前言

阅读此教程需要一定的C++和JUCE基础。
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上次我们搭完了合成器的基本框架，这一章我们讲ADSR的搭建。

来到SynthVoice.h中的private，添加如下：

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...
private:
    juce::dsp::Oscillator<float> osc { [](float x) {return std::sin(x);} };
    juce::dsp::Gain<float> gain;
};
...

在SynthVoice的renderNextBlock前补充prepareToPlay函数同时补充isPrepared用于判断：

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//SynthVoice.h
class SynthVoice : public juce::SynthesiserVoice
{
public:
    bool canPlaySound (juce::SynthesiserSound* sound) override;
    void startNote (int midiNoteNumber, float velocity, juce::SynthesiserSound* sound, int currentPitchWheelPosition) override;
    void stopNote (float velocity, bool allowTailOff) override;
    void controllerMoved (int controllerNumber, int newControllerValue) override;
    void pitchWheelMoved (int newPitchWheelValue) override;
    void prepareToPlay (double sampleRate, int samplesPerBlock, int outputChannels);
    void renderNextBlock (juce::AudioBuffer< float > &outputBuffer, int startSample, int numSamples) override;
private:
    juce::dsp::Oscillator<float> osc { [](float x) {return std::sin(x);} };
    juce::dsp::Gain<float> gain;
    bool isPrepared { false };  //判断是否prepare
};
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//SynthVoice.cpp
...
void SynthVoice::renderNextBlock (juce::AudioBuffer< float > &outputBuffer, int startSample, int numSamples)
{
    jassert (isPrepared);  //如果没有prepared会抛出问题
    
    juce::dsp::AudioBlock<float> audioBlock { outputBuffer };
    osc.process(juce::dsp::ProcessContextReplacing<float> (audioBlock));
    gain.process(juce::dsp::ProcessContextReplacing<float> (audioBlock));
}
...

来的PluginProcessor.cpp完成prepareToPlay

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//PluginProcessor.cpp
...
void SynthTutorialAudioProcessor::prepareToPlay (double sampleRate, int samplesPerBlock)
{
    synth.setCurrentPlaybackSampleRate (sampleRate);
    
    for (int i = 0; i < synth.getNumVoices(); i++)
    {
        if (auto voice = dynamic_cast<SynthVoice*>(synth.getVoice(i)))
        {
            voice->prepareToPlay (sampleRate, samplesPerBlock, getTotalNumInputChannels());
        }
    }
}
...

创建ADSR及其对应Params：

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//SynthVoice.h
...
private:
    juce::ADSR adsr;
    juce::ADSR::Parameters adsrParams;
...

设定adsr的sampleRate：

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void SynthVoice::prepareToPlay (double sampleRate, int samplesPerBlock, int outputChannels)
{
    adsr.setSampleRate (sampleRate);
    
    juce::dsp::ProcessSpec spec;
    spec.maximumBlockSize = samplesPerBlock;
    spec.sampleRate = sampleRate;
    spec.numChannels = outputChannels;
    
    osc.prepare (spec);
    gain.prepare ( spec );
    
    gain.setGainLinear (0.01f);
    
    isPrepared = true;
...
}

把adsr应用到buffer中的数据：

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void SynthVoice::renderNextBlock (juce::AudioBuffer< float > &outputBuffer, int startSample, int numSamples)
{
    jassert (isPrepared);  //如果没有prepared会抛出问题
    
    juce::dsp::AudioBlock<float> audioBlock { outputBuffer };
    osc.process(juce::dsp::ProcessContextReplacing<float> (audioBlock));
    gain.process(juce::dsp::ProcessContextReplacing<float> (audioBlock));
    
    adsr.applyEnvelopeToBuffer(outputBuffer, startSample, numSamples);
}
...

补充noteOn/noteOff：

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void SynthVoice::startNote (int midiNoteNumber, float velocity, juce::SynthesiserSound* sound, int currentPitchWheelPosition)
{
    adsr.noteOn();
}

void SynthVoice::stopNote (float velocity, bool allowTailOff)
{
    adsr.noteOff();
}

使用传入的midiNoteNumber来控制Osc振荡频率：

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void SynthVoice::startNote (int midiNoteNumber, float velocity, juce::SynthesiserSound* sound, int currentPitchWheelPosition)
{
    osc.setFrequency (juce::MidiMessage::getMidiNoteInHertz(midiNoteNumber));
    adsr.noteOn();
}