Android 7.0 Audio的Resample过程详解
Qidi 2017.02.23 (Markdown & Haroopad)
【前言】
处理过音频文件的工程师都知道音频数据存在采样率(Sample Rate)这个指标。在位深度(Bit Depth)一定的情况下,采样率越高,理论上来说播放出来的声音就越细腻,录制的声音也就越保真,反之亦然。
但在较早的Android系统版本上,不管音频文件原来的采样率几何,统统都被重采样(Resample)到44.1KHz进行播放,录制的时候则是被固定为8KHz进行采样。尽管这样的处理方式被广大音质爱好者所诟病,但在当时它确实是一种实现设备兼容的有效方法。
作为Android Audio BSP工程师,有必要了解系统实现Resample的过程。现在Android系统已经发布到了7.0版本,一起看看在最新的版本上这个Resample的过程是怎样实现的吧。
【背景知识】
我们知道在Android系统中,当应用层APP播放一个音频文件时,Framework层的AudioPolicyService(APS)会接收上层APP传递来的音频参数(例如格式、声道、采样率等),并调用AudioFlinger的createTrack()
方法对应创建1个Track,再调用openOutput()
方法来打开1个outputStream,然后使用这个outputStream来创建相应的Playback线程(依据应用场景可以是OffloadThread、DirectOutputThread、MixerThread),最终在Playback线程中匹配之前创建的Track,开始自APP至HAL的数据传输。
【Resample过程分析】
那么我们对Android Audio Resample过程的分析就从AudioFlinger开始。在AudioFlinger::openOutput()
中可以看到,在Playback线程被成功创建之后,即被加入到mPlaybackThreads向量中进行管理了。具体代码如下:
sp<AudioFlinger::PlaybackThread> AudioFlinger::openOutput_l(audio_module_handle_t module,
audio_io_handle_t *output,
audio_config_t *config,
audio_devices_t devices,
const String8& address,
audio_output_flags_t flags)
{
......
AudioStreamOut *outputStream = NULL;
status_t status = outHwDev->openOutputStream(
&outputStream,
*output,
devices,
flags,
config,
address.string());
mHardwareStatus = AUDIO_HW_IDLE;
if (status == NO_ERROR) {
PlaybackThread *thread;
if (flags & AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD) {
thread = new OffloadThread(this, outputStream, *output, devices, mSystemReady);
ALOGV("openOutput_l() created offload output: ID %d thread %p", *output, thread);
} else if ((flags & AUDIO_OUTPUT_FLAG_DIRECT)
|| !isValidPcmSinkFormat(config->format)
|| !isValidPcmSinkChannelMask(config->channel_mask)) {
thread = new DirectOutputThread(this, outputStream, *output, devices, mSystemReady);
ALOGV("openOutput_l() created direct output: ID %d thread %p", *output, thread);
} else {
thread = new MixerThread(this, outputStream, *output, devices, mSystemReady);
ALOGV("openOutput_l() created mixer output: ID %d thread %p", *output, thread);
}
mPlaybackThreads.add(*output, thread);
return thread;
}
return 0;
}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
随后Playback线程运行,对应的AudioFlinger::Playback::threadLoop()
方法被执行,在该方法中调用了prepareTracks_l()
函数。这个函数实际上是对应于AudioFlinger::MixerThread::prepareTracks_l()
这个函数。threadLoop()函数代码细节如下:
bool AudioFlinger::PlaybackThread::threadLoop()
{
Vector< sp<Track> > tracksToRemove;
......
while (!exitPending())
{
cpuStats.sample(myName);
Vector< sp<EffectChain> > effectChains;
{
Mutex::Autolock _l(mLock);
processConfigEvents_l();
......
saveOutputTracks();
......
if ((!mActiveTracks.size() && systemTime() > mStandbyTimeNs) ||
isSuspended()) {
if (shouldStandby_l()) {
threadLoop_standby();
mStandby = true;
}
......
