WASAPI：以最小延迟播放正弦波声音，无毛刺（独占事件驱动模式）

我尝试在独占模式下使用 Windows 音频会话 API (WASAPI) 播放简单的正弦波形，但无论我做什么，都会遇到声音故障。我一直在使用MSDN 独占模式流示例 https://msdn.microsoft.com/en-us/library/windows/desktop/dd370844(v=vs.85).aspx作为参考点，下面是稍微修改过的代码当前的样子。

设置代码：

-- <variable declarations, incl. "HRESULT hr; BYTE *pData;" > --

// also, hr is checked for errors every step of the way

hr = CoCreateInstance(
    CLSID_MMDeviceEnumerator, NULL,
    CLSCTX_ALL, IID_IMMDeviceEnumerator,
    (void**)&pEnumerator);

hr = pEnumerator->GetDefaultAudioEndpoint(
    eRender, eConsole, &pDevice);

hr = pDevice->Activate(
    IID_IAudioClient, CLSCTX_ALL,
    NULL, (void**)&pAudioClient);


REFERENCE_TIME DefaultDevicePeriod = 0, MinimumDevicePeriod = 0;
hr = pAudioClient->GetDevicePeriod(&DefaultDevicePeriod, &MinimumDevicePeriod);

WAVEFORMATEX wave_format = {};
wave_format.wFormatTag = WAVE_FORMAT_PCM;
wave_format.nChannels = 2;
wave_format.nSamplesPerSec = 44100;
wave_format.nAvgBytesPerSec = 44100 * 2 * 16 / 8;
wave_format.nBlockAlign = 2 * 16 / 8;
wave_format.wBitsPerSample = 16;

hr = pAudioClient->IsFormatSupported(
    AUDCLNT_SHAREMODE_EXCLUSIVE,
    &wave_format,
    NULL // can't suggest a "closest match" in exclusive mode
    );

hr = pAudioClient->Initialize(
    AUDCLNT_SHAREMODE_EXCLUSIVE,
    AUDCLNT_STREAMFLAGS_EVENTCALLBACK,
    MinimumDevicePeriod,
    MinimumDevicePeriod,
    &wave_format,
    NULL);


// Get the actual size of the allocated buffer.
hr = pAudioClient->GetBufferSize(&bufferFrameCount);

INT32 FrameSize_bytes = bufferFrameCount * wave_format.nChannels * wave_format.wBitsPerSample / 8;

hr = pAudioClient->GetService(
    IID_IAudioRenderClient,
    (void**)&pRenderClient);

hEvent = CreateEvent(nullptr, false, false, nullptr);
if (hEvent == INVALID_HANDLE_VALUE) { printf("CreateEvent failed\n");  return -1; }

hr = pAudioClient->SetEventHandle(hEvent);

缓冲区设置：

const size_t num_samples = FrameSize_bytes / sizeof(unsigned short);

unsigned short *samples = new unsigned short[num_samples];

float min = (float)(std::numeric_limits<unsigned short>::min)();
float max = (float)(std::numeric_limits<unsigned short>::max)();
float halfmax = max / 2.0;
float dt = 1.0 / (float)wave_format.nSamplesPerSec;

float freq = (float)wave_format.nSamplesPerSec / (float)bufferFrameCount;

for (int i = 0; i < num_samples/2; ++i) {
    float t = (float)i * dt;
    samples[2*i] = sin_minmax_Hz(min, max, freq, t);
    samples[2*i + 1] = sin_minmax_Hz(min, max, freq, t);
}

hr = pRenderClient->GetBuffer(bufferFrameCount, &pData);

memcpy(pData, samples, FrameSize_bytes);

hr = pRenderClient->ReleaseBuffer(bufferFrameCount, flags);

DWORD taskIndex = 0;
hTask = AvSetMmThreadCharacteristics(TEXT("Pro Audio"), &taskIndex);

if (hTask == NULL) {
    hr = E_FAIL;
    IF_ERROR_EXIT(hr);
}

功能sin_minmax_Hz定义如下：

#define TWO_PI (3.14159265359*2)

static inline float sin01(float alpha) {
    return 0.5*sin(alpha) + 0.5;
}

static inline float sin_minmax_Hz(float min, float max, float freq_Hz, float t) {
    return (max - min) / 2.0 * sin01(t * freq_Hz * TWO_PI);
}

回放：

hr = pAudioClient->Start();  // Start playing.
IF_ERROR_EXIT(hr);

// just play indefinitely

while (true) {
    WaitForSingleObject(hEvent, INFINITE);

    hr = pRenderClient->GetBuffer(bufferFrameCount, &pData);
    memcpy(pData, samples, FrameSize_bytes);
    hr = pRenderClient->ReleaseBuffer(bufferFrameCount, 0);
}

问题是，在最短的延迟下，正弦波通常会平稳播放大约 2 秒，然后开始出现大量混叠，听起来几乎像锯齿波。我在这里错过了什么吗？

（整个工作示例可以找到here https://gist.github.com/anonymous/6359a8273ab33bcf24bd705ea779a09f.)

刚刚尝试了完整的示例，IAudioClient::Initialize 失败并显示 AUDCLNT_E_BUFFER_SIZE_NOT_ALIGNED。此错误是在 Windows 7 中引入的，您运行的是 Vista 吗？校正周期大小以进行对齐后，我得到了完美的正弦波。如果您的系统没有生成此错误，请尝试在 128 字节（而非位）边界上手动对齐缓冲区。否则，这是对齐代码：

hr = pAudioClient->Initialize(
        AUDCLNT_SHAREMODE_EXCLUSIVE,
        AUDCLNT_STREAMFLAGS_EVENTCALLBACK,
        MinimumDevicePeriod,
        MinimumDevicePeriod,
        &wave_format,
        NULL);

  if(hr == AUDCLNT_E_BUFFER_SIZE_NOT_ALIGNED) {
    UINT32 nFramesInBuffer;
    REFERENCE_TIME hnsPeriod;
    hr = pAudioClient->GetBufferSize(&nFramesInBuffer);
    IF_ERROR_EXIT(hr);
    hnsPeriod = (REFERENCE_TIME)(REFTIMES_PER_SEC * nFramesInBuffer / wave_format.nSamplesPerSec + 0.5);
    hr = pAudioClient->Release();
    IF_ERROR_EXIT(hr);
    hr = pDevice->Activate(__uuidof(IAudioClient), CLSCTX_ALL, NULL, (void**)&pAudioClient);
    IF_ERROR_EXIT(hr);
    hr = pAudioClient->Initialize(AUDCLNT_SHAREMODE_EXCLUSIVE, AUDCLNT_STREAMFLAGS_EVENTCALLBACK, hnsPeriod, hnsPeriod, &wave_format, NULL);
    IF_ERROR_EXIT(hr);
  }