Interview: Geert Bevin on Developing Moog Music Model 15 Synth for iOS

We went behind the scenes to talk with Moog Senior Software Engineer Geert Bevin about their highly acclaimed Model 15 iPad and iPhone synth app. A riveting read for those interested in Moog & synths.

AskAudio: What was Moogs guiding philosophy for building the Moog Model 15 app?

Geert Bevin: First and foremost, we wanted to create a modular synthesizer that provided the absolute best sound and user experience possible. Modular synthesizers have quite a high barrier to entry. The price and complexity can be extremely daunting for people. We wanted to create something that would reduce the learning curve, allowing users to explore and learn at their own pace anywhere in the world.

The Model 15 app allows anyone to be guided through the principles of modular synthesis. There are tutorial presets included that are designed to encourage learning through step-by-step instruction. You learn by doing, and the app ensures you are covered every step of the way.

In addition to this, we wanted people to be able to share their discoveries, knowledge and experience with others. So, we built in a comprehensive file and sound sharing system that allows anyone to share just about anything from within the app. There is even a special manual that teaches people how to create their own tutorial presets.

You can get the same sounds if you dial them in correctly as on the original Model 15.

Ask: It’s interesting that you chose to emulate the Model 15, which is a highly limited edition $10,000 modular synthesizer. Was the thought process behind that about making something which is exclusive, accessible to everyone?

GB:  We did want it to be accessible, yes. The Moog Model 15 hardware is already quite portable and its layout is welcoming without being “too much”. There’s something about this form factor with these large modules that makes it easier to navigate. You have space around the knobs and space around the jack connections so that you don’t feel so constrained when you’re looking at an existing patch, or when you’re creating a patch yourself.

Ask: Why have you focused on an iOS app and not a Mac or PC plug-in and Android?

GB: This all starts with the experience we want our users to have. Our goal is to make the screen go away, and to have people be immersed into the experience of the instrument. That emotional and sonic connection is critical. To achieve that level of user immersion, the technology has to be at a certain level, which is one of the reasons the Moog Model 15 app is limited to a specific set of iOS devices.  We needed a lot of CPU power and a number of new technologies that modern iOS devices offer.

We also added an overdubbing recorder that allowed people to experiment with different sounds and layers, and build up a looping performance within the app

Ask: I guess that the number of different devices makes it very hard to make the user experience consistent across all devices.

GB: A small screen has different rules than a large screen. A portrait orientation has different rules than a landscape orientation. We actually had to design for each iOS device that we support—each screen size and orientation specifically so that the app becomes an instrument on every supported device.

Ask: Can we talk about the process of developing the app? The procedure, the different stages, if you’d like.

GB: One of the key turning points was when we were bringing the visual and sound engines together. We had developed a prototype sound engine that we were very happy with and we had also spent a long time designing the GUI [graphical user interface], but when we brought them together we realized that we had a problem. The technological choices we had made beforehand wouldn’t allow us to retain the quality of sound, the quality of the graphics and the speed of interaction we needed. We had to either decide to reduce the possibilities of the app, to skimp on our vision, or find a new technological solution that would work.

We did quite a bit of prototyping around that time and found that Apple’s Metal Graphics engine would allow us to realize our vision for the Model 15 App. They basically created it for games. We needed that kind of performance on the graphics side, and the Metal engine allows you to offload almost all heavy tasks straight to the graphical processor, instead of in the central processor. Leveraging Apple’s Metal Graphics engine was the only way we were able to move forward with that quality of sound engine paired with the visuals and interactions of the GUI.

Moog Music Model 15 App - better than the real thing?


Ask: Apple’s Metal was a game changer for you. I guess that when the iPad Pro came out you probably looked at that with sparkly eyes?

GB: Well, yes. When the iPad Pro arrived we were 75% through this huge trench and it was obvious that we would need to support it as a first level device and not just an upscaled version of a regular iPad. We went back and did a redesign specifically for the iPad Pro. We also did some experimenting with the Apple Pencil, because it does provide you with a three-dimensional stylophone playing experience.

Ask: Interesting. Could you also use the Apple Pencil to more easily patch cables?

GB: Some people preferred that because your hand is not in front of the screen. It allows you to point more easily. The actual touch interaction is also faster so that it’s a higher sample rate for the interaction events. If you aren’t very sensitive to these things you might not notice it, but it is quite a big deal. When you play the Animoog keyboard with the Apple Pencil, even though at that point it’s inherently monophonic, it’s still extremely emotional. You can actually press down and articulate notes—up, down, left, right and through pressure.

Ask: Interesting, so, it seems like the app is a very faithful emulation of the original Model 15, but you did modify some things, didn’t you?

GB: The app was developed alongside an original 1979 Moog Model 15 modular synthesizer. We actually spent an excessive amount of time looking at an oscilloscope while listening through high-end monitors. We had listeners A/B the original to the app, building patches to compare behavior.  We also brought in our legacy modular engineering and manufacturing team as part of the development process. 

