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Diffstat (limited to 'third_party/libwebrtc/common_audio/smoothing_filter.cc')
| -rw-r--r-- | third_party/libwebrtc/common_audio/smoothing_filter.cc | 147 |
1 files changed, 147 insertions, 0 deletions
diff --git a/third_party/libwebrtc/common_audio/smoothing_filter.cc b/third_party/libwebrtc/common_audio/smoothing_filter.cc new file mode 100644 index 0000000000..eaaf3a0033 --- /dev/null +++ b/third_party/libwebrtc/common_audio/smoothing_filter.cc @@ -0,0 +1,147 @@ +/* + * 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 <math.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 : std::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 = std::exp(last_state_time_ms_ - time_ms); + } else { + multiplier = std::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} |