}
mMixerStatus = prepareTracks_l(&tracksToRemove);
......
lockEffectChains_l(effectChains);
}
......
unlockEffectChains(effectChains);
threadLoop_removeTracks(tracksToRemove);
tracksToRemove.clear();
clearOutputTracks();
effectChains.clear();
}
threadLoop_exit();
if (!mStandby) {
threadLoop_standby();
mStandby = true;
}
......
return false;
}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 42
- 43
- 44
- 45
- 46
- 47
- 48
- 49
- 50
- 51
- 52
- 53
- 54
- 55
- 56
- 57
- 58
- 59
- 60
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 42
- 43
- 44
- 45
- 46
- 47
- 48
- 49
- 50
- 51
- 52
- 53
- 54
- 55
- 56
- 57
- 58
- 59
- 60
Resample的过程就发生在prepareTracks_l()
函数中,所以我们来好好阅读一下。在该函数中,通过一个for循环遍历所有处于active状态的track。每一次循环中,都要进行如下2步操作:
1. 通过reqSampleRate = track->mAudioTrackServerProxy->getSampleRate()
来获取硬件设备所支持的采样率;
2. 之后调用mAudioMixer->setParameter(name, AudioMixer::RESAMPLE, AudioMixer::SAMPLE_RATE, (void*)(uintptr_t)reqSampleRate)
,通过对比音频文件采样率和音频设备支持的采样率,判断是否创建新的Resampler对象,或者从已有的Resampler对象列表中返回1个;
prepareTracks_l()函数代码细节如下:
AudioFlinger::PlaybackThread::mixer_state AudioFlinger::MixerThread::prepareTracks_l(
Vector< sp<Track> > *tracksToRemove)
{
......
size_t count = mActiveTracks.size();
......
for (size_t i=0 ; i<count ; i++) {
const sp<Track> t = mActiveTracks[i].promote();
if (t == 0) {
continue;
}
Track* const track = t.get();
......
audio_track_cblk_t* cblk = track->cblk();
int name = track->name();
......
if ((framesReady >= minFrames) && track->isReady() &&
!track->isPaused() && !track->isTerminated())
{
......
int param = AudioMixer::VOLUME;
if (track->mFillingUpStatus == Track::FS_FILLED) {
track->mFillingUpStatus = Track::FS_ACTIVE;
if (track->mState == TrackBase::RESUMING) {
track->mState = TrackBase::ACTIVE;
param = AudioMixer::RAMP_VOLUME;
}
mAudioMixer->setParameter(name, AudioMixer::RESAMPLE, AudioMixer::RESET, NULL);
} else if (cblk->mServer != 0) {
param = AudioMixer::RAMP_VOLUME;
}
......
......
mAudioMixer->setBufferProvider(name, track);
mAudioMixer->enable(name);
mAudioMixer->setParameter(name, param, AudioMixer::VOLUME0, &vlf);
mAudioMixer->setParameter(name, param, AudioMixer::VOLUME1, &vrf);
mAudioMixer->setParameter(name, param, AudioMixer::AUXLEVEL, &vaf);
mAudioMixer->setParameter(
name,
AudioMixer::TRACK,
AudioMixer::FORMAT, (void *)track->format());
mAudioMixer->setParameter(
name,
AudioMixer::TRACK,
AudioMixer::CHANNEL_MASK, (void *)(uintptr_t)track->channelMask());
mAudioMixer->setParameter(
name,
AudioMixer::TRACK,
AudioMixer::MIXER_CHANNEL_MASK, (void *)(uintptr_t)mChannelMask);
uint32_t maxSampleRate = mSampleRate * AUDIO_RESAMPLER_DOWN_RATIO_MAX;
uint32_t reqSampleRate = track->mAudioTrackServerProxy->getSampleRate();
if (reqSampleRate == 0) {
reqSampleRate = mSampleRate;
} else if (reqSampleRate > maxSampleRate) {
reqSampleRate = maxSampleRate;
}
mAudioMixer->setParameter(
name,
AudioMixer::RESAMPLE,
AudioMixer::SAMPLE_RATE,
(void *)(uintptr_t)reqSampleRate);
AudioPlaybackRate playbackRate = track->mAudioTrackServerProxy->getPlaybackRate();
mAudioMixer->setParameter(
name,
AudioMixer::TIMESTRETCH,
AudioMixer::PLAYBACK_RATE,
&playbackRate);
if (mMixerBufferEnabled
&& (track->mainBuffer() == mSinkBuffer
|| track->mainBuffer() == mMixerBuffer)) {
mAudioMixer->setParameter(
name,
AudioMixer::TRACK,
AudioMixer::MIXER_FORMAT, (void *)mMixerBufferFormat);
mAudioMixer->setParameter(
name,
AudioMixer::TRACK,
AudioMixer::MAIN_BUFFER, (void *)mMixerBuffer);
mMixerBufferValid = true;
} else {
......
