Android音频架构概览

FROM:http://www.bobbog.com/archives/147

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一、架构图

二、MediaServer初始化

所有的media服务都在进程mediaserver里。其代码在framework/base/media/mediaserver/main_mediaserver.cpp中。初始化过程如下。

1. 先初始化AudioFlinger

其初始化通过AudioFlinger的父类BindService创建唯一的AudioFlinger实例。

2. 然后初始化MediaPlayerService和CameraService

3. 最后初始化AudioPolicyService

三、 AudioFlinger分析

AudioFlinger负责管理每个音轨AudioTrack及RecordTrack，主音量控制，每种声音流的属性设置，设备控制，音效控制。

四、 AudioPolicyService分析

AudioPolicyService用于声音策略管理

客户端代码在framework/base/media/libmedia/AudioSystem.cpp中，其实该客户端只是监测AudioPolicyService是否死亡（binderDied)。真正的客户端为BpAudioPolicyService类，在framework/base/media/libmedia/IAudioPolicyService.cpp中。

接口

adjustVolume
adjustSuggestedStreamVolume
adjustStreamVolume
setStreamVolume
setStreamSolo
setStreamMute
getStreamVolume
getStreamMaxVolume
setRingerMode
getRingerMode
setVibrateSetting
getVibrateSetting
shouldVibrate
setMode
getMode
playSoundEffect
playSoundEffectVolume
loadSoundEffects
unloadSoundEffects
reloadAudioSettings
setSpeakerphoneOn
isSpeakerphoneOn
setBluetoothScoOn
isBluetoothScoOn
requestAudioFocus
abandonAudioFocus
unregisterAudioFocusClient
registerMediaButtonEventReceiver
unregisterMediaButtonEventReceiver
startBluetoothSco
stopBluetoothSco

服务端代码在framework/base/services/audioflinger/AudioPolicyService.cpp中，实现了AudioPolicyService本地代码及服务内部使用的AudioPolicyClientInterface接口，该接口被AudioPolicyManager调用。

openOutput
openDuplicateOutput
closeOutput
suspendOutput
restoreOutput
openInput
loseInput
setStreamVolume
setStreamOutput
setParameters
getParameters
startTone
stopTone
setVoiceVolume
moveEffects

AudioPolicyService会启动两个AudioCommandThread类的线程mTonePlaybackThread和mAudioCommandThread，然后构造AudioPolicyManager的实例mpPolicyManager。基本上所有的服务接口都会在检查权限后调用mpPolicyManager的相应实现。

mTonePlaybackThread用户播放通话中的通知音

mAudioCommandThread用于设置AudioFlinger的一些设置(为什么使用AudioCommandThread，然后调用AudioSystem中接口控制AudioFlinger?)

五、 AudioManager分析

1. 音频焦点转

requestAudioFocus 请求焦点
abandonAudioFocus     放弃焦点

requestAudioFocus的第一参数是Listener,可以是下面枚举的后三个值

第二个参数是当前流类型

最后一个参数可以是以下的前三个值

AUDIOFOCUS_GAIN 1
AUDIOFOCUS_GAIN_TRANSIENT 2
AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK = 3;
AUDIOFOCUS_LOSS = -1 * AUDIOFOCUS_GAIN;
AUDIOFOCUS_LOSS_TRANSIENT = -1 * AUDIOFOCUS_GAIN_TRANSIENT;
AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK =
 -1 * AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK;

六、 AudioPolicyManager分析

流类型stream_type(在 AudioSystem中定义)

TYPE VALUE STRATEGY
DEFAULT(->MUSIC) -1 STRATEGY_MEDIA(0)
VOICE_CALL 0 STRATEGY_PHONE(1)
SYSTEM 1  STRATEGY_MEDIA(0)
RING 2 STRATEGY_SONIFICATION(2)
MUSIC 3   STRATEGY_MEDIA(0)
ALARM 4 STRATEGY_SONIFICATION(2)
NOTIFICATION 5 STRATEGY_SONIFICATION(2)
BLUETOOTH_SCO 6 STRATEGY_PHONE(1)
// Sounds that cannot be muted by user and must be routed to speaker
ENFORCED_AUDIBLE 7 STRATEGY_SONIFICATION(2)
DTMF 8 STRATEGY_DTMF(3)
TTS 9  STRATEGY_MEDIA(0)
HDMI 10 MV的自定义类型 STRATEGY_MEDIA(0)
FM 11 MV的自定义类型 STRATEGY_MEDIA(0)

