implement AudioManagerMac::GetPreferredLowLatencyOutputStreamParameters(

media/audio/mac/audio_manager_mac.cc

 // Copyright (c) 2012 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 <CoreAudio/AudioHardware.h>
 
 #include <string>
 
 #include "base/command_line.h"
 #include "base/mac/mac_logging.h"
 #include "base/mac/scoped_cftyperef.h"
 #include "base/sys_string_conversions.h"
+#include "media/audio/audio_util.h"
 #include "media/audio/mac/audio_input_mac.h"
 #include "media/audio/mac/audio_low_latency_input_mac.h"
 #include "media/audio/mac/audio_low_latency_output_mac.h"
 #include "media/audio/mac/audio_manager_mac.h"
 #include "media/audio/mac/audio_output_mac.h"
 #include "media/audio/mac/audio_synchronized_mac.h"
 #include "media/audio/mac/audio_unified_mac.h"
 #include "media/base/limits.h"
 #include "media/base/media_switches.h"
 
 namespace media {
 
 // Maximum number of output streams that can be open simultaneously.
 static const int kMaxOutputStreams = 50;
 
+// Maximum buffer size CoreAudio can support.
+static const int kMaxBufferSize = 2047;

[DaleCurtis, 2012/10/01 17:37:36]

Add some comment about low latency specifically. Maybe kMaxLowLatencyBufferSize.

[no longer working on chromium, 2012/10/02 08:06:22]

Done.

+
 static bool HasAudioHardware(AudioObjectPropertySelector selector) {
   AudioDeviceID output_device_id = kAudioObjectUnknown;
   const AudioObjectPropertyAddress property_address = {
     selector,
     kAudioObjectPropertyScopeGlobal,            // mScope
     kAudioObjectPropertyElementMaster           // mElement
   };
   UInt32 output_device_id_size = static_cast<UInt32>(sizeof(output_device_id));
   OSStatus err = AudioObjectGetPropertyData(kAudioObjectSystemObject,
                                             &property_address,
@@ skipping 288 matching lines @@
   if (audio_device_id != kAudioObjectUnknown)
     stream = new AUAudioInputStream(this, params, audio_device_id);
 
   return stream;
 }
 
 AudioManager* CreateAudioManager() {
   return new AudioManagerMac();
 }
 
+AudioParameters AudioManagerMac::GetPreferredLowLatencyOutputStreamParameters(
+    const AudioParameters& params) {
+  // Applications should use their own preferred buffer size when no resampler
+  // is needed, and Apple CoreAudio can accept any buffer size up to 2047.
+  int native_sample_rate = GetAudioHardwareSampleRate();
+  int buffer_size = GetAudioHardwareBufferSize();
+  if (native_sample_rate == params.sample_rate() &&
+      params.frames_per_buffer() <= kMaxBufferSize) {
+    buffer_size = params.frames_per_buffer();
+  }
+
+  return AudioParameters(AudioParameters::AUDIO_PCM_LOW_LATENCY,
+                         params.channel_layout(),
+                         native_sample_rate,
+                         16,
+                         buffer_size);
+}
+
 }  // namespace media