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FM'ing 3 or more different Oscillators ( Is it Possible )
MUFF WIGGLER Forum Index -> Modular Synth General Discussion  
Author FM'ing 3 or more different Oscillators ( Is it Possible )
PietroC
Hello
Little Noob Dilemma
Let's say i have 3 Oscillators
i wanted them all to have the same amount of frequency modulation

Option 1 ( The Only way i know of )
Mult my FM Signal to each FM Input for 3 OSC
Adjust each attenuator independently

Option 2
Is FM'ing Externally possible to achieve ????

Example
3 OSC--->Mixer
FM'ing the SUM

Thanks To All for the Countless Help and Knowledge
( Also I believe this post should have been in Modular Synth Discussion but topic classifying is a bit confusing to me )
authorless
To make this a little more clear, you want three carriers all modulated with the same modulator by the same index?

I would send the modulator through a VCA or attenuator then mult the output to each carrier. That way you have a single control for all three indices.
Arneb
PietroC wrote:
Option 2
Is FM'ing Externally possible to achieve ????

Example
3 OSC--->Mixer
FM'ing the SUM

I'm not exactly an FM expert, but I don't think this works. IIUC FM is not a form of waveshaping - it physically works by manipulating the VCO itself, not the waveform generated by the VCO.
ranix
Not sure what you mean by "FMing externally" but check out the Happy Nerding FM Aid. It's my favorite module
oberdada
Why is it important to have the same amount of modulation?

If you feed one modulator into the FM inputs of three oscillators, it would be really important to adjust the carrier frequencies of all three oscillators, whereas the difference due to minor variations in modulation amount would be less noticeable.

Mixing three oscillators and using that signal as FM input to a single oscillator is a completely different setup.

FM of sorts can be applied to any audio signal. You have to use a delay, then modulate the delay time.
cornutt
oberdada wrote:


FM of sorts can be applied to any audio signal. You have to use a delay, then modulate the delay time.


Technically that's phase modulation rather than frequency modulation. It can achieve similar effects, though.
starthief
The first method (multing the modulator to 3 carriers) is the correct way. Other methods are going to give you interesting results though and they're worth trying.

As for "external FM", or rather PM, here are your choices, in approximate decreasing order of accuracy and sanity:

- Imitate phase modulation with a delay. No feedback, 100% wet, short delay time and attenuate the modulation amount a lot. The delay has to doppler shift (not crossfade) with delay time changes, and it has to accept audio rate modulation -- some delays will slew or filter the CV and that won't work. Disting's delay algorithm works for this.

- Something like FM Aid or Sinulator. This is also a kind of phase modulation. It's meant to work with sawtooth waves for the input but it will still do something interesting with most signals (not square/pulse).

- You can imitate FM Aid by mixing your carrier and modulator and running the result into a wavefolder, or better yet, a sine lookup table -- something like Piston Honda's external input, WMD Geiger Counter or perhaps Starling Via Scanner. The relative gain of your carrier and modulator are going to be very important here.

- If you have a VCO that will go down to low LFO rates but will phase modulate strongly enough, or which supports thru-zero FM, you can use it as a "silent carrier" -- or really, like a wavefolder! -- and mix your audible carrier and modulator into the modulation input. I've done that with Synth Tech E352 and E370.
ranix
oberdada wrote:
Why is it important to have the same amount of modulation?

If you feed one modulator into the FM inputs of three oscillators, it would be really important to adjust the carrier frequencies of all three oscillators, whereas the difference due to minor variations in modulation amount would be less noticeable.


When stacking modulators it becomes important to sync the carriers and the modulators all to the same source, or you get detune that injects unpleasant inharmonic content. Like, if you have one carrier and one modulator and the modulator is a few cents off that's not a big deal. But if you have a carrier, a modulator affecting the carrier, and another modulator modulating the modulator, the effects start to get noticeable. Or in anything more complex than that. Filtering out the unpleasantness (I've been yelled at around here before for calling this "noise") is tough to do without also filtering the high frequencies you want to hear.

In my patches I work around this in various ways. This is where music theory starts to matter. I don't know how to communicate these ideas very well. I'm not really a musician myself I just watched a bunch of youtube videos... but I'll try.

Sometimes I want more than one carrier running at the (exact) same frequency. Sometimes I want a carrier running (exactly, no detune) a fourth or a fifth up or down from the first carrier. Or a modulator that's exactly a fifth up or down from the carrier, or from another modulator. On a DX style synth, these intervals are represented by frequency ratios, i.e. a fifth is a 2:3 (or a 3:2) ratio. The carrier could be peaking exactly 2 times for every 3 times the modulator peaks, or vice-versa.

One way I get an exact 2:3 ratio is by sending an oscillator's square wave output through an audio-rate clock divider. The oscillator is running at frequency 2. I can get a square wave with frequency 1 from the /2 output of the clock divider.

