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      1 /*
      2  * Copyright (C) 2013 Google Inc. All rights reserved.
      3  *
      4  * Redistribution and use in source and binary forms, with or without
      5  * modification, are permitted provided that the following conditions are
      6  * met:
      7  *
      8  *     * Redistributions of source code must retain the above copyright
      9  * notice, this list of conditions and the following disclaimer.
     10  *     * Redistributions in binary form must reproduce the above
     11  * copyright notice, this list of conditions and the following disclaimer
     12  * in the documentation and/or other materials provided with the
     13  * distribution.
     14  *     * Neither the name of Google Inc. nor the names of its
     15  * contributors may be used to endorse or promote products derived from
     16  * this software without specific prior written permission.
     17  *
     18  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
     19  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
     20  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
     21  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
     22  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
     23  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
     24  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
     25  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
     26  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     27  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
     28  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     29  */
     30 
     31 #include "config.h"
     32 
     33 #if ENABLE(WEB_AUDIO)
     34 
     35 #include "core/platform/audio/DownSampler.h"
     36 
     37 #include "wtf/MathExtras.h"
     38 
     39 namespace WebCore {
     40 
     41 DownSampler::DownSampler(size_t inputBlockSize)
     42     : m_inputBlockSize(inputBlockSize)
     43     , m_reducedKernel(DefaultKernelSize / 2)
     44     , m_tempBuffer(inputBlockSize / 2)
     45     , m_inputBuffer(inputBlockSize * 2)
     46     , m_convolver(inputBlockSize / 2) // runs at 1/2 source sample-rate
     47 {
     48     initializeKernel();
     49 }
     50 
     51 void DownSampler::initializeKernel()
     52 {
     53     // Blackman window parameters.
     54     double alpha = 0.16;
     55     double a0 = 0.5 * (1.0 - alpha);
     56     double a1 = 0.5;
     57     double a2 = 0.5 * alpha;
     58 
     59     int n = DefaultKernelSize;
     60     int halfSize = n / 2;
     61 
     62     // Half-band filter.
     63     double sincScaleFactor = 0.5;
     64 
     65     // Compute only the odd terms because the even ones are zero, except
     66     // right in the middle at halfSize, which is 0.5 and we'll handle specially during processing
     67     // after doing the main convolution using m_reducedKernel.
     68     for (int i = 1; i < n; i += 2) {
     69         // Compute the sinc() with offset.
     70         double s = sincScaleFactor * piDouble * (i - halfSize);
     71         double sinc = !s ? 1.0 : sin(s) / s;
     72         sinc *= sincScaleFactor;
     73 
     74         // Compute Blackman window, matching the offset of the sinc().
     75         double x = static_cast<double>(i) / n;
     76         double window = a0 - a1 * cos(2.0 * piDouble * x) + a2 * cos(4.0 * piDouble * x);
     77 
     78         // Window the sinc() function.
     79         // Then store only the odd terms in the kernel.
     80         // In a sense, this is shifting forward in time by one sample-frame at the destination sample-rate.
     81         m_reducedKernel[(i - 1) / 2] = sinc * window;
     82     }
     83 }
     84 
     85 void DownSampler::process(const float* sourceP, float* destP, size_t sourceFramesToProcess)
     86 {
     87     bool isInputBlockSizeGood = sourceFramesToProcess == m_inputBlockSize;
     88     ASSERT(isInputBlockSizeGood);
     89     if (!isInputBlockSizeGood)
     90         return;
     91 
     92     size_t destFramesToProcess = sourceFramesToProcess / 2;
     93 
     94     bool isTempBufferGood = destFramesToProcess == m_tempBuffer.size();
     95     ASSERT(isTempBufferGood);
     96     if (!isTempBufferGood)
     97         return;
     98 
     99     bool isReducedKernelGood = m_reducedKernel.size() == DefaultKernelSize / 2;
    100     ASSERT(isReducedKernelGood);
    101     if (!isReducedKernelGood)
    102         return;
    103 
    104     size_t halfSize = DefaultKernelSize / 2;
    105 
    106     // Copy source samples to 2nd half of input buffer.
    107     bool isInputBufferGood = m_inputBuffer.size() == sourceFramesToProcess * 2 && halfSize <= sourceFramesToProcess;
    108     ASSERT(isInputBufferGood);
    109     if (!isInputBufferGood)
    110         return;
    111 
    112     float* inputP = m_inputBuffer.data() + sourceFramesToProcess;
    113     memcpy(inputP, sourceP, sizeof(float) * sourceFramesToProcess);
    114 
    115     // Copy the odd sample-frames from sourceP, delayed by one sample-frame (destination sample-rate)
    116     // to match shifting forward in time in m_reducedKernel.
    117     float* oddSamplesP = m_tempBuffer.data();
    118     for (unsigned i = 0; i < destFramesToProcess; ++i)
    119         oddSamplesP[i] = *((inputP - 1) + i * 2);
    120 
    121     // Actually process oddSamplesP with m_reducedKernel for efficiency.
    122     // The theoretical kernel is double this size with 0 values for even terms (except center).
    123     m_convolver.process(&m_reducedKernel, oddSamplesP, destP, destFramesToProcess);
    124 
    125     // Now, account for the 0.5 term right in the middle of the kernel.
    126     // This amounts to a delay-line of length halfSize (at the source sample-rate),
    127     // scaled by 0.5.
    128 
    129     // Sum into the destination.
    130     for (unsigned i = 0; i < destFramesToProcess; ++i)
    131         destP[i] += 0.5 * *((inputP - halfSize) + i * 2);
    132 
    133     // Copy 2nd half of input buffer to 1st half.
    134     memcpy(m_inputBuffer.data(), inputP, sizeof(float) * sourceFramesToProcess);
    135 }
    136 
    137 void DownSampler::reset()
    138 {
    139     m_convolver.reset();
    140     m_inputBuffer.zero();
    141 }
    142 
    143 size_t DownSampler::latencyFrames() const
    144 {
    145     // Divide by two since this is a linear phase kernel and the delay is at the center of the kernel.
    146     return m_reducedKernel.size() / 2;
    147 }
    148 
    149 } // namespace WebCore
    150 
    151 #endif // ENABLE(WEB_AUDIO)
    152