With the rise of software-based music production, integrating MIDI controllers into audio processing applications has become a popular feature. MIDI (Musical Instrument Digital Interface) controllers allow musicians to control various parameters in real-time, offering an interactive and hands-on experience.

In this blog post, we will explore how to implement MIDI controllers in audio processing using C++. We will cover the basics of MIDI, how to handle MIDI input, and how to map MIDI events to audio processing parameters.

What is MIDI?

MIDI is a protocol that enables electronic musical instruments, computers, and other devices to communicate and synchronize with each other. It consists of a series of messages that carry information about musical events such as note-on/off, pitch bend, modulation, and more.

Handling MIDI Input

To handle MIDI input in our C++ audio processing application, we can use a library like RtMidi. It provides a simple and cross-platform interface for sending and receiving MIDI messages.

First, we need to initialize an instance of RtMidi and open a MIDI input port:

#include <RtMidi.h>

RtMidiIn midiIn;
midiIn.openPort(0); // Open the first MIDI input port

Next, we can set up a callback function that will be called whenever a MIDI message is received:

void midiCallback(double deltaTime, std::vector<unsigned char> *message, void *userData) {
  // Extract MIDI message data
  unsigned int messageType = message->at(0) & 0xF0;
  unsigned int channel = message->at(0) & 0x0F;
  unsigned int data1 = message->at(1);
  unsigned int data2 = message->at(2);

  // Handle MIDI message based on message type
  switch (messageType) {
    case 0x80: // Note off
      // Process note-off event
      break;
    case 0x90: // Note on
      // Process note-on event
      break;
    case 0xB0: // Control change
      // Process control change event
      break;
    // Handle other MIDI message types
  }
}

// Set the callback function
midiIn.setCallback(&midiCallback);

We are now ready to receive and handle MIDI messages in our audio processing application.

Mapping MIDI Events to Audio Parameters

To control audio processing parameters with MIDI, we need to map MIDI events to the desired parameters. For example, we can map a MIDI control change event to the cutoff frequency of a filter.

Let’s say we have a Filter class with a setCutoffFrequency() method. We can modify our midiCallback function to update the cutoff frequency based on MIDI control change events:

void midiCallback(double deltaTime, std::vector<unsigned char> *message, void *userData) {
  // ...

  if (messageType == 0xB0) { // Control change
    unsigned int controlNumber = data1;
    unsigned int controlValue = data2;

    if (controlNumber == 1) { // MIDI control change number 1
      // Map control value (0-127) to cutoff frequency range (20Hz-20kHz)
      float cutoffFrequency = (controlValue / 127.0f) * (20000.0f - 20.0f) + 20.0f;

      // Update the cutoff frequency of the filter
      filter.setCutoffFrequency(cutoffFrequency);
    }
  }
}

By mapping MIDI control change events to audio processing parameters, we can dynamically control various aspects of our audio application in real-time.

Conclusion

Integrating MIDI controllers in audio processing applications opens up new possibilities for musicians and producers. By handling MIDI input using libraries like RtMidi and mapping MIDI events to audio parameters, we can create interactive and expressive music applications.

With this guide, you should have a basic understanding of how to implement MIDI controllers in your C++ audio processing projects. So go ahead and start exploring the exciting world of MIDI-controlled audio applications!