To get 3, we have to get a signal that's running exactly 3 times the speed of 1. Since our 1 is a 50% square wave we know it contains odd harmonics, so each harmonic is going to be i.e. 3 times, 5 times, 7 times, etc. the frequency of the fundamental (aka 1). So we can get 3 by sending the square wave to a band-pass filter that isolates only the third harmonic. That gives us a sine wave running at frequency 3, which we can use as our modulator. I have a state variable filter I use for that which seems to work well.

If you're using an FM Aid it's driven from our oscillator which is running at speed 2. The modulator will be the output of our band-pass filter, which is a sine wave of speed 3. That gives us a perfect 2:3 ratio between the carrier and the modulator.

If you're using an FM oscillator instead of FM Aids you could use an output from your first oscillator to sync the FM oscillator. This FM oscillator would be your carrier which would be running at speed 2. But I don't know how sync would affect the modulation of such an oscillator - if the sync interferes with the modulation you'd have problems. This isn't a problem with a module like the FM Aid, since the modulation of the FM Aid does not affect the base frequency.

There are an awful lot of things you can do to get various frequency relationships between signals using the above tools, and an awful lot of frequency ratios you can get from them. And you can use clock multipliers instead of dividers, and multipliers and dividers sent through filters track at different speeds, and the phase of any synced oscillators matters... there are lots of considerations and options.
PietroC
authorless wrote:

I would send the modulator through a VCA or attenuator then mult the output to each carrier. That way you have a single control for all three indices.


Reverse Engineering ( Thats Beautiful ) I did not think of that
It would simplify the process of adjusting evenly 3 attenuverters

So i guess reverse my question to be clearer

Oscillators are only Frequency Modulatable Internally
I would not be able to Group many voices then apply a tremolo after they are mixed

but

Arneb wrote:
IIUC FM is not a form of waveshaping - it physically works by manipulating the VCO itself, not the waveform generated by the VCO.


This pretty much clears up my question
Makes sense since cv signals cannot be configured to target only pitch
Hence the FM Input on Oscillators

starthief wrote:

ranix wrote:


Thanks So Much for all this info
Im new to this and your explanations are very clear but obviously
I have much Learning / Explorations to do

My Dilemma Began cause
- i have only on Buff Mult
- I want to avoid further purchases
- Curiousity
- Trying to get the sounds perfect for sound layering / sculpting

Weapons Of Choice
https://www.modulargrid.net/e/racks/view/951326
Blairio
Yamaha's 6 operator DX7 had about thirty different 'operator as carrier or modulator' algorithms.

Even the current Reface DX has twelve such 4 operator algorithms,

I guess that in theory (at least) any of these multiple operator algorithms could be implemented in Eurorack?
cornutt
Blairio wrote:


I guess that in theory (at least) any of these multiple operator algorithms could be implemented in Eurorack?


Sure. You can do it with any VCOs, if you have enough. HOWEVER... you will only get "mathematically correct" results if your VCOs have linear FM inputs. Now, "artistically correct" is another matter, and you can get many interesting things using the ordinary volt/octave inputs. So grab some VCOs, patch them up, and see what you get.

I'll add this: In FM, if you use harmonic-rich waveforms (pulse, sawtooth) as operators, the results you get will trend strongly towards noise. Start with sines and triangles. Remember, the DX7 operators only produced sine waves.
pricklyrobot
PietroC wrote:

Oscillators are only Frequency Modulatable Internally
I would not be able to Group many voices then apply a tremolo after they are mixed


If tremolo was what you were after, this approach would work, but FM is basically vibrato. So you either have to modulate the pitch at the source (VCO), or you do it with a delay line (this is how Vibrato FX units do it).
Blairio
cornutt wrote:
Blairio wrote:


I guess that in theory (at least) any of these multiple operator algorithms could be implemented in Eurorack?


Sure. You can do it with any VCOs, if you have enough. HOWEVER... you will only get "mathematically correct" results if your VCOs have linear FM inputs. Now, "artistically correct" is another matter, and you can get many interesting things using the ordinary volt/octave inputs. So grab some VCOs, patch them up, and see what you get.

I'll add this: In FM, if you use harmonic-rich waveforms (pulse, sawtooth) as operators, the results you get will trend strongly towards noise. Start with sines and triangles. Remember, the DX7 operators only produced sine waves.


Yes, the DX7s (and all their antecedents) operators only produced sine waves, but the Reface DX operators can produce a range of waveforms from square through sine through sawtooth. The result is harmonically richer than with straight sines. I spent years programming DX's back in the 80s. The Reface DX can achieve timbres more complex than the 4 or 6 operator DX7's and DX9's etc in very short order.
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