Fixes issue where Web Speech API drops a frame every 5.1 seconds

content/browser/speech/speech_recognizer_impl.cc

Index: content/browser/speech/speech_recognizer_impl.cc
diff --git a/content/browser/speech/speech_recognizer_impl.cc b/content/browser/speech/speech_recognizer_impl.cc
index a08ffe17661af3b5ecaeffa381fa22b8a0c6308f..b38963dc91e90e7be68b8493f2bfc0884ce2e277 100644
--- a/content/browser/speech/speech_recognizer_impl.cc
+++ b/content/browser/speech/speech_recognizer_impl.cc
@@ -42,6 +42,8 @@ class SpeechRecognizerImpl::OnDataConverter
   // |output_parameters_|.
   scoped_refptr<AudioChunk> Convert(const AudioBus* data);
 
+  bool data_was_converted() const { return data_was_converted_; }
+
  private:
   // media::AudioConverter::InputCallback implementation.
   double ProvideInput(AudioBus* dest, base::TimeDelta buffer_delay) override;
@@ -54,7 +56,7 @@ class SpeechRecognizerImpl::OnDataConverter
   scoped_ptr<AudioBus> output_bus_;
   const AudioParameters input_parameters_;
   const AudioParameters output_parameters_;
-  bool waiting_for_input_;
+  bool data_was_converted_;
 
   DISALLOW_COPY_AND_ASSIGN(OnDataConverter);
 };
@@ -119,8 +121,9 @@ SpeechRecognizerImpl::OnDataConverter::OnDataConverter(
       output_bus_(AudioBus::Create(output_params)),
       input_parameters_(input_params),
       output_parameters_(output_params),
-      waiting_for_input_(false) {
+      data_was_converted_(false) {
   audio_converter_.AddInput(this);
+  audio_converter_.PrimeWithSilence();
 }
 
 SpeechRecognizerImpl::OnDataConverter::~OnDataConverter() {
@@ -132,12 +135,18 @@ SpeechRecognizerImpl::OnDataConverter::~OnDataConverter() {
 scoped_refptr<AudioChunk> SpeechRecognizerImpl::OnDataConverter::Convert(
     const AudioBus* data) {
   CHECK_EQ(data->frames(), input_parameters_.frames_per_buffer());
-
+  data_was_converted_ = false;
+  // Copy recorded audio to the |input_bus_| for later use in ProvideInput().
   data->CopyTo(input_bus_.get());
-
-  waiting_for_input_ = true;
+  // Convert the audio and place the result in |output_bus_|. This call will
+  // result in a ProvideInput() callback where the actual input is provided.
+  // However, it can happen that the converter contains enough cached data
+  // to return a result without calling ProvideInput(). The caller of this
+  // method should check the state of data_was_converted_() and make an
+  // additional call if it is set to false at return.
+  // See http://crbug.com/506051 for details.
   audio_converter_.Convert(output_bus_.get());
-
+  // Create an audio chunk based on the converted result.
   scoped_refptr<AudioChunk> chunk(
       new AudioChunk(output_parameters_.GetBytesPerBuffer(),
                      output_parameters_.bits_per_sample() / 8));
@@ -149,16 +158,10 @@ scoped_refptr<AudioChunk> SpeechRecognizerImpl::OnDataConverter::Convert(
 
 double SpeechRecognizerImpl::OnDataConverter::ProvideInput(
     AudioBus* dest, base::TimeDelta buffer_delay) {
-  // The audio converted should never ask for more than one bus in each call
-  // to Convert(). If so, we have a serious issue in our design since we might
-  // miss recorded chunks of 100 ms audio data.
-  CHECK(waiting_for_input_);
-
   // Read from the input bus to feed the converter.
   input_bus_->CopyTo(dest);
-
-  // |input_bus_| should only be provide once.
-  waiting_for_input_ = false;
+  // Indicate that the recorded audio has in fact been used by the converter.
+  data_was_converted_ = true;
   return 1;
 }
 
@@ -273,10 +276,20 @@ void SpeechRecognizerImpl::OnData(AudioInputController* controller,
   // Convert audio from native format to fixed format used by WebSpeech.
   FSMEventArgs event_args(EVENT_AUDIO_DATA);
   event_args.audio_data = audio_converter_->Convert(data);
-
   BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
                           base::Bind(&SpeechRecognizerImpl::DispatchEvent,
                                      this, event_args));
+  // See http://crbug.com/506051 regarding why one extra convert call can
+  // sometimes be required. It should be a rare case.
+  if (!audio_converter_->data_was_converted()) {
+    event_args.audio_data = audio_converter_->Convert(data);
+    BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
+                            base::Bind(&SpeechRecognizerImpl::DispatchEvent,
+                                       this, event_args));
+  }
+  // Something is seriously wrong here and we are most likely missing some
+  // audio segments.
+  CHECK(audio_converter_->data_was_converted());
 }
 
 void SpeechRecognizerImpl::OnAudioClosed(AudioInputController*) {}
@@ -523,6 +536,8 @@ SpeechRecognizerImpl::StartRecording(const FSMEventArgs&) {
   AudioParameters output_parameters = AudioParameters(
       AudioParameters::AUDIO_PCM_LOW_LATENCY, kChannelLayout, kAudioSampleRate,
       kNumBitsPerAudioSample, frames_per_buffer);
+  DVLOG(1) << "SRI::output_parameters: "
+           << output_parameters.AsHumanReadableString();
 
   // Audio converter will receive audio based on these parameters as input.
   // On Windows we start by verifying that Core Audio is supported. If not,
@@ -543,17 +558,17 @@ SpeechRecognizerImpl::StartRecording(const FSMEventArgs&) {
     // We rely on internal buffers in the audio back-end to fulfill this request
     // and the idea is to simplify the audio conversion since each Convert()
     // call will then render exactly one ProvideInput() call.
-    // Due to implementation details in the audio converter, 2 milliseconds
-    // are added to the default frame size (100 ms) to ensure there is enough
-    // data to generate 100 ms of output when resampling.
+    // in_params.sample_rate()
     frames_per_buffer =
-        ((in_params.sample_rate() * (chunk_duration_ms + 2)) / 1000.0) + 0.5;
+        ((in_params.sample_rate() * chunk_duration_ms) / 1000.0) + 0.5;
     input_parameters.Reset(in_params.format(),
                            in_params.channel_layout(),
                            in_params.channels(),
                            in_params.sample_rate(),
                            in_params.bits_per_sample(),
                            frames_per_buffer);
+    DVLOG(1) << "SRI::input_parameters: "
+             << input_parameters.AsHumanReadableString();
   }
 
   // Create an audio converter which converts data between native input format