}
mAudioMixer->setParameter(
name,
AudioMixer::TRACK,
AudioMixer::AUX_BUFFER, (void *)track->auxBuffer());
track->mRetryCount = kMaxTrackRetries;
if (mMixerStatusIgnoringFastTracks != MIXER_TRACKS_READY ||
mixerStatus != MIXER_TRACKS_ENABLED) {
mixerStatus = MIXER_TRACKS_READY;
}
} else {
......
}
}
......
......
removeTracks_l(*tracksToRemove);
......
......
......
return mixerStatus;
}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 42
- 43
- 44
- 45
- 46
- 47
- 48
- 49
- 50
- 51
- 52
- 53
- 54
- 55
- 56
- 57
- 58
- 59
- 60
- 61
- 62
- 63
- 64
- 65
- 66
- 67
- 68
- 69
- 70
- 71
- 72
- 73
- 74
- 75
- 76
- 77
- 78
- 79
- 80
- 81
- 82
- 83
- 84
- 85
- 86
- 87
- 88
- 89
- 90
- 91
- 92
- 93
- 94
- 95
- 96
- 97
- 98
- 99
- 100
- 101
- 102
- 103
- 104
- 105
- 106
- 107
- 108
- 109
- 110
- 111
- 112
- 113
- 114
- 115
- 116
- 117
- 118
- 119
- 120
- 121
- 122
- 123
- 124
- 125
- 126
- 127
- 128
- 129
- 130
- 131
- 132
- 133
- 134
- 135
- 136
- 137
- 138
- 139
- 140
- 141
- 142
- 143
- 144
- 145
- 146
- 147
- 148
- 149
- 150
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 42
- 43
- 44
- 45
- 46
- 47
- 48
- 49
- 50
- 51
- 52
- 53
- 54
- 55
- 56
- 57
- 58
- 59
- 60
- 61
- 62
- 63
- 64
- 65
- 66
- 67
- 68
- 69
- 70
- 71
- 72
- 73
- 74
- 75
- 76
- 77
- 78
- 79
- 80
- 81
- 82
- 83
- 84
- 85
- 86
- 87
- 88
- 89
- 90
- 91
- 92
- 93
- 94
- 95
- 96
- 97
- 98
- 99
- 100
- 101
- 102
- 103
- 104
- 105
- 106
- 107
- 108
- 109
- 110
- 111
- 112
- 113
- 114
- 115
- 116
- 117
- 118
- 119
- 120
- 121
- 122
- 123
- 124
- 125
- 126
- 127
- 128
- 129
- 130
- 131
- 132
- 133
- 134
- 135
- 136
- 137
- 138
- 139
- 140
- 141
- 142
- 143
- 144
- 145
- 146
- 147
- 148
- 149
- 150
在确认要使用的Resampler对象存在后,调用invalidateState(1 << name)
使设置生效,开始执行重采样。invalidateState()函数会调用AudioMixer::process_validate()
,在该函数中首先通过语句t.hook = getTrackHook(TRACKTYPE_RESAMPLE, t.mMixerChannelCount, t.mMixerInFormat, t.mMixerFormat);
获取执行重采样操作的函数,随后通过state->hook = process_resampling;
中的t.hook(&t, outTemp, numFrames, state->resampleTemp, aux)
语句进行调用。
setParameter()函数代码如下:
void AudioMixer::setParameter(int name, int target, int param, void *value)
{
......