But there are a few things we did change. The original Model 15 has a Sync input to the oscillators that was meant to keep different oscillators in tune with each other. Running this type of sync on a digital device without that much drift between the oscillators (we did model the drift) didn’t require that kind of sync anymore. We decided to re-envision the sync behavior while keeping it true to the original spirit of the phase-lock loop design. You can get the same sounds if you dial them in correctly as on the original Model 15, but there were a lot of areas where the sync didn’t influence the sounds very much or at all. So, the way that we reimplemented it was to make the whole range of the sync settings and knobs musically useful. This actually brings out an aggressiveness and a growl in the instrument that wasn’t available in the original, which a lot of people seem to like.

We wanted to be able to use the instrument as a processor for sounds as well as use sounds from other applications as sources for the instrument.

Ask: So in a way Moog Model 15 app is its own instrument too?

GB:  Yes, the app has become its own instrument. We added an extension cabinet to the Model 15 app that adds various functions. Being modular, we felt it was extremely important that it be able to interact with almost anything on an iOS device. We wanted to be able to use the instrument as a processor for sounds as well as use sounds from other applications as sources for the instrument.

The same thing went for control. The MIDI implementation is very robust and versatile. MIDI learn, for example, is quite extensive. The app can learn any knob or switch via MIDI and includes both 7 bit and 14 bit support. We also added a module called MIDI Bridge, which functions sort of like a fixed set of patch points that you know your gear will be connected to. So you can connect a specific controller to one of these patch points or even an external digital low frequency oscillator, or any kind of other modulation source. You can really make it part of your own set up.

Ask: So, basically you can connect a controller just to specific functions of the app that will only control that and then everything else could be controlled by another MIDI controller?

GB: Yes.

Ask: That’s incredible. And then you’ve also got inter-app audio—you’ve got Ableton Link support?

GB: Yes, both are there. We also included a stereo delay after the trunk lines. You can modulate the mix level, time, and the feedback of the delay. It goes all the way from being extremely long with eternal feedback to being super short and almost creating flanging and phasing effects, to chorusing. That delay is quite powerful.

Something we realized during sound design was that everyone wanted more attenuators and more basic amplifiers. So we added two reversible attenuators and two amplifiers. The reversible attenuators can be CV controlled at audio rates, which turns them into ring modulators. Just adding those two attenuators made a big difference.

There’s a lot of listening, looking, and measuring in order to impart the character of an analog instrument.


The amplifiers were designed in such a way that they can also be used as multiples because that was the other part everyone was asking for. There were only two multiples in the original Model 15, so combining that together with a set of amplifiers covered most of the needs for sound design.

We also added an overdubbing recorder that allowed people to experiment with different sounds and layers, and build up a looping performance within the app, which has been a fun addition as well.

Ask: You mentioned the drift, and that’s one thing I wanted to ask about. How do you go about emulating that analog drift? That kind of, almost, randomness. Is it, randomized within the app or you have some kind of function that does that?

GB: It’s much more complex than that. I can’t comment on the algorithms, but there’s a lot of listening, a lot of looking, and a lot of measuring that goes on in order to impart the character of an analog instrument while still keeping the control, which is what people expect from a digital device.

Ask: What would you say were the main, or the biggest, challenges for Moog emulating the original into an app?

GB:  Making it feel like a real instrument. Capturing the responsiveness of a sound or how the instrument reacts when you turn a knob. Or creating the emotional feeling you have when you interact with different controls, even though it’s a glass surface and you can’t possibly have the same experience as touching a hardware control.

There is something about the linearity or non-linearity of a knob—how the potentiometer interacts with the voltages. The smoothness or the quickness with which you can change the value of a particular function… all those things have to come together with the visual representation and the overall sound—and at a very low latency. Getting that to work with older devices like the iPad Air 1 was a challenge. Accurately creating the sound engine was a major undertaking, but our team did a great job coming up with algorithms that were respectful of the CPU on one end, while offering four-voice polyphony and accurately emulating analog character in a very faithful way; both of which are kind of a big deal.

Ask: Is this possibly the start of more iPad app versions of Moog synths?

GB: I am not at liberty to comment on product development...

Ask: In terms of making decisions for the app in the design and programming process, were you using a beta test team outside of the programmers?

GB: We have an amazing team at Moog. We’ve got people that have been here for a very, very long time. They have all this knowledge and history behind them to help out with all the decisions and approaches that we’ve taken with the app.  

We spent a long time with our beta testers. From people interested in MIDI sequencing to folks from analog backgrounds who had never touched an iOS device. People that just want to make music to people that love creating sounds. And then, of course, folks that are gear-heads that hook up everything that they can imagine. 

As we went through the initial beta test, we watched people use the app.  Putting the device into someone’s hands and [without directions] saying, “go ahead” and then watching them interact with it is so revealing. That led into a whole series of revisions and tweaks that we believe have made the app much better. 

Download Model 15 for iOS here.

Read our review of Moog's Model 15 here.

More about Moog Music here


Synthesis 101
The Filter
by Bob Moog Foundation

"Rounik is the Executive Editor for Ask.Audio & macProVideo. He's built a crack team of professional musicians and writers to create one of the most visited online resources for news, review, tutorials and interviews for modern musician and producer. As an Apple Certified Trainer for Logic Pro Rounik has taught teachers, professional..." Read More


Soooo.... Apple devices are good...... i definitely had that drilled into me.

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