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Using a MIDI Control Surface to Interact with a Simulink Model

This example shows how to use a MIDI control surface as a physical user interface to a Simulink model, allowing you to use knobs, sliders and buttons to interact with that model. It can be used in Simulink as well as with generated code running on a workstation.

Introduction

Although MIDI is best known for its use in audio applications, this example illustrates that MIDI control surfaces have uses in many other applications besides audio. In this example, we use a MIDI controller to provide a user configurable value that can vary at runtime, we use it to control the amplitude of signals, and for several other illustrative purposes. This example is not comprehensive, but rather can provide inspiration for other creative uses of the control surface to interact with a model.

By "MIDI control surfaces", we mean a physical device that

  1. has knobs, sliders and push buttons,

  2. and uses the MIDI (Musical Instrument Digital Interface) protocol.

Many MIDI controllers plug into the USB port on a computer and make use of the MIDI support built into modern operating systems. Specific MIDI control surfaces that we have used include the Korg nanoKONTROL and the Behringer BCF2000. An advantage of the Korg device is its cost: it is readily available online at prices comparable to that of a good mouse. The Behringer device is more costly, but has the enhanced capability to both send and receive MIDI signals (the Korg can only send signals). This ability can be used to send data back from a model to keep a control surface in sync with changes to the model. We use this capability to bring a control surface in sync with the starting point of a model, so that initially changes to a specific control do not produce abrupt changes in the block output.

To use your own controller with this example, plug it into the USB port on the computer and run the model dspmidi. Be sure that the model is not running when you plug in the control device. The model is originally configured such that it responds to movement of any control on the default MIDI device. This construction is meant to make it easier and more likely that this example works out of the box for all users. In a real use case, you would probably want to tie individual controls to each sub-portion of the model. For that purpose, you can use the midiid function to explicitly set the MIDI device parameter on the appropriate blocks in the model to recognize a specific control. For example, running midiid with the Korg nanoKONTROL device produces the following information:

>> [ctl device]=midiid
Move the control you wish to identify; type ^C to abort.
Waiting for control message... done
ctl =
      1002
device =
nanoKONTROL

The actual value of ctl depends on which control you moved.

If you will be using a particular controller repeatedly, you may want to use the setpref command to set that controller as the default midi device:

>> setpref('midi','DefaultDevice','nanoKONTROL')

This capability is particularly helpful on Linux, where your control surface may not be immediately recognized as the default device.

After the controller is plugged in, hit the play button on dspmidi. Now move any knob or slider. You should see variations in the signals that are plotted in the various scopes in the model as you move any knob or slider. The model is initially configured to respond to any control.

Examples

Next, several example use cases are provided. Each example uses the basic MIDI Controls block to accomplish a different task. Look under the mask of the appropriate block in each example to see how that use case was accomplished. To reuse these in your own model, just drag a copy of the desired block into your model.

Example 1: MIDI Controls as a User Defined Source

In example 1 of the model, we see the simplest use of this control. It can act as a source that is under user control. The original block MIDI Controls (in the DSP sources block library), outputs a value between 0 and 1. We have also created a slightly modified block, by placing a mask on the original block to output a source with values that cover a user defined range.

Example 2: MIDI Controls to Adjust the Level of a Single Signal

In this example, a straightforward application of the MIDI controls block uses the 0 to 1 range as an amplitude control on a given signal.

Example 3: MIDI Controls to Split a Signal Into Two Streams With User Controlled Relative Amplitudes.

In this example, we see an example where a signal is split into two streams: $\alpha u$ and $\left ( 1-\alpha\right ) u$ where $\alpha$ can be interactively controlled by the user with the control surface.

Example 4: MIDI Controls to Mix Two Signals Into One

In this example, we create an arbitrary linear combination of two inputs: $y = \alpha u_1 + (1-\alpha) u_2$ with $\alpha$ being set interactively by the user with the control surface.

Example 5: MIDI Controls to Generate a Sinuoid with Arbitrary Phase

Lastly, example 5 allows the user input a desired phase with the control surface. A sinusoid with that phase is then generated. The phase can be interactively varied as the model runs.

Conclusions

This model is provided to give inspiration for how the MIDI Controls block can be used to interact with a model. Other uses are possible and encouraged, including use with generated code.

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