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// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "third_party/blink/renderer/platform/audio/audio_array.h"
#include "third_party/blink/renderer/platform/platform_export.h"
namespace blink {
class PLATFORM_EXPORT IIRFilter final {
// The maximum IIR filter order. This also limits the number of feedforward
// coefficients. The maximum number of coefficients is 20 according to the
// spec.
const static size_t kMaxOrder = 19;
IIRFilter(const AudioDoubleArray* feedforward_coef,
const AudioDoubleArray* feedback_coef);
void Process(const float* source_p,
float* dest_p,
uint32_t frames_to_process);
void Reset();
void GetFrequencyResponse(int n_frequencies,
const float* frequency,
float* mag_response,
float* phase_response);
// Compute the tail time of the IIR filter
double TailTime(double sample_rate, bool is_filter_stable);
// Reset the internal state of the IIR filter to the initial state.
void ResetState();
// Filter memory
// For simplicity, we assume |m_xBuffer| and |m_yBuffer| have the same length,
// and the length is a power of two. Since the number of coefficients has a
// fixed upper length, the size of xBuffer and yBuffer is fixed. |m_xBuffer|
// holds the old input values and |m_yBuffer| holds the old output values
// needed to compute the new output value.
// m_yBuffer[m_bufferIndex] holds the most recent output value, say, y[n].
// Then m_yBuffer[m_bufferIndex - k] is y[n - k]. Similarly for m_xBuffer.
// To minimize roundoff, these arrays are double's instead of floats.
AudioDoubleArray x_buffer_;
AudioDoubleArray y_buffer_;
// Index into the xBuffer and yBuffer arrays where the most current x and y
// values should be stored. xBuffer[bufferIndex] corresponds to x[n], the
// current x input value and yBuffer[bufferIndex] is where y[n], the current
// output value.
int buffer_index_;
// Coefficients of the IIR filter. To minimize storage, these point to the
// arrays given in the constructor.
const AudioDoubleArray* feedback_;
const AudioDoubleArray* feedforward_;
} // namespace blink