Direct Sound: How do I read captured data from a small buffer? - c++

Question

I'm trying to capture waveforms of floating point PCM data from a microphone. The application is only asking for a small number of samples each cycle (For 20'000Hz and a frame size of 0.003s, it would ask for 60 samples)
I would like to set the buffer size depending on how many ms the app is interested in but it seems that dwBufferBytes has to be a certain size. Instead, I set it to nAvgBytesPerSec and only lock/copy 60 samples each time (even though much more data would be available to read)
Is this a valid approach or is there a different way to throttle the sound driver? Is there a way to reduce the size of the buffer to only give me as much data as the app is requesting? I don't want to get a ton of sound
data if the application only wants 60 values.
Using this approach, I certainly will run into problems if the buffer catches up with my (slow) read cursor.
unsigned short channelNum = 2;
unsigned short bitsPerSample = 32;
unsigned long sampleRate = 20000;
unsigned short blockAlign = (channelNum * bitsPerSample) / 8;
unsigned long avgBytesPerSec = sampleRate * blockAlign;
WAVEFORMATEX wfx = { WAVE_FORMAT_IEEE_FLOAT, channelNum, sampleRate, avgBytesPerSec, blockAlign, bitsPerSample, 0 };
unsigned int mSampleBufferSize = 60; // 1400
DSCBUFFERDESC bufferDesc;
bufferDesc.dwSize = sizeof(DSCBUFFERDESC);
bufferDesc.dwFlags = 0;
bufferDesc.dwBufferBytes = wfx.nAvgBytesPerSec;
bufferDesc.dwReserved = 0;
bufferDesc.lpwfxFormat = &wfx;
bufferDesc.dwFXCount = 0;
bufferDesc.lpDSCFXDesc = NULL;
IDirectSoundCaptureBuffer *buffer = 0;
bool bufferRunning = false;
if (directSound && capture)
{
hr = capture->CreateCaptureBuffer(&bufferDesc, &buffer, NULL);
if (FAILED(hr))
std::cout << "SampleThread() -- Error creating DirectSoundCaptureBuffer " << endl;
else
{
hr = buffer->Start(DSCBSTART_LOOPING);
if (SUCCEEDED(hr)) {
bufferRunning = true;
}
}
}
void* primaryBuffer = NULL;
unsigned long primaryBufferSizeBytes = 0;
void* secondaryBuffer = NULL;
unsigned long secondaryBufferSize = 0;
bool mStopExecution = false;
unsigned long lastReadPosition = 0;
if (directSound && capture && buffer)
{
while (!mStopExecution)
{
DWORD readPos;
WORD remainingSize = 0;
DWORD capturePos;
hr = buffer->GetCurrentPosition(&capturePos, &readPos);
if (FAILED(hr))
{
cout << "SampleThread() -- Error GetCurrentPosition" << endl;
return 0;
}
buffer->Lock(lastReadPos, mSampleBufferSize, &primaryBuffer, &primaryBufferSizeBytes, &secondaryBuffer, &secondaryBufferSize, NULL);
memcpy(mBuffer, (float*)primaryBuffer, primaryBufferSizeBytes / sizeof(float));
// .... copy secondary buffer
hr = buffer->Unlock(primaryBuffer, primaryBufferSizeBytes, secondaryBuffer, secondaryBufferSize);
lastReadPosition = (lastReadPosition + mSampleBufferSize) % bufferDesc.dwBufferBytes;
}
}