音频状态audio_mode(在 AudioSystem中定义)

enum audio_mode {
 MODE_INVALID = -2,
 MODE_CURRENT = -1,
 MODE_NORMAL = 0,
 MODE_RINGTONE,
 MODE_IN_CALL,
 MODE_IN_COMMUNICATION,
 NUM_MODES // not a valid entry, denotes end-of-list
 };

策略类型routing_strategy：

STRATEGY_MEDIA 0
STRATEGY_PHONE 1
STRATEGY_SONIFICATION 2
STRATEGY_DTMF       3

设备类型audio_devices：

enum audio_devices {
 // output devices
 DEVICE_OUT_EARPIECE = 0x1,
 DEVICE_OUT_SPEAKER = 0x2,
 DEVICE_OUT_WIRED_HEADSET = 0x4,
 DEVICE_OUT_WIRED_HEADPHONE = 0x8,
 DEVICE_OUT_BLUETOOTH_SCO = 0x10,
 DEVICE_OUT_BLUETOOTH_SCO_HEADSET = 0x20,
 DEVICE_OUT_BLUETOOTH_SCO_CARKIT = 0x40,
 DEVICE_OUT_BLUETOOTH_A2DP = 0x80,
 DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES = 0x100,
 DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER = 0x200,
 DEVICE_OUT_AUX_DIGITAL = 0x400,
 DEVICE_OUT_DEFAULT = 0x8000,
 DEVICE_OUT_ALL = (DEVICE_OUT_EARPIECE | DEVICE_OUT_SPEAKER | DEVICE_OUT_WIRED_HEADSET |
 DEVICE_OUT_WIRED_HEADPHONE | DEVICE_OUT_BLUETOOTH_SCO | DEVICE_OUT_BLUETOOTH_SCO_HEADSET |
 DEVICE_OUT_BLUETOOTH_SCO_CARKIT | DEVICE_OUT_BLUETOOTH_A2DP | DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES |
 DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER | DEVICE_OUT_AUX_DIGITAL | DEVICE_OUT_DEFAULT),
 DEVICE_OUT_ALL_A2DP = (DEVICE_OUT_BLUETOOTH_A2DP | DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES |
 DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER),

 // input devices
 DEVICE_IN_COMMUNICATION = 0x10000,
 DEVICE_IN_AMBIENT = 0x20000,
 DEVICE_IN_BUILTIN_MIC = 0x40000,
 DEVICE_IN_BLUETOOTH_SCO_HEADSET = 0x80000,
 DEVICE_IN_WIRED_HEADSET = 0x100000,
 DEVICE_IN_AUX_DIGITAL = 0x200000,
 DEVICE_IN_VOICE_CALL = 0x400000,
 DEVICE_IN_BACK_MIC = 0x800000,
 DEVICE_IN_DEFAULT = 0x80000000,

 DEVICE_IN_ALL = (DEVICE_IN_COMMUNICATION | DEVICE_IN_AMBIENT | DEVICE_IN_BUILTIN_MIC |
 DEVICE_IN_BLUETOOTH_SCO_HEADSET | DEVICE_IN_WIRED_HEADSET | DEVICE_IN_AUX_DIGITAL |
 DEVICE_IN_VOICE_CALL | DEVICE_IN_BACK_MIC | DEVICE_IN_DEFAULT)
 };

七、其它：

1.音量控制

initStreamVolume

setStreamVolumeIndex

setPhoneState

2.播放控制

入口函数

startOutput

getStrategy

getDeviceForStrategy

setOutputDevice

applyStreamVolume

checkAndSetVolume

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