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
37namespace blink {
38
39// DownSampler down-samples the source stream by a factor of 2x.
40
41class PLATFORM_EXPORT DownSampler {
42public:
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
53private:
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