Line data    Source code

       1             : /*
       2             :  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
       3             :  *
       4             :  *  Use of this source code is governed by a BSD-style license
       5             :  *  that can be found in the LICENSE file in the root of the source
       6             :  *  tree. An additional intellectual property rights grant can be found
       7             :  *  in the file PATENTS.  All contributing project authors may
       8             :  *  be found in the AUTHORS file in the root of the source tree.
       9             :  */
      10             : 
      11             : #ifndef WEBRTC_COMMON_AUDIO_SMOOTHING_FILTER_H_
      12             : #define WEBRTC_COMMON_AUDIO_SMOOTHING_FILTER_H_
      13             : 
      14             : #include "webrtc/base/constructormagic.h"
      15             : #include "webrtc/base/optional.h"
      16             : #include "webrtc/system_wrappers/include/clock.h"
      17             : 
      18             : namespace webrtc {
      19             : 
      20           0 : class SmoothingFilter {
      21             :  public:
      22           0 :   virtual ~SmoothingFilter() = default;
      23             :   virtual void AddSample(float sample) = 0;
      24             :   virtual rtc::Optional<float> GetAverage() = 0;
      25             :   virtual bool SetTimeConstantMs(int time_constant_ms) = 0;
      26             : };
      27             : 
      28             : // SmoothingFilterImpl applies an exponential filter
      29             : //   alpha = exp(-1.0 / time_constant_ms);
      30             : //   y[t] = alpha * y[t-1] + (1 - alpha) * sample;
      31             : // This implies a sample rate of 1000 Hz, i.e., 1 sample / ms.
      32             : // But SmoothingFilterImpl allows sparse samples. All missing samples will be
      33             : // assumed to equal the last received sample.
      34           0 : class SmoothingFilterImpl final : public SmoothingFilter {
      35             :  public:
      36             :   // |init_time_ms| is initialization time. It defines a period starting from
      37             :   // the arriving time of the first sample. During this period, the exponential
      38             :   // filter uses a varying time constant so that a smaller time constant will be
      39             :   // applied to the earlier samples. This is to allow the the filter to adapt to
      40             :   // earlier samples quickly. After the initialization period, the time constant
      41             :   // will be set to |init_time_ms| first and can be changed through
      42             :   // |SetTimeConstantMs|.
      43             :   SmoothingFilterImpl(int init_time_ms, const Clock* clock);
      44             :   ~SmoothingFilterImpl() override;
      45             : 
      46             :   void AddSample(float sample) override;
      47             :   rtc::Optional<float> GetAverage() override;
      48             :   bool SetTimeConstantMs(int time_constant_ms) override;
      49             : 
      50             :   // Methods used for unittests.
      51             :   float alpha() const { return alpha_; }
      52             : 
      53             :  private:
      54             :   void UpdateAlpha(int time_constant_ms);
      55             :   void ExtrapolateLastSample(int64_t time_ms);
      56             : 
      57             :   const int init_time_ms_;
      58             :   const float init_factor_;
      59             :   const float init_const_;
      60             :   const Clock* const clock_;
      61             : 
      62             :   rtc::Optional<int64_t> init_end_time_ms_;
      63             :   float last_sample_;
      64             :   float alpha_;
      65             :   float state_;
      66             :   int64_t last_state_time_ms_;
      67             : 
      68             :   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(SmoothingFilterImpl);
      69             : };
      70             : 
      71             : }  // namespace webrtc
      72             : 
      73             : #endif  // WEBRTC_COMMON_AUDIO_SMOOTHING_FILTER_H_