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 #ifndef DownSampler_h 32 #define DownSampler_h 33 34 #include "platform/audio/AudioArray.h" 35 #include "platform/audio/DirectConvolver.h" 36 37 namespace blink { 38 39 // DownSampler down-samples the source stream by a factor of 2x. 40 41 class PLATFORM_EXPORT DownSampler { 42 public: 43 DownSampler(size_t inputBlockSize); 44 45 // The destination buffer |destP| is of size sourceFramesToProcess / 2. 46 void process(const float* sourceP, float* destP, size_t sourceFramesToProcess); 47 48 void reset(); 49 50 // Latency based on the destination sample-rate. 51 size_t latencyFrames() const; 52 53 private: 54 enum { DefaultKernelSize = 256 }; 55 56 size_t m_inputBlockSize; 57 58 // Computes ideal band-limited half-band filter coefficients. 59 // In other words, filter out all frequencies higher than 0.25 * Nyquist. 60 void initializeKernel(); 61 AudioFloatArray m_reducedKernel; 62 63 // Half-band filter. 64 DirectConvolver m_convolver; 65 66 AudioFloatArray m_tempBuffer; 67 68 // 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. 69 AudioFloatArray m_inputBuffer; 70 }; 71 72 } // namespace blink 73 74 #endif // DownSampler_h 75
