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/*
* Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#include "common_audio/smoothing_filter.h"
#include <cmath>
#include "rtc_base/checks.h"
#include "rtc_base/time_utils.h"
namespace webrtc {
SmoothingFilterImpl::SmoothingFilterImpl(int init_time_ms)
: init_time_ms_(init_time_ms),
// Duing the initalization time, we use an increasing alpha. Specifically,
// alpha(n) = exp(-powf(init_factor_, n)),
// where |init_factor_| is chosen such that
// alpha(init_time_ms_) = exp(-1.0f / init_time_ms_),
init_factor_(init_time_ms_ == 0
? 0.0f
: powf(init_time_ms_, -1.0f / init_time_ms_)),
// |init_const_| is to a factor to help the calculation during
// initialization phase.
init_const_(init_time_ms_ == 0
? 0.0f
: init_time_ms_ -
powf(init_time_ms_, 1.0f - 1.0f / init_time_ms_)) {
UpdateAlpha(init_time_ms_);
}
SmoothingFilterImpl::~SmoothingFilterImpl() = default;
void SmoothingFilterImpl::AddSample(float sample) {
const int64_t now_ms = rtc::TimeMillis();
if (!init_end_time_ms_) {
// This is equivalent to assuming the filter has been receiving the same
// value as the first sample since time -infinity.
state_ = last_sample_ = sample;
init_end_time_ms_ = now_ms + init_time_ms_;
last_state_time_ms_ = now_ms;
return;
}
ExtrapolateLastSample(now_ms);
last_sample_ = sample;
}
absl::optional<float> SmoothingFilterImpl::GetAverage() {
if (!init_end_time_ms_) {
// |init_end_time_ms_| undefined since we have not received any sample.
return absl::nullopt;
}
ExtrapolateLastSample(rtc::TimeMillis());
return state_;
}
bool SmoothingFilterImpl::SetTimeConstantMs(int time_constant_ms) {
if (!init_end_time_ms_ || last_state_time_ms_ < *init_end_time_ms_) {
return false;
}
UpdateAlpha(time_constant_ms);
return true;
}
void SmoothingFilterImpl::UpdateAlpha(int time_constant_ms) {
alpha_ = time_constant_ms == 0 ? 0.0f : exp(-1.0f / time_constant_ms);
}
void SmoothingFilterImpl::ExtrapolateLastSample(int64_t time_ms) {
RTC_DCHECK_GE(time_ms, last_state_time_ms_);
RTC_DCHECK(init_end_time_ms_);
float multiplier = 0.0f;
if (time_ms <= *init_end_time_ms_) {
// Current update is to be made during initialization phase.
// We update the state as if the |alpha| has been increased according
// alpha(n) = exp(-powf(init_factor_, n)),
// where n is the time (in millisecond) since the first sample received.
// With algebraic derivation as shown in the Appendix, we can find that the
// state can be updated in a similar manner as if alpha is a constant,
// except for a different multiplier.
if (init_time_ms_ == 0) {
// This means |init_factor_| = 0.
multiplier = 0.0f;
} else if (init_time_ms_ == 1) {
// This means |init_factor_| = 1.
multiplier = exp(last_state_time_ms_ - time_ms);
} else {
multiplier =
exp(-(powf(init_factor_, last_state_time_ms_ - *init_end_time_ms_) -
powf(init_factor_, time_ms - *init_end_time_ms_)) /
init_const_);
}
} else {
if (last_state_time_ms_ < *init_end_time_ms_) {
// The latest state update was made during initialization phase.
// We first extrapolate to the initialization time.
ExtrapolateLastSample(*init_end_time_ms_);
// Then extrapolate the rest by the following.
}
multiplier = powf(alpha_, time_ms - last_state_time_ms_);
}
state_ = multiplier * state_ + (1.0f - multiplier) * last_sample_;
last_state_time_ms_ = time_ms;
}
} // namespace webrtc
// Appendix: derivation of extrapolation during initialization phase.
// (LaTeX syntax)
// Assuming
// \begin{align}
// y(n) &= \alpha_{n-1} y(n-1) + \left(1 - \alpha_{n-1}\right) x(m) \\*
// &= \left(\prod_{i=m}^{n-1} \alpha_i\right) y(m) +
// \left(1 - \prod_{i=m}^{n-1} \alpha_i \right) x(m)
// \end{align}
// Taking $\alpha_{n} = \exp(-\gamma^n)$, $\gamma$ denotes init\_factor\_, the
// multiplier becomes
// \begin{align}
// \prod_{i=m}^{n-1} \alpha_i
// &= \exp\left(-\sum_{i=m}^{n-1} \gamma^i \right) \\*
// &= \begin{cases}
// \exp\left(-\frac{\gamma^m - \gamma^n}{1 - \gamma} \right)
// & \gamma \neq 1 \\*
// m-n & \gamma = 1
// \end{cases}
// \end{align}
// We know $\gamma = T^{-\frac{1}{T}}$, where $T$ denotes init\_time\_ms\_. Then
// $1 - \gamma$ approaches zero when $T$ increases. This can cause numerical
// difficulties. We multiply $T$ (if $T > 0$) to both numerator and denominator
// in the fraction. See.
// \begin{align}
// \frac{\gamma^m - \gamma^n}{1 - \gamma}
// &= \frac{T^\frac{T-m}{T} - T^\frac{T-n}{T}}{T - T^{1-\frac{1}{T}}}
// \end{align}