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 6 * are met: 7 * 8 * 1. Redistributions of source code must retain the above copyright 9 * notice, this list of conditions and the following disclaimer. 10 * 2. Redistributions in binary form must reproduce the above copyright 11 * notice, this list of conditions and the following disclaimer in the 12 * documentation and/or other materials provided with the distribution. 13 * 3. Neither the name of Apple Inc. ("Apple") nor the names of 14 * its contributors may be used to endorse or promote products derived 15 * from this software without specific prior written permission. 16 * 17 * THIS SOFTWARE IS PROVIDED BY APPLE AND ITS CONTRIBUTORS "AS IS" AND ANY 18 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED 19 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 20 * DISCLAIMED. IN NO EVENT SHALL APPLE OR ITS CONTRIBUTORS BE LIABLE FOR ANY 21 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 22 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 23 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND 24 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 26 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 27 */ 28 29#ifndef DownSampler_h 30#define DownSampler_h 31 32#include "AudioArray.h" 33#include "DirectConvolver.h" 34 35namespace WebCore { 36 37// DownSampler down-samples the source stream by a factor of 2x. 38 39class DownSampler { 40public: 41 DownSampler(size_t inputBlockSize); 42 43 // The destination buffer |destP| is of size sourceFramesToProcess / 2. 44 void process(const float* sourceP, float* destP, size_t sourceFramesToProcess); 45 46 void reset(); 47 48 // Latency based on the destination sample-rate. 49 size_t latencyFrames() const; 50 51private: 52 enum { DefaultKernelSize = 256 }; 53 54 size_t m_inputBlockSize; 55 56 // Computes ideal band-limited half-band filter coefficients. 57 // In other words, filter out all frequencies higher than 0.25 * Nyquist. 58 void initializeKernel(); 59 AudioFloatArray m_reducedKernel; 60 61 // Half-band filter. 62 DirectConvolver m_convolver; 63 64 AudioFloatArray m_tempBuffer; 65 66 // Used as delay-line (FIR filter history) for the input samples to account for the 0.5 term right in the middle of the kernel. 67 AudioFloatArray m_inputBuffer; 68}; 69 70} // namespace WebCore 71 72#endif // DownSampler_h 73