third_party/WebKit/Source/platform/audio/UpSampler.cpp

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#include "platform/audio/UpSampler.h"
#include "wtf/MathExtras.h"

namespace blink {

UpSampler::UpSampler(size_t inputBlockSize)
    : m_inputBlockSize(inputBlockSize),
      m_kernel(DefaultKernelSize),
      m_convolver(inputBlockSize),
      m_tempBuffer(inputBlockSize),
      m_inputBuffer(inputBlockSize * 2) {
  initializeKernel();
}

void UpSampler::initializeKernel() {
  // Blackman window parameters.
  double alpha = 0.16;
  double a0 = 0.5 * (1.0 - alpha);
  double a1 = 0.5;
  double a2 = 0.5 * alpha;

  int n = m_kernel.size();
  int halfSize = n / 2;
  double subsampleOffset = -0.5;

  for (int i = 0; i < n; ++i) {
    // Compute the sinc() with offset.
    double s = piDouble * (i - halfSize - subsampleOffset);
    double sinc = !s ? 1.0 : sin(s) / s;

    // Compute Blackman window, matching the offset of the sinc().
    double x = (i - subsampleOffset) / n;
    double window =
        a0 - a1 * cos(twoPiDouble * x) + a2 * cos(twoPiDouble * 2.0 * x);

    // Window the sinc() function.
    m_kernel[i] = sinc * window;
  }
}

void UpSampler::process(const float* sourceP,
                        float* destP,
                        size_t sourceFramesToProcess) {
  bool isInputBlockSizeGood = sourceFramesToProcess == m_inputBlockSize;
  ASSERT(isInputBlockSizeGood);
  if (!isInputBlockSizeGood)
    return;

  bool isTempBufferGood = sourceFramesToProcess == m_tempBuffer.size();
  ASSERT(isTempBufferGood);
  if (!isTempBufferGood)
    return;

  bool isKernelGood = m_kernel.size() == DefaultKernelSize;
  ASSERT(isKernelGood);
  if (!isKernelGood)
    return;

  size_t halfSize = m_kernel.size() / 2;

  // Copy source samples to 2nd half of input buffer.
  bool isInputBufferGood = m_inputBuffer.size() == sourceFramesToProcess * 2 &&
                           halfSize <= sourceFramesToProcess;
  ASSERT(isInputBufferGood);
  if (!isInputBufferGood)
    return;

  float* inputP = m_inputBuffer.data() + sourceFramesToProcess;
  memcpy(inputP, sourceP, sizeof(float) * sourceFramesToProcess);

  // Copy even sample-frames 0,2,4,6... (delayed by the linear phase delay)
  // directly into destP.
  for (unsigned i = 0; i < sourceFramesToProcess; ++i)
    destP[i * 2] = *((inputP - halfSize) + i);

  // Compute odd sample-frames 1,3,5,7...
  float* oddSamplesP = m_tempBuffer.data();
  m_convolver.process(&m_kernel, sourceP, oddSamplesP, sourceFramesToProcess);

  for (unsigned i = 0; i < sourceFramesToProcess; ++i)
    destP[i * 2 + 1] = oddSamplesP[i];

  // Copy 2nd half of input buffer to 1st half.
  memcpy(m_inputBuffer.data(), inputP, sizeof(float) * sourceFramesToProcess);
}

void UpSampler::reset() {
  m_convolver.reset();
  m_inputBuffer.zero();
}

size_t UpSampler::latencyFrames() const {
  // Divide by two since this is a linear phase kernel and the delay is at the
  // center of the kernel.
  return m_kernel.size() / 2;
}

}  // namespace blink