Changed mixing to be done at the minimal possible frequency.

webrtc/modules/audio_mixer/audio_mixer_impl.cc

Index: webrtc/modules/audio_mixer/audio_mixer_impl.cc
diff --git a/webrtc/modules/audio_mixer/audio_mixer_impl.cc b/webrtc/modules/audio_mixer/audio_mixer_impl.cc
index a7af8c33d74a85df86a237cb299f5dcd12ce97d0..1421df321e7ecd7c316c6e7757dd3167b36f0117 100644
--- a/webrtc/modules/audio_mixer/audio_mixer_impl.cc
+++ b/webrtc/modules/audio_mixer/audio_mixer_impl.cc
@@ -137,6 +137,32 @@ AudioMixerImpl::SourceStatusList::iterator FindSourceInList(
       });
 }
 
+// Rounds the maximal audio source frequency up to an APM-native
+// frequency.
+int CalculateMixingFrequency(
+    AudioMixerImpl::SourceStatusList* const audio_source_list) {
+  if (audio_source_list->empty()) {
+    return AudioMixerImpl::kDefaultFrequency;
+  }
+  using APM = AudioProcessing::NativeRate;
+  int maximal_frequency = 0;
+  for (const auto& source_status : *audio_source_list) {
+    const int source_needed_frequency =
+        source_status->audio_source->PreferredSampleRate();
+    RTC_DCHECK_LE(APM::kSampleRate8kHz, source_needed_frequency);

[ivoc, 2016/10/31 15:39:34]

To get better error messages, I think the thing that is being checked should be
the first argument. So this should be RTC_DCHECK_GE(source_needed_frequency,
APM::kSampleRate8kHz).

[hlundin-webrtc, 2016/10/31 15:48:25]

I'm sure this applies to gtest macros, like EXPECT_LT, because it prints out
something like "expected X, got Y which is not X (duh)". But for the (D)CHECKS,
I'm not so sure. I tend not to care with the (D)CHECKS.

+    RTC_DCHECK_LE(source_needed_frequency, APM::kSampleRate48kHz);
+    maximal_frequency = std::max(maximal_frequency, source_needed_frequency);
+  }
+
+  static const std::vector<int> native_rates = {
+      APM::kSampleRate8kHz, APM::kSampleRate16kHz, APM::kSampleRate32kHz,
+      APM::kSampleRate48kHz};
+  const auto rounded_up_index = std::lower_bound(
+      native_rates.begin(), native_rates.end(), maximal_frequency);
+  RTC_DCHECK(rounded_up_index != native_rates.end());
+  return *rounded_up_index;
+}
+
 }  // namespace
 
 AudioMixerImpl::AudioMixerImpl(std::unique_ptr<AudioProcessing> limiter)
@@ -186,12 +212,16 @@ rtc::scoped_refptr<AudioMixerImpl> AudioMixerImpl::Create() {
       new rtc::RefCountedObject<AudioMixerImpl>(std::move(limiter)));
 }
 
-void AudioMixerImpl::Mix(int sample_rate,
-                         size_t number_of_channels,
+void AudioMixerImpl::Mix(size_t number_of_channels,
                          AudioFrame* audio_frame_for_mixing) {
   RTC_DCHECK(number_of_channels == 1 || number_of_channels == 2);
   RTC_DCHECK_RUNS_SERIALIZED(&race_checker_);
 
+  const int sample_rate = [this]() -> int {
+    rtc::CritScope lock(&crit_);
+    return CalculateMixingFrequency(&audio_source_list_);
+  }();
+
   if (OutputFrequency() != sample_rate) {
     SetOutputFrequency(sample_rate);
   }