Add cosine volume slew.

Raised cosine add less audible artifacts when used as volume slew
curve, compared to linear one.

Recurrent calculation of slew cosine is based on:
https://basesandframes.files.wordpress.com/2016/05/rgreenfastermath_gdc02.pdf

Bug: internal b/167251783
Change-Id: Ia9262700ecc0b9f4ee5e3895cc9864480eabab73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391894Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Auto-Submit: Michael Montvelishsky <mont@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804478}

chromecast/media/base/slew_volume.cc

@@ -68,9 +68,13 @@ namespace media {
 SlewVolume::SlewVolume() : SlewVolume(kMaxSlewTimeMs) {}
 
 SlewVolume::SlewVolume(int max_slew_time_ms)
+    : SlewVolume(max_slew_time_ms, false) {}
+
+SlewVolume::SlewVolume(int max_slew_time_ms, bool use_cosine_slew)
     : sample_rate_(kDefaultSampleRate),
       max_slew_time_ms_(max_slew_time_ms),
-      max_slew_per_sample_(1000.0 / (max_slew_time_ms_ * sample_rate_)) {}
+      max_slew_per_sample_(1000.0 / (max_slew_time_ms_ * sample_rate_)),
+      use_cosine_slew_(use_cosine_slew) {}
 
 void SlewVolume::SetSampleRate(int sample_rate) {
   CHECK_GT(sample_rate, 0);
@@ -92,6 +96,16 @@ void SlewVolume::SetVolume(double volume_scale) {
       std::max(0.1, std::fabs(volume_scale_ - current_volume_));
   max_slew_per_sample_ =
       volume_diff * 1000.0 / (max_slew_time_ms_ * sample_rate_);
+
+  if (use_cosine_slew_) {
+    // Set initial state for cosine slew. Cosine fading always lasts
+    // max_slew_time_ms_.
+    slew_counter_ = max_slew_time_ms_ * 0.001 * sample_rate_;
+    slew_angle_ = sin(M_PI / slew_counter_);
+    slew_offset_ = (current_volume_ + volume_scale_) * 0.5;
+    slew_cos_ = (current_volume_ - volume_scale_) * 0.5;
+    slew_sin_ = 0.0;
+  }
 }
 
 float SlewVolume::LastBufferMaxMultiplier() {
@@ -163,7 +177,24 @@ void SlewVolume::ProcessData(bool repeat_transition,
     return;
   }
 
-  if (current_volume_ < volume_scale_) {
+  if (use_cosine_slew_) {
+    int slew_frames = std::min(slew_counter_, frames);
+    frames -= slew_frames;
+    slew_counter_ -= slew_frames;
+    for (; slew_frames > 0; --slew_frames) {
+      slew_cos_ -= slew_sin_ * slew_angle_;
+      slew_sin_ += slew_cos_ * slew_angle_;
+      current_volume_ = std::min(1.0, std::max(0.0, slew_offset_ + slew_cos_));
+      for (int i = 0; i < channels; ++i) {
+        Traits::ProcessSingleDatum(src, current_volume_, dest);
+        ++src;
+        ++dest;
+      }
+    }
+    if (!slew_counter_) {
+      current_volume_ = volume_scale_;
+    }
+  } else if (current_volume_ < volume_scale_) {
     do {
       for (int i = 0; i < channels; ++i) {
         Traits::ProcessSingleDatum(src, current_volume_, dest);

chromecast/media/base/slew_volume.h

@@ -18,6 +18,9 @@ class SlewVolume {
  public:
   SlewVolume();
   explicit SlewVolume(int max_slew_time_ms);
+  // Use raised negative cosine function when |use_cosine_slew| is true and
+  // linear otherwise.
+  SlewVolume(int max_slew_time_ms, bool use_cosine_slew);
   ~SlewVolume() = default;
 
   void SetSampleRate(int sample_rate);
@@ -76,7 +79,12 @@ class SlewVolume {
   double max_slew_time_ms_;
   double max_slew_per_sample_;
   bool interrupted_ = true;
-
+  bool use_cosine_slew_ = false;
+  int slew_counter_;
+  double slew_angle_;
+  double slew_offset_;
+  double slew_cos_;
+  double slew_sin_;
   DISALLOW_COPY_AND_ASSIGN(SlewVolume);
 };
 

chromecast/media/cma/backend/mixer/mixer_input.cc

@@ -48,7 +48,7 @@ MixerInput::MixerInput(Source* source, FilterGroup* filter_group)
       primary_(source->primary()),
       device_id_(source->device_id()),
       content_type_(source->content_type()),
-      slew_volume_(kDefaultSlewTimeMs),
+      slew_volume_(kDefaultSlewTimeMs, true),
       volume_applied_(false),
       previous_ended_in_silence_(false),
       first_buffer_(true),