.info/hurleur/patch.png

I'll have to check out your patch tomorrow, but at a glance you might not want to use vcf~, it is basically a band-pass filter, and the monotron uses a low pass filter, try the moog~ filter to see if that's nearer the sound.
If you right-click on the filters (this works with all boxes) they have help files in which it describes what it does, and also has a small patch where you can try it out.I remember the moog~ one has the filter modulated by an LFO, exactly what you want
some people have done other, less Moog-sounding resonant low pass filters, I can't remember any off the top of my head.
anyway, until tomorrow....

The moog~ filter produces nice sounds, but generates a lot of clipping. You said it's possible to build our own filter, in my case it would some kind of low pass filter with added resonance, how should I proceed ?

You can try the filters I posted here. They're implementations of Butterworth filters.

http://puredata.hurleur.com/sujet-5342-signal-controlled-filters

You could have a slow rising phasor controlling the LFO speed, so the modulation would speed up with time.
Other than that there is not much to do with the patch you have, you'd probably find it more rewarding to build an open ended synth.
Try hooking it up to a MIDI keyboard, then you can take full advantage of envelope generators, and mod amount via velocity.
Maelstorm knows his stuff with filters, he helped me out a while ago when I was trying to design biquads from filter co-efficients.
I looked into creating filter abstractions, I've done bilinear transforms and impulsive invariant methods in theory situations, but when it comes to designing one in PD you have to write it in C, something I can't do. Lot's of oversampling, non-linearities.....
Let me know how you get along, I've got pdf files of patents for ARP filters, I think I might have SEM or curtis chip from the Prophet 5, they have circuit diagrams and stuff. I also have a paper discussing ways of modeling the filter from an EMS VSC3 in digital form.
www.acoustics.hut.fi/dafx08/papers/dafx08_06.pdf
If they are any use to you I will put them up

Hello, I was wondering how did this one turned out in the end. Maybe LaurentC is still around

thanks

Here is my try.

Heres the link:
https://www.dropbox.com/s/blwkk8ouv0uezqs/monotron.pd

Just bought a Korg Monotron and got amazed. I'm also interested on learning how it works and what's behind it's ~space magic~ so I found this thread.

I haven't tried your patch, LaurentC, but I found this project which is based on a PD patch: https://github.com/jkandzi/Monotone-Delay

The youtube demo sounds quite nice!