Swift AudioKit: How to build an VCO who's pitch is controlled by an AR-envelope?

问题

I am new to AudioKit. I want to build an VCO who's pitch is controlled by a re-triggerable A(H)R-envelope (with adjustable attenuation) and I am looking for the best or most common way to do it.

After some research (and wrong directions) it looks to me that I have to go with AKOperationGenerator and AKOperation.

Best example to follow was the Filter Envelope example from the playgrounds.

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
    }
}

I trigger the generator via a sequencer track, triggering a calback

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)
)

which works fine. But I just hear an endless tone (with the correct frequency). Only the first trigger swells in pitch. The other triggers don't re-trigger the envelopes. How comes - what get I wrong? THNX

回答1:

It seems like I needed to find the correct length of the gate-on time (note length of sequencer step duration) and also send a gate-off event to set the gate to zero.

...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
}

it also seems not to work when the duration time is too short, means < 0.03 - then I guess the gate-on is faster set to off than the attack time was finished. also the decay time I had to simulate using the release time instead

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
}

EDIT: ok - the correct answer I found I posted here

来源：https://stackoverflow.com/questions/49140013/swift-audiokit-how-to-build-an-vco-whos-pitch-is-controlled-by-an-ar-envelope