Announcing: Sapphire Tube Unit

Announcing: Sapphire Tube Unit

I’m excited to announce a fun new gadget for you to play with. My latest VCV Rack module Sapphire Tube Unit has just been released.

Tube Unit is primarily an effect synthesizer, although it can be used as a filter also.It is a simulation of a resonant tube as shown in this schematic:

Sounds are generated by air flowing through a spring-loaded piston valve. The resulting vibrations of air pressure feed into a tubular waveguide, and the sound waves reflect back and forth in the tube.

Although inspired by physics, the simulation departs from realistic physics in an interesting way. Instead of modeling air pressure as a scalar field, Tube Unit uses a complex-valued field. That means the air pressure at any time and place inside the tube has both a real part and an imaginary part. In some ways the signal is more like an electromagnetic wave with circular polarization than a pressure wave, but moving at sonic speed and oscillating at audio frequencies.

Using a complex-valued signal provides two interesting opportunities:

  • The output is stereo. The real part of the output signal becomes the left channel, and the imaginary part becomes the right channel.
  • Complex arithmetic operations like addition and multiplication have interesting effects when applied as variable parameters in the feedback path. For example, multiplication with a variable value e^{i \theta} when reflected from the open end of the tube causes a rotation in the returned wave’s “polarization”, causing interesting spectral effects. (See the VORTEX and ANGLE parameters in the documentation for more about this topic.)

The Tube Unit documentation page has a more detailed explanation of the physics and control parameters.

Here are some demo videos. Headphones are recommended to best experience the stereo field.

First, a basic demo showing the controls and how they affect the sound. It also shows using Tube Unit as a filter.

Tube Unit is polyphonic across all inputs. Here is a demo of processing CV inputs across 8 simultaneous channels.

As always, I’m eager to hear what you think. Please post any questions or comments below. I always receive constructive feedback on this forum about what is confusing, what can be improved, or what needs to be fixed.

And above all, I hope you have a great time making some wild sounds with this thing!


This is cool! :star_struck:


Wow, that is wild. Very cool idea. Nice work!

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This module is really really nicely done, Don. Both fun and unique.

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Wow Don… The module looks great! It also seems to be capable of some really interesting sounds and I look forward to playing with it. My first thought was that it seems like a distant, cosmic twin to Elements, and that I would love an expander with a strike input and envelope :slight_smile: Which would make it kind of like an Elements/Rings with a whole new range of crazy sounds possible.

Thank you and well done!

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I’ve had a quick play, and it makes some really interesting sounds. Some settings are reminiscent of MI Elements, and others are quite unique! However, with an external input, I can’t seem to get any of the original signal through. That’s not a problem as I can mix some in on another channel, but using guitar as an input, everything I play comes out at the same pitch. I scanned through the manual quickly, and it does say the root freq control isn’t a V/Oct input, but is there any way of getting different pitches out of it? Sorry if I’ve missed something really obvious, but I couldn’t seem to tune it!

Yes, Tube Unit is not going to work very well as a tunable instrument. It’s not really an oscillator in the usual sense. I intended it more as a sound effect generator that could be used for drones or percussive effects.

And when you connect an audio input, only the highlighted controls (root, decay, angle) have any effect on the input. Acting as a filter like this was something I added toward the end of development, because it was fairly easy and I was curious what would happen.

You can try turning airflow all the way to zero to focus only on the audio input. Then adjust root and angle for different spectral effects. The closest analog here would be for a comb filter or a delay with a very short time lag.

Without any audio input, I’m planning on experimenting with carefully hand-tuning Tube Unit to a middle C note (using something like HotTuna), then recording it with SickoPlayer, which should allow V/OCT pitch control. I will have to find out how that sounds, and how far above or below the original pitch it can go.


Ah I wondered about that. Making drones, tuning it roughly then sampling it is always an option!

drone througt always a good option…

I don’t use “drone” word…My word is ‘ambiance’

Wow, Tube Unit is really a masterstroke. Elastica is just so interesting but a little challenging to use in patches because a lot of its interest lies in how it tapers and gently “gongs” after being activated, so any competing frequencies tend to mask its incredible beauty easily, you have to carve a space for it to really shine, but its fire in terms of thoughtful design and colorful sound. But Tube Unit just gets up in your face with gritty fat character and with fun modulation can just shape the world of your patch. Ha now clicking away at this, I think maybe I should just stack the 2, Elastica and Tube Unit, oh boy, tonight I’m gonna make a real resonating racket!!


Amazing stuff you create(d)!

Physical Modelling and Resonators are scarse concepts in VCV Rack and synthesis/synthesizers in general.

Which is strange. Since few will question the added value and popularity of Mutable / Audible Instruments Rings / Resonator and Elements / Modal Synthesizer.

Especially valuable in the “slower” more “ambient” or “drone based” genres that rely heavily on “spectral” or “timbral” aspects of sound/music.

Both Elastika and Tube Unit are great additions. Very unique and versatile. And very nicely implemented.

Both Tube Unit and Elastika are in effect pretty complex units with fixed routings. Personally I would love to see some elements from these modules in isolation or in some selectable configuration. Either as seperate modules or the current modules with options to (re)route or in/exclude subprocesses/elements.

But hey, no complaining here. I’m very happy and thankfull that people like you provide us with these great (and free!) modules!

Alas, my c++ coding skills are pretty limited. As is my knowledge of the “math” needed to actually (efficiently) implement stuff that I could dream up. My deplorable math skills do not even allow me to read and understand most of the abundance of (more technical) information on Waveguides and Resonators available on the web.

Actually I’m having trouble even reading and understanding lots of DSP code. I’m even having trouble understanding some of the implementations in VCV Rack modules enough to adapt them or reuse their (sub)solutions. Although I recently started to delve deeper into c++ and VCV Rack Development.


I (and many with me) rely on people like you to provide us with the tools to further explore the wonderful world of (modular) synthesis.

Thanks again…and…lookin forward to new creations?


Awesome !

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I had Tube Unit connected to an array of Krell generators playing in the background now for a couple of hours, getting some awesome soundscapes.


I’d love to hear that if you are willing to post a video/audio recording!


I second that!

This thing is neat, I found I can use it like a really peculiar reverb. Costs a little more CPU than most, but sounds extra cool

When I made this patch I had no idea I would be uploading a video, so I decided to use a couple of unreleased modules as Krell generators. If you want to play with the patch you will need to use my development branch, but its a fews days away from being release ready.

tube space.vcv (2.9 KB)

Update, a more traditional krell sound, with an envelope on the output


An updated version of the Sapphire plugin (version 2.2.2) was released today. The most significant change was in Tube Unit. It now includes a menu option to toggle the VENT port to a SEAL port, where the input gate has the opposite effect. Instead of a high VENT gate making Tube Unit quiet, a high SEAL gate makes Tube Unit produce sound.

Here is a demo video:

Also, there was a minor graphics bug that could happen if you had more than one Tube Unit in the same patch, with at least one connected to input audio and at least one not connected. Sometimes a Tube Unit would incorrectly display a yellow path around DECAY, ANGLE, and ROOT when there was no audio input, or would incorrectly not show it when there was audio input. This has been corrected.