android audio 音量设置分析

最新推荐文章于 2024-06-06 09:00:59 发布

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本文详细解析了Android系统中音量调整的两种实现方式：软件混音调整PCM数据及控制DAC硬件增益。介绍了AudioFlinger组件如何通过PlaybackThread和DirectOutputThread进行音量设置，并深入到HAL层和内核层的实现细节。

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转载自：http://blog.youkuaiyun.com/wan8180192/article/details/50705882

audiod 中经常遇到的场景是音量调整与输出设备的切换，下面两篇文章针对这两个场景分别分析一下
1，音量调整场景
android 音量调整，可以使用两种方式：
软件mixer的时候修改PCM data
控制DAC硬件的增益

第一种情况，如果是多路mix的情况，就是MixerThread进行软件mixer，然后在mixer计算的时候来缩放PCM data，
首先，JNI层调用了AudioFlinger::setStreamVolume。

[cpp] view plain copy print ?

status_t AudioFlinger::setStreamVolume(audio_stream_type_t stream, float value,
audio_io_handle_t output)
{
AutoMutex lock(mLock);
PlaybackThread *thread = NULL;
if (output) {
thread = checkPlaybackThread_l(output); //获得对应的PlaybackThread
if (thread == NULL) {
return BAD_VALUE;
}
}
if (thread == NULL) {
for (size_t i = 0; i < mPlaybackThreads.size(); i++) {
mPlaybackThreads.valueAt(i)->setStreamVolume(stream, value);
}
} else {
thread->setStreamVolume(stream, value); //继续向下层设置
}
return NO_ERROR;
}

status_t AudioFlinger::setStreamVolume(audio_stream_type_t stream, float value,
        audio_io_handle_t output)
{
    AutoMutex lock(mLock);
    PlaybackThread *thread = NULL;
    if (output) {
        thread = checkPlaybackThread_l(output);  //获得对应的PlaybackThread
        if (thread == NULL) {
            return BAD_VALUE;
        }
    }
    if (thread == NULL) {
        for (size_t i = 0; i < mPlaybackThreads.size(); i++) {
            mPlaybackThreads.valueAt(i)->setStreamVolume(stream, value);
        }
    } else {
        thread->setStreamVolume(stream, value);  //继续向下层设置
    }
    return NO_ERROR;
}

可以看到，最终是调用了PlaybackThread：：setStreamVolume来继续设置音量

[cpp] view plain copy print ?

void AudioFlinger::PlaybackThread::setStreamVolume(audio_stream_type_t stream, float value)
{
Mutex::Autolock _l(mLock);
mStreamTypes[stream].volume = value;//把音量数据存起来
broadcast_l();
}

void AudioFlinger::PlaybackThread::setStreamVolume(audio_stream_type_t stream, float value)
{
    Mutex::Autolock _l(mLock);
    mStreamTypes[stream].volume = value;//把音量数据存起来
    broadcast_l();
}

[cpp] view plain copy print ?

AudioFlinger::PlaybackThread::mixer_state AudioFlinger::MixerThread::prepareTracks_l()
{
float typeVolume = mStreamTypes[track->streamType()].volume; //取出暂存的音量数据
float v = masterVolume * typeVolume;
AudioTrackServerProxy *proxy = track->mAudioTrackServerProxy;
uint32_t vlr = proxy->getVolumeLR();
vl = vlr & 0xFFFF;
vr = vlr >> 16;
vl = (uint32_t)(v * vl) << 12;
vr = (uint32_t)(v * vr) << 12;
mAudioMixer->setParameter(name, param, AudioMixer::VOLUME0, (void *)vl); //设置给audioMixer
mAudioMixer->setParameter(name, param, AudioMixer::VOLUME1, (void *)vr);
}

AudioFlinger::PlaybackThread::mixer_state AudioFlinger::MixerThread::prepareTracks_l()
{
                float typeVolume = mStreamTypes[track->streamType()].volume;  //取出暂存的音量数据
                float v = masterVolume * typeVolume;
                AudioTrackServerProxy *proxy = track->mAudioTrackServerProxy;
                uint32_t vlr = proxy->getVolumeLR();
                vl = vlr & 0xFFFF;
                vr = vlr >> 16;
                vl = (uint32_t)(v * vl) << 12;
                vr = (uint32_t)(v * vr) << 12;
            mAudioMixer->setParameter(name, param, AudioMixer::VOLUME0, (void *)vl);  //设置给audioMixer
            mAudioMixer->setParameter(name, param, AudioMixer::VOLUME1, (void *)vr);
}

可以看到上述函数把参数设置到了audioMixer里面，在audioMixer章节里面，
我们介绍过在audioMixer的实际操作函数是track__16BitsStereo这种函数，
对track__16BitsStereo的分析中，我们可以看到根据音量对PCM data进行实际的缩放
不再继续赘述了。

对于第二种方式，控制DAC硬件的增益，主要用在了DirectOutputThread，中，因为DirectOutputThread只有一路音频，直接写入HAL层，直接写入硬件的，
所以需要直接调用硬件DAC芯片的控制接口来调整音量。
其主要流程如下：
和mixerThread的流程一样，上层在调用了AudioFlinger::setStreamVolume之后，会调用prepareTracks_l函数

[cpp] view plain copy print ?

