Android2.01 音量调节详解

lixw668

Android2.01 音量调节详解

调用详细路径如下：

文件	原型
RingerVolumePreference.java	onCheckedChanged(CompoundButton buttonView, boolean isChecked)
AudioManager.java	setStreamVolume(int streamType, int index, int flags)
AudioService.java	setStreamVolume(int streamType, int index, int flags)
AudioSystem.java	native setStreamVolumeIndex(int stream, int index);
android_media_AudioSystem.cpp	android_media_AudioSystem_setStreamVolumeIndex
AudioSystem.cpp	setStreamVolumeIndex(stream_type stream, int index)
AudioPolicyService.cpp	setStreamVolumeIndex(AudioSystem::stream_type stream, int index)
AudioPolicyManager.cpp	setStreamVolumeIndex(AudioSystem::stream_type stream, int index)
AudioPolicyService.cpp	setStreamVolume(AudioSystem::stream_type stream, float volume, audio_io_handle_t output, int delayMs = 0);
AudioPolicyService.cpp	AudioCommandThread::volumeCommand(int stream, float volume, int output, int delayMs)
AudioPolicyService.cpp	AudioCommandThread::threadLoop()
AudioSystem.cpp	setStreamVolume(int stream, float value, int output)
AudioFlinger.cpp	setStreamVolume(int stream, float value, int output)
AudioFlinger.cpp	MixerThread::threadLoop()
AudioFlinger.cpp	MixerThread::prepareTracks_l(const SortedVector< wp<Track> >& activeTracks, Vector< sp<Track> > *tracksToRemove)
AudioMixer.cpp	process(void* output)

详细步骤：
1.当按音量调节按钮时

会调用RingerVolumePreference::onCheckedChanged，见文件
packages/apps/Settings/res/layout/sound_and_display_settings.xml,
然后就会调用AudioManager::setStreamVolume,一路跟踪直至文件AudioPolicyManager.cpp中的函数setStreamVolumeIndex，它会调用checkAndSetVolume，进而调用mpClientInterface->setStreamVolume，这个mpClientInterface应该是由AudioPolicyService来实现的，所以调用AudioPolicyService.cpp中的setStreamVolume，一直往下看至AudioFlinger.cpp中的setStreamVolume，改变的值保存在数组mStreamTypes中。
另外，当播放音乐时，MixerThread::threadLoop()是一直在运行的，当检测到待播放的track时，就调用函数prepareTracks_l，这个函数里会计算左右声道的音量，并且设置到AudioMixer里，代码如下：
......
                float typeVolume = mStreamTypes[track->type()].volume;
                float v = mMasterVolume * typeVolume;
                float v_clamped = v * cblk->volume[0];
                if (v_clamped > MAX_GAIN) v_clamped = MAX_GAIN;
                left = int16_t(v_clamped);
                v_clamped = v * cblk->volume[1];
                if (v_clamped > MAX_GAIN) v_clamped = MAX_GAIN;
                right = int16_t(v_clamped);
......
                 mAudioMixer->setParameter(AudioMixer::RAMP_VOLUME, AudioMixer::VOLUME0, left);
                 mAudioMixer->setParameter(AudioMixer::RAMP_VOLUME, AudioMixer::VOLUME1, right);            
......
调用函数prepareTracks_l后，继续调用
mAudioMixer->process(curBuf);
从而综合处理音频数据
由上可见，android是利用软件来调节音量，仅仅设置一个硬件音量的最大值(完).