1.qq通话，微信通话，打电话，铃声想起时，为何铃声只在手机端响起？而蓝牙耳机里只有嘟嘟声？

(1)来电铃声播放

streamType=2(AUDIO_STREAM_RING)

APM::AudioPolicyManager: startOutput() output 18, stream 2, session 24

(2)Engine::getStrategyForStream()函数可得stream 2对应

strategy 2 (STRATEGY_SONIFICATION)

(3)选设备

APM::AudioPolicyEngine: getDeviceForStrategy() strategy 2, device 82

结果为AUDIO_DEVICE_OUT_BLUETOOTH_A2DP | AUDIO_DEVICE_OUT_SPEAKER

这地方就奇怪了，明明选择的设备是蓝牙设备和扬声器，为什么只有扬声器有铃声？

log中可以发现

bt_a2dp_hw: out_set_parameters: state 5

5 对应 AUDIO_A2DP_STATE_STANDBY

经过一番费力的查找，可以发现

铃声响起时运行

adb shell dumpsys media.audio_flinger结果中

蓝牙设备对应的：

Output thread 0xecec0000 type 0 (MIXER):

Thread name: AudioOut_441

I/O handle: 1089

TID: 2033

Standby: yes

Sample rate: 44100 Hz

HAL frame count: 2560

HAL format: 0x1 (pcm16)

HAL buffer size: 10240 bytes

Channel count: 2

Channel mask: 0x00000003 (front-left, front-right)

Format: 0x1 (pcm16)

Frame size: 4 bytes

Pending config events: none

Output device: 0x80 (BLUETOOTH_A2DP)

小提示

多输出设备的时候，每个设备对应一个Mixthread,然后对应着一个总的DuplicatingThread。

蓝牙设备对应的Output thread在铃声播放的时候处于Standby状态。而这个状态赋值的地方很好找到(Threads.cpp)：

if ((!mActiveTracks.size() && systemTime() > mStandbyTimeNs) ||

isSuspended()) {

// put audio hardware into standby after short delay

if (shouldStandby_l()) {

threadLoop_standby();

mStandby = true;

}

经过一番寻找，真相了：

AudioPolicyManager setPhoneState会调用checkA2dpSuspend()函数，最终导致蓝牙Thread standBy.然后整个蓝牙输出相关的MixThread都暂停工作了，此时肯定不会去调用out_write函数往下写铃声数据，也就不会发出任何声音了。

看官且看checkA2dpSuspend函数中注释：

// suspend A2DP output if:

// (NOT already suspended) &&

// ((SCO device is connected &&

// (forced usage for communication || for record is SCO))) ||

// (phone state is ringing || in call)//响铃的时候要把a2dp挂起，注意是a2dp

//

// restore A2DP output if:

// (Already suspended) &&

// ((SCO device is NOT connected ||

// (forced usage NOT for communication && NOT for record is SCO))) &&

// (phone state is NOT ringing && NOT in call)

//

以在下区区四级的英文水平观之,大意为:打电话或者is ringing(setPhonestate 0之后)的时候(sco要连接成功)要用sco,挂起a2dp.

百度搜搜sco和a2dp的区别：

蓝牙一般有两种语音相关的模式是A2DP和SCO，前者是高质量音乐播放(俗称：只进不出)，后者是语音通话(俗称：有进有出)。

摘自：http://blog.csdn.net/kangear/article/details/38780185(如有冒犯，请作者原谅)

2. 通话过程中，如果开启免提，声音不再从蓝牙设备中输出，而从手机端输出，这是为什么？

我们且看看

电话接通之后，电话应用正常会调用

setBluetoothScoOn()切换蓝牙设备为Sco模式

public void setBluetoothScoOnInt(boolean on) {

if (on) {

//通话时强制使用BT_SCO

mForcedUseForComm = AudioSystem.FORCE_BT_SCO;

} else if (mForcedUseForComm == AudioSystem.FORCE_BT_SCO) {

mForcedUseForComm = AudioSystem.FORCE_NONE;

}

sendMsg(mAudioHandler, MSG_SET_FORCE_USE, SENDMSG_QUEUE,

AudioSystem.FOR_COMMUNICATION, mForcedUseForComm, null, 0);

sendMsg(mAudioHandler, MSG_SET_FORCE_USE, SENDMSG_QUEUE,

AudioSystem.FOR_RECORD, mForcedUseForComm, null, 0);

}

，点击“扬声器”之后setSpeakerphoneOn()

public void setSpeakerphoneOn(boolean on){

if (!checkAudioSettingsPermission("setSpeakerphoneOn()")) {

return;

}

if (on) {

if (mForcedUseForComm == AudioSystem.FORCE_BT_SCO) {

//如果之前设置了通话强制使用BT_SCO

//先取消强制使用(FORCE_NONE)

sendMsg(mAudioHandler, MSG_SET_FORCE_USE, SENDMSG_QUEUE,

AudioSystem.FOR_RECORD, AudioSystem.FORCE_NONE, null, 0);

}

//设置强制使用SPEAKER

mForcedUseForComm = AudioSystem.FORCE_SPEAKER;

} else if (mForcedUseForComm == AudioSystem.FORCE_SPEAKER){

mForcedUseForComm = AudioSystem.FORCE_NONE;

}

sendMsg(mAudioHandler, MSG_SET_FORCE_USE, SENDMSG_QUEUE,

AudioSystem.FOR_COMMUNICATION, mForcedUseForComm, null, 0);

}

case MSG_SET_FORCE_USE:

case MSG_SET_FORCE_BT_A2DP_USE:

setForceUse(msg.arg1, msg.arg2);

break;

//往下，AudioPolicyManager setForceUse的调用就略过不提了。

所以。。。就。。。不多说了