Audio effects form an essential part of audio processing and music production. They can transform the sound, adding depth, texture, and richness to the audio. One key component used in audio effects is envelope generators.

Envelopes define the shape of a sound over time, controlling parameters such as volume, pitch, and filter cutoff. In this article, we will explore how to implement audio effects using envelope generators in C++.

What is an Envelope Generator?

An envelope generator, also known as an ADSR envelope, is a module that controls the amplitude of a sound over time. It consists of four stages: Attack, Decay, Sustain, and Release. Each stage determines how the volume of the sound changes over time.

The Attack stage defines the time it takes for the sound to reach its maximum volume. The Decay stage determines the time it takes for the sound to reach the sustain level from the maximum volume. The Sustain level is the volume maintained during the duration of the sound. Lastly, the Release stage controls the time it takes for the sound to fade out after releasing a key or stopping the sound.

Implementing an Envelope Generator

To implement an envelope generator in C++, we can define a class that represents the envelope. We will provide functions to set the values for each stage of the envelope and retrieve the current envelope value at any given time.

class EnvelopeGenerator {
private:
    float attackTime;
    float decayTime;
    float sustainLevel;
    float releaseTime;
    
public:
    void setAttackTime(float time) {
        attackTime = time;
    }
    
    void setDecayTime(float time) {
        decayTime = time;
    }
    
    void setSustainLevel(float level) {
        sustainLevel = level;
    }
    
    void setReleaseTime(float time) {
        releaseTime = time;
    }
    
    float getEnvelopeValue(float currentTime) {
        // Calculate the envelope value based on current time and stage parameters
        // ...
        // Return the calculated envelope value
        return envelopeValue;
    }
};

In the getEnvelopeValue function, you would need to implement the logic to calculate the envelope value based on the current time and the envelope stage parameters. This could involve interpolating between the stages to create a smooth transition.

Applying Envelope Generators to Audio Effects

Once you have implemented the envelope generator, you can apply it to various audio effects. For example, you could use the envelope in a filter effect to control the cutoff frequency over time, creating dynamic and evolving sounds.

void applyFilterWithEnvelope(float* audioBuffer, int bufferSize, EnvelopeGenerator envelope) {
    for (int i = 0; i < bufferSize; i++) {
        // Apply the envelope to control the filter cutoff frequency
        float envelopeValue = envelope.getEnvelopeValue(i);
        float filteredSample = applyFilter(audioBuffer[i], envelopeValue);
        
        // Store the filtered sample back into the audio buffer
        audioBuffer[i] = filteredSample;
    }
}

In the example above, we apply the envelope generator to control the filter cutoff frequency. The applyFilter function takes the input audio sample and the envelope value to produce the filtered output sample. This creates a dynamic filter effect that modifies the audio based on the envelope shape.

Conclusion

Envelope generators play a crucial role in creating dynamic and expressive audio effects. By implementing an envelope generator in C++ and applying it to various audio effects, you can add a new dimension to your audio processing capabilities.