Test Patch (updated v5): Linear FM & Phase Modulation VCOs with External Sync

(Edit: Please skip to the latest version below.)

Here’s a test patch I put together that uses Algomorph to put VCOs through their paces.

AlgomorphVCO_TestPatch_Tetradecahedron.vcv (16.3 KB)

Quick demo video, all synth no talking (recorded with the Rack engine running at 48 kHz):

I set each group of oscillators to matching frequencies and FM modulation depths. Each operator’s frequency and modulation depth, along with the set of three algorithms which all groups smoothly fade between in lock-step, are chosen to elicit interesting but not excessively noisy results from all the groups collectively. (Hence: the low modulation attenuators.)

I only include operators if they include both a PM / TZ / LFM input as well as a sync input. The included oscillators are:

Phase Modulation

  • Bogaudio – VCO (sine and triangle)
  • Squinky Labs – Kitchen Sink (sine and triangle)
  • Valley – Terrorform (sine and triangle)

True Frequency Modulation

  • Fundamental – VCO and WT-VCO (TZ) (sine and triangle, each)
  • Valley – Terrorform (TZ) (sine and triangle)

The settings for each operator:

Operator 4: [Freq - 65.407 Hz, i.e. Octave -2 & Ratio x1] [Modulation Depth - 6.6265%]
Operator 3: [Freq - 196.22 Hz, i.e. Octave -2 & Ratio x3] [Modulation Depth - 6.988%]
Operator 2: [Freq - 163.52 Hz, i.e. Octave -3 & Ratio x5] [Modulation Depth - 5.1807%]
Operator 1: [Freq - 114.46 Hz, i.e. Octave -4 & Ratio x7] [Modulation Depth - 4.5783%]

Wildcard:   [Freq - 16.352 Hz, i.e. Octave -4 & Ratio x1] [Modulation Depth - N/A] [Oscillator Amplitude - 0.20482%]

And these are the three algorithms (a horizontal red line indicates a silenced operator):

Among the phase modulation oscillators (in the context of this patch and the chosen algorithms/frequencies), I find that the resulting sound can easily be grouped into 2 categories among which I believe the primary difference is the implementation of oscillator sync:

Bogaudio and Squinky Labs
NYSTHI and Valley

For now, Algomorph for VCV Rack 2 is only available as source code. That can be found through here:

See also the Algoseq version of this patch below.

Developers: Want me to add your module to this patch? Am I using your module incorrectly? Please let me know!

Everyone: If you find an interesting set of algorithms, frequencies, and modulation depths for use in this patch, please share!


This is a really great idea. Most videos about VCV and FM where someone states that FM is “so super nice” then just show a simple two or three operators-in-serial patch, which is IMHO (!) the “boring” part of FM. I would love to see two improvements here: First is a VCA (with an envelope input) on the modulator side and the second is a way to route an input directly to a fifth output (which is then routed to the mixer). That way you could easily setup an algorythm, where an operator is a carrier and a modulator at the same time. In terms of useful algorythms have a look at the digitone manual / Page 84, those are really great 4-Operator-Algos.

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This is available: while no operator is selected (no flashing lights), click the Modulator Button corresponding to the operator which you want to mark as a Designated Carrier.

This is available in the larger version of the module: Algomorph Advance. There are also many other options available for the 5 assignable Auxiliary Inputs.

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:heart: Awesome, where should I send my money?

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I sincerely appreciate that!

In the future, Delexander Volume 1 (the plugin which features these modules) might be sold on the VCV Library as a premium open source plugin. For now there’s a donation link that can also be found on the library.

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I will buy it in an instant, the moment it is released. That is a promise! No more matrix-router-mixing for my digitone emulation. Thanks for your work!

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Why the love for FM-OP? what does it do that Kitchen Sink doesn’t do?

Why the shade for FM-OP specifically? :sweat_smile: Kitchen Sink doesn’t need your help to set itself apart; it speaks for itself.

Bogaudio’s FM-OP has one of my favorite interfaces among any of these modules, and it started me on this whole journey with FM and Algomorph, and it sounds great. I wish it had sync, though.

Truth be told, I’m weak in the knees for any oscillators like this. And I’m always looking for more. Every oscillator featured in this patch is a favorite of mine, too!

because it is my nemesis. everyone loves it, but it’s nothing that special. so sue me!

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Because Ratios.

FM-OP let me put in a 1.75 Ratio. In KitchenSink that’s essentially turning three knobs. “Octave” the same as the other operators, Ratio = 1, and Fine = 9.6882. Now let’s try 2.75 on the KitchenSink → Ratio = 2 and Fine = 5.5127.


Oh but it is! My lawyers will be in touch. :wink:


in TZOP ratios are same as DX series (plus extras on the sidelines) ratios are DISCRETE

I you want more complex ratios you modify the base tuning of the OP

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@robert.kock might put in an option to allow ratio to be continuous if you ask him.

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Thank you for tagging me. Yes, please raise a Issue on github. I don’t have access to my development environment at the moment due to family issues, but I hope to be able to pick this up in the new year.

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I already did that a few weeks ago. I copied my feature requests from squinky over to your github.

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Here’s a video of a version of this patch which uses the still-in-development Algoseq (a 32-step, multi-sequence, knobless version of Algomorph). This allows for more densely varied testing of oscillator characteristics. The oscillator frequencies and modulation depths are the same as in the OP:

Operator 4: [Freq - 65.407 Hz, i.e. Octave -2 & Ratio x1] [Modulation Depth - 6.6265%]
Operator 3: [Freq - 196.22 Hz, i.e. Octave -2 & Ratio x3] [Modulation Depth - 6.988%]
Operator 2: [Freq - 163.52 Hz, i.e. Octave -3 & Ratio x5] [Modulation Depth - 5.1807%]
Operator 1: [Freq - 114.46 Hz, i.e. Octave -4 & Ratio x7] [Modulation Depth - 4.5783%]

Wildcard:   [Freq - 16.352 Hz, i.e. Octave -4 & Ratio x1] [Modulation Depth - N/A] [Oscillator Amplitude - 0.20482%]

I added one new oscillator:

  • NYSTHI µOPERATOR (the sound is not the same as TZOP!)

Also, because triangle-wave FM also sounds good to me within the same parameters and serves as an additional differentiator between oscillators, I added triangle-core versions of these modules:

  • Fundamental VCO & WT-VCO
  • Bogaudio VCO
  • Squinky Labs Kitchen Sink
  • Valley Terrorform (edited to add)

I think this sequence of 32 algorithms does a better job showing what I like about the Fundamental oscillators in this context. They behave very differently from all the rest, but the resulting sound is still strongly correlated with that of the rest and I think it sounds particularly interesting to layer Fundamental with any of the other oscillators here.

The original post’s patch and video have been updated similarly (with an Algomorph Pocket version).

This video was recorded with the Rack engine running at 48 kHz.

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BTW, regarding Bogaudio FM-OP and sync:

In my testing, all Bogaudio sine VCOs sound identical and respond to LFM identically. That includes these modules:

  • VCO
  • LVCO
  • SINE
  • FM-OP

The first three all have sync, and it too behaves identically on all three.

For that reason, I think it’s safe to say we already have “FM-OP w/ sync w/o envelope”, and it is featured in both patches here, and I’m content with that. :+1:

I find this a very interesting thread, tried to open the patch to check it out for myself but the algomorph stuff is missing. Are you guys building that yourselves?


Yeah, the VCV Rack v2 version of Algomorph is only available as source code for now. You can find that, as well subscribe to the Rack v1 version, via the library:

Me too - great work Anthony!

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