Swift AudioKit：如何构建由 AR 包络控制音调的 VCO？

我是 AudioKit 的新手。我想构建一个 VCO，其音调由可重新触发的 A(H)R 包络（具有可调衰减）控制，并且我正在寻找最好或最常见的方法来实现它。

经过一番研究（和错误的方向 https://stackoverflow.com/questions/49094157/how-to-build-an-oscillator-with-an-envelope-on-its-frequency-pitch-in-audiokit）在我看来，我必须使用 AKOperationGenerator 和 AKOperation。

最好的例子是滤波器包络示例 http://audiokit.io/playgrounds/Synthesis/Filter%20Envelope/从游乐场。

I tried:

enum PitchEnvVCOSynthParameter: Int {
    case frequency, decayTime, gate
}

struct PitchEnvVCO {
    static var frequency: AKOperation {
        return AKOperation.parameters[PitchEnvVCOSynthParameter.frequency.rawValue]
    }
    static var decayTime: AKOperation {
        return AKOperation.parameters[PitchEnvVCOSynthParameter.decayTime.rawValue]
    }
    static var gate: AKOperation {
        return AKOperation.parameters[PitchEnvVCOSynthParameter.gate.rawValue]
    }
}

extension AKOperationGenerator {
    var frequency: Double {
        get { return self.parameters[PitchEnvVCOSynthParameter.frequency.rawValue] }
        set(newValue) { self.parameters[PitchEnvVCOSynthParameter.frequency.rawValue] = newValue }
    }
    var decayTime: Double {
        get { return self.parameters[PitchEnvVCOSynthParameter.decayTime.rawValue] }
        set(newValue) { self.parameters[PitchEnvVCOSynthParameter.decayTime.rawValue] = newValue }
    }
    var gate: Double {
        get { return self.parameters[PitchEnvVCOSynthParameter.gate.rawValue] }
        set(newValue) { self.parameters[PitchEnvVCOSynthParameter.gate.rawValue] = newValue }
    }
}

with

class SimpleGenerator: AKNode {

    var generator = AKOperationGenerator { parameters in

        var oscillator = AKOperation.squareWave(frequency: PitchEnvVCO.frequency.triggeredWithEnvelope(
           trigger: PitchEnvVCO.gate,
           attack: 0.01,
           hold: 0.01,
           release: 0.2
        )).triggeredWithEnvelope(trigger: PitchEnvVCO.gate, attack: 0.1, hold: 0.01, release: 0.5)

        return oscillator
    }

    var vco1Freq: Double = 440.0 {
        didSet {
            generator.parameters[PitchEnvVCOSynthParameter.frequency.rawValue] = vco1Freq
        }
    }

    var pitchDecay: Double = 0.1 {
        didSet {
            generator.parameters[PitchEnvVCOSynthParameter.decayTime.rawValue] = pitchDecay
        }
    }
    var sourceMixer = AKMixer()

    func start() {
        generator.parameters = [vco1Freq, pitchDecay, 0] // Initialize the array
        generator.start()
    }

    func triggerGate(_ freq: Double) {
        vco1FreqOffset = freq
        generator.parameters[PitchEnvVCOSynthParameter.gate.rawValue] = 1
        // or generator.trigger(1)
    }

    override init() {
        sourceMixer = AKMixer(generator)
        super.init()
        avAudioNode = sourceMixer.avAudioNode
    }
}

我通过音序器轨道触发生成器，触发回调

let seqPositioncallbackTrack = sequencer.newTrack()
seqPositioncallbackTrack?.setMIDIOutput(sequencerCallbackInst.midiIn)
sequencerCallbackInst.callback = { status, noteNumber, velocity in
    self.onSequencerStepChange(status: status, noteNumber: noteNumber, velocity: velocity)
}
//...for i in 0 ..< numberOfSteps...etc.
seqPositioncallbackTrack?.add(noteNumber: MIDINoteNumber(i), 
    velocity: 100, 
    position: AKDuration(beats: Double(i)), 
    duration: AKDuration(beats: 0.5)
)

效果很好。但我只是听到无尽的声音（频率正确）。只有第一个触发器的音调会增强。其他触发器不会重新触发包络。怎么会-我错了什么？ THNX

看来我需要找到门开启时间的正确长度（注意音序器步骤持续时间的长度），并发送门关闭事件以将门设置为零。

...duration: AKDuration(seconds: 0.03)


private func onSequencerStepChange(status: AKMIDIStatus, noteNumber: MIDINoteNumber, velocity: MIDIVelocity) {

    if status == .noteOn {
        core.triggerGateOn(seq1Values[Int(noteNumber)])
    }
    else if status == .noteOff {
        core.triggerGateOff()
    }
    else { return }

}
...
// splitting func triggerGate(_ freq: Double) into on/off:
func triggerGateOn(_ freq: Double) {
    generator.parameters[PitchEnvVCOSynthParameter.gate.rawValue] = 1
}

func triggerGateOff() {
    generator.parameters[PitchEnvVCOSynthParameter.gate.rawValue] = 0
}

当持续时间太短（意味着

let generator = AKOperationGenerator { parameters in

    let oscillator = AKOperation.squareWave(
        frequency: PitchEnvVCO.frequency.triggeredWithEnvelope(
            trigger: PitchEnvVCO.gate,
            attack: 0.01,
            hold: 0.0,
            release: PitchEnvVCO.freqDecayTime
        ),
        amplitude: PitchEnvVCO.amplitude.triggeredWithEnvelope(
            trigger: PitchEnvVCO.gate,
            attack: 0.01,
            hold: 0.0,
            release: PitchEnvVCO.ampDecayTime
        )
    )
    return oscillator
}

编辑：好的 - 我发现我发布的正确答案here https://stackoverflow.com/questions/49241328/audiokit-how-to-operate-with-akoperation-parameter-values-as-double/49277777#49277777