AudioFlinger::PlaybackThread::mixer_state AudioFlinger::DirectOutputThread::prepareTracks_l(
Vector< sp<Track> > *tracksToRemove
)
{
// compute volume for this track
processVolume_l(track, last);
}

AudioFlinger::PlaybackThread::mixer_state AudioFlinger::DirectOutputThread::prepareTracks_l(
    Vector< sp<Track> > *tracksToRemove
)
{
            // compute volume for this track
            processVolume_l(track, last);
}

AT章节中，我们提到过prepareTracks_l函数，其中会调用processVolume_l来处理音量

[cpp] view plain copy print ?

void AudioFlinger::DirectOutputThread::processVolume_l(Track *track, bool lastTrack)
{
audio_track_cblk_t* cblk = track->cblk();
float left, right;
float typeVolume = mStreamTypes[track->streamType()].volume; //和mixerThread一样，也是从mStreamTypes里面取出音量数据
float v = mMasterVolume * typeVolume;
AudioTrackServerProxy *proxy = track->mAudioTrackServerProxy;
uint32_t vlr = proxy->getVolumeLR();
float v_clamped = v * (vlr & 0xFFFF);
if (v_clamped > MAX_GAIN) v_clamped = MAX_GAIN;
left = v_clamped/MAX_GAIN;
v_clamped = v * (vlr >> 16);
if (v_clamped > MAX_GAIN) v_clamped = MAX_GAIN;
right = v_clamped/MAX_GAIN;
if (lastTrack) {
if (left != mLeftVolFloat || right != mRightVolFloat) {
mLeftVolFloat = left;
mRightVolFloat = right;
uint32_t vl = (uint32_t)(left * (1 << 24));
uint32_t vr = (uint32_t)(right * (1 << 24));
if (mOutput->stream->set_volume) {
mOutput->stream->set_volume(mOutput->stream, left, right); //向下层设置音量
}
}
}
}

void AudioFlinger::DirectOutputThread::processVolume_l(Track *track, bool lastTrack)
{
    audio_track_cblk_t* cblk = track->cblk();
    float left, right;
        float typeVolume = mStreamTypes[track->streamType()].volume;  //和mixerThread一样，也是从mStreamTypes里面取出音量数据
        float v = mMasterVolume * typeVolume;
        AudioTrackServerProxy *proxy = track->mAudioTrackServerProxy;
        uint32_t vlr = proxy->getVolumeLR();
        float v_clamped = v * (vlr & 0xFFFF);
        if (v_clamped > MAX_GAIN) v_clamped = MAX_GAIN;
        left = v_clamped/MAX_GAIN;
        v_clamped = v * (vlr >> 16);
        if (v_clamped > MAX_GAIN) v_clamped = MAX_GAIN;
        right = v_clamped/MAX_GAIN;
    if (lastTrack) {
        if (left != mLeftVolFloat || right != mRightVolFloat) {
            mLeftVolFloat = left;
            mRightVolFloat = right;
            uint32_t vl = (uint32_t)(left * (1 << 24));
            uint32_t vr = (uint32_t)(right * (1 << 24));
            if (mOutput->stream->set_volume) {
                mOutput->stream->set_volume(mOutput->stream, left, right);  //向下层设置音量
            }
        }
    }
}

mOutput->stream->set_volume实际上调用的是 libhardware_legacy中的函数

[cpp] view plain copy print ?

static int out_set_volume(struct audio_stream_out *stream, float left, //调用libhardware_legacy中的函数
float right)
{
struct legacy_stream_out *out =
reinterpret_cast<struct legacy_stream_out *>(stream);
return out->legacy_out->setVolume(left, right);
}

static int out_set_volume(struct audio_stream_out *stream, float left,  //调用libhardware_legacy中的函数
                          float right)
{
    struct legacy_stream_out *out =
        reinterpret_cast<struct legacy_stream_out *>(stream);
    return out->legacy_out->setVolume(left, right);
}

然后就进入了HAL层代码，HAL层代码中最终调用了 ioctl(ctl->mixer->fd, SNDRV_CTL_IOCTL_ELEM_WRITE, &ev);
写入驱动
由于商业机密，HAL层代码不能贴出来。
到了内核中，则是按照以下调用序列，最终通过IIC总线，将音量控制命令写入了DAC芯片中

[cpp] view plain copy print ?

ioctl(ctl->mixer->fd, SNDRV_CTL_IOCTL_ELEM_WRITE, &ev);
snd_ctl_elem_write_user
snd_ctl_elem_write_user
snd_ctl_elem_write
wm8523_controls
snd_soc_put_volsw
snd_soc_update_bits_locked

ioctl(ctl->mixer->fd, SNDRV_CTL_IOCTL_ELEM_WRITE, &ev);
snd_ctl_elem_write_user
snd_ctl_elem_write_user
snd_ctl_elem_write
wm8523_controls
snd_soc_put_volsw
snd_soc_update_bits_locked

IIC总线写入命令
关于驱动的流程我们在这里不展开讲，后续会单独讲解

其他系列可参考

http://blog.youkuaiyun.com/WAN8180192/article/category/6103460