[chromecast] Slew stream volume changes in StreamMixerAlsa.

chromecast/media/cma/backend/alsa/slew_volume.cc

+// 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 "chromecast/media/cma/backend/alsa/slew_volume.h"
+
+#include <algorithm>
+
+#include "base/logging.h"
+#include "media/base/vector_math.h"
+
+namespace {
+
+// The time to slew from 0.0 to 1.0.
+const int kMaxSlewTimeMs = 100;
+const int kDefaultSampleRate = 44100;
+// ::media::base::FMAC requires |src| to be 16-byte aligned
+const int kRequiredAlignment = 16;
+}
+
+namespace chromecast {
+namespace media {
+
+SlewVolume::SlewVolume() : SlewVolume(kMaxSlewTimeMs, kMaxSlewTimeMs) {}
+
+SlewVolume::SlewVolume(int max_slew_time_up_ms, int max_slew_time_down_ms)
+    : max_slew_time_up_ms_(max_slew_time_up_ms),
+      max_slew_time_down_ms_(max_slew_time_down_ms),
+      max_slew_up_(1000.0 / (max_slew_time_up_ms * kDefaultSampleRate)),
+      max_slew_down_(1000.0 / (max_slew_time_down_ms * kDefaultSampleRate)) {}
+
+// Slew rate should be 1 / (slew_time * sample_rate)
+void SlewVolume::SetSampleRate(int sample_rate) {
+  max_slew_up_ = (1000.0 / (max_slew_time_up_ms_ * sample_rate));
+  max_slew_down_ = (1000.0 / (max_slew_time_down_ms_ * sample_rate));
+}
+
+void SlewVolume::SetVolume(double volume_scale) {
+  volume_scale_ = volume_scale;
+}
+
+bool SlewVolume::ProcessFMAC(const float* src, int frames, float* dest) {

[kmackay, 2016/09/14 20:48:15]

Do we need to return bool? It always returns true

[jyw, 2016/09/15 01:12:15]

Done.

+  DCHECK(src);
+  DCHECK(dest);
+  if (!frames) {
+    return true;
+  }

[kmackay, 2016/09/14 20:48:15]

if (current_volume_ == volume_scale_) {
  ::media::vector_math::FMAC(src, volume_scale_, frames, dest);
  return;
}

(could also check for volume_scale_ == 0.0/1.0)

[jyw, 2016/09/15 01:12:15]

Done.

+
+  if (current_volume_ < volume_scale_) {
+    do {
+      (*dest) += (*src) * current_volume_;
+      ++src;
+      ++dest;
+      --frames;
+      current_volume_ += max_slew_up_;
+    } while (current_volume_ < volume_scale_ && frames);
+    current_volume_ = std::min(current_volume_, volume_scale_);
+  } else {
+    do {
+      (*dest) += (*src) * current_volume_;
+      ++src;
+      ++dest;
+      --frames;
+      current_volume_ -= max_slew_down_;
+    } while (current_volume_ > volume_scale_ && frames);
+    current_volume_ = std::max(current_volume_, volume_scale_);
+  }
+
+  if (current_volume_ == 0.0) {
+    return true;
+  }
+
+  int unaligned_frames =

[kmackay, 2016/09/14 20:48:14]

prefer to just do the simple naive method here rather than trying to align and
use FMAC. The common case can use FMAC above.

[jyw, 2016/09/15 01:12:15]

Done.

+      (reinterpret_cast<uintptr_t>(src) & (kRequiredAlignment - 1)) /
+      sizeof(float);
+  if (unaligned_frames > 0) {
+    unaligned_frames = kRequiredAlignment / sizeof(float) - unaligned_frames;
+  }
+  if (frames >= unaligned_frames) {
+    for (int f = 0; f < unaligned_frames; ++f) {
+      (*dest) += (*src) * current_volume_;
+      ++src;
+      ++dest;
+    }
+    frames -= unaligned_frames;
+  }
+
+  if (frames) {
+    ::media::vector_math::FMAC(src, volume_scale_, frames, dest);
+  }
+  return true;
+}
+
+// Scaling samples naively like this takes 0.2% of the CPU's time @ 44100hz
+// (profiled with waves_standalone).
+// Assumes 2 channel audio.
+bool SlewVolume::ProcessInterleaved(int32_t* data, int frames) {
+  DCHECK(data);
+
+  if (!frames) {
+    return true;
+  }

[kmackay, 2016/09/14 20:48:14]

if (current_volume_ == volume_scale_) {
  ::media::vector_math::FMUL(src, volume_scale_, frames * 2, dest);
  return;
}

(could also check for volume_scale_ == 0.0/1.0)

[jyw, 2016/09/15 01:12:15]

can't use FMUL, which requires floats. data is an int32_t array.

+
+  if (current_volume_ < volume_scale_) {
+    do {
+      (*data) *= current_volume_;

[kmackay, 2016/09/14 20:48:14]

It might be nice to have a helper function/macro for these 5 lines, since they
are repeated 3 times.

+      ++data;
+      (*data) *= current_volume_;
+      ++data;
+      --frames;
+      current_volume_ += max_slew_up_;
+    } while (current_volume_ < volume_scale_ && frames);
+    current_volume_ = std::min(current_volume_, volume_scale_);
+  } else {
+    do {
+      (*data) *= current_volume_;
+      ++data;
+      (*data) *= current_volume_;
+      ++data;
+      --frames;
+      current_volume_ -= max_slew_down_;
+    } while (current_volume_ > volume_scale_ && frames);
+    current_volume_ = std::max(current_volume_, volume_scale_);
+  }
+
+  if (current_volume_ == 1.0) {
+    return true;
+  }
+
+  if (current_volume_ == 0.0) {
+    std::fill_n(data, frames * 2, 0);
+    return true;
+  }
+
+  while (frames) {
+    (*data) *= current_volume_;
+    ++data;
+    (*data) *= current_volume_;
+    ++data;
+    --frames;
+  }
+
+  return true;
+}
+
+}  // namespace media
+}  // namespace chromecast