简单的VST合成器以随机间隔更改音高

Question

I'm learning how to use the Steinberg VST 2.4 SDK (or rather, the 2.x portion that comes with the 3.6.0 version). I've created a simple synthesizer designed to play a sine wave at a constant frequency during its lifetime. Here is the code for this synth:

static const float TAU = 3.14159f * 2;

AudioEffect* createEffectInstance(audioMasterCallback audioMaster) {
    return new VSTTest(audioMaster);
}

VSTTest::VSTTest(audioMasterCallback audioMaster) : AudioEffectX(audioMaster, 0, NUM_PARAMS), //NUM_PARAMS is 0
                                                    fDeltaTime(1.0f / getSampleRate()), //time per sample (1.0 / 44100, presumably)
                                                    fFrequency(100.0f), //frequency of the wave (100 Hz)
                                                    fAmplitude(0.5f), //amplitude of the wave
                                                    fPos(0.0f) { //position of the wave in the x direction
    setNumInputs(0);
    setNumOutputs(2);
    canProcessReplacing();
    isSynth();
}

VSTTest::~VSTTest(void) {

}

void VSTTest::processReplacing(float** input, float** output, VstInt32 numFrames) {
    for (VstInt32 i = 0; i < numFrames; i++) {
        output[0][i] = fAmplitude * sin(fPos);
        output[1][i] = fAmplitude * sin(fPos);

        fPos += fFrequency * TAU * fDeltaTime;

        if (fPos >= TAU) {
            fPos = fmod(fPos, TAU);
        }
    }
}

void VSTTest::setSampleRate(float fSampleRate) {
    fDeltaTime = 1.0f / fSampleRate;
}

The problem is that when the VST is loaded as a channel in FL Studio, I can hear (and see) it changing pitch a couple of times over about 20 seconds until it settles on a final pitch that isn't even correct (deviates by about 10 Hz). Why is this happening?

Answer 1

您正在向fPos添加一个巨大的数字，应该将其增加1。您正在寻找的公式是：

sinf(2.0 * M_PI * fPos++ * fFrequency / fSampleRate);

Answer 2

Well this isn't exactly a complete answer, but this was solved by calculating the delta time in resume().

void VSTTest::resume(void) {
    fDeltaTime = 1.0 / getSampleRate();
}

I'm not sure why this needs to be done or how it solves the problem, but it does. Also, I should note that fDeltaTime should not be initialized in the constructor since there's no guarantee getSampleRate() is correct at that time.