int valueInt = static_cast<int>(reinterpret_cast<uintptr_t>(value));
int32_t *valueBuf = reinterpret_cast<int32_t*>(value);
switch (target) {
......
case RESAMPLE:
switch (param) {
case SAMPLE_RATE:
ALOG_ASSERT(valueInt > 0, "bad sample rate %d", valueInt);
if (track.setResampler(uint32_t(valueInt), mSampleRate)) {
ALOGV("setParameter(RESAMPLE, SAMPLE_RATE, %u)",
uint32_t(valueInt));
invalidateState(1 << name);
}
break;
case RESET:
track.resetResampler();
invalidateState(1 << name);
break;
case REMOVE:
delete track.resampler;
track.resampler = NULL;
track.sampleRate = mSampleRate;
invalidateState(1 << name);
break;
default:
LOG_ALWAYS_FATAL("setParameter resample: bad param %d", param);
}
break;
}
}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
invalidateState()函数代码如下:
void AudioMixer::invalidateState(uint32_t mask)
{
if (mask != 0) {
mState.needsChanged |= mask;
mState.hook = process__validate;
}
}
process__validate()函数代码如下:
void AudioMixer::process__validate(state_t* state)
{
......
uint32_t en = state->enabledTracks;
while (en) {
......
if (n & NEEDS_MUTE) {
......
} else {
......
if (n & NEEDS_RESAMPLE) {
all16BitsStereoNoResample = false;
resampling = true;
t.hook = getTrackHook(TRACKTYPE_RESAMPLE, t.mMixerChannelCount,
t.mMixerInFormat, t.mMixerFormat);
ALOGV_IF((n & NEEDS_CHANNEL_COUNT__MASK) > NEEDS_CHANNEL_2,
"Track %d needs downmix + resample", i);
} else {
......
}
}
}
state->hook = process__nop;
if (countActiveTracks > 0) {
if (resampling) {
if (!state->outputTemp) {
state->outputTemp = new int32_t[MAX_NUM_CHANNELS * state->frameCount];
}
if (!state->resampleTemp) {
state->resampleTemp = new int32_t[MAX_NUM_CHANNELS * state->frameCount];
}
state->hook = process__genericResampling; // 在需要重采样操作的情况下,调用process_genericResampling()函数
} else {
......
}
}
......
// Now that the volume ramp has been done, set optimal state and
// track hooks for subsequent mixer process
......
}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 42
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 25
- 26
- 27
- 28
- 29
- 30
- 31
- 32
- 33
- 34
- 35
- 36
- 37
- 38
- 39
- 40
- 41
- 42
process_genericResampling()函数代码如下:
void AudioMixer::process__genericResampling(state_t* state)
{
......
uint32_t e0 = state->enabledTracks;
while (e0) {
......
while (e1) {
......
if (t.needs & NEEDS_RESAMPLE) {
t.hook(&t, outTemp, numFrames, state->resampleTemp, aux);
} else {
......
}
}
convertMixerFormat(out, t1.mMixerFormat,
outTemp, t1.mMixerInFormat, numFrames * t1.mMixerChannelCount);
}
}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
至此,Android系统播放音频时的Resample过程就分析完成了。
具体的Resample处理实质是数字信号处理,是个数学运算过程。Android系统中提供的算法有线性插值、三次插值、FIR滤波 3种。感兴趣的工程师同仁可以自行查阅相关资料书籍,这里不对数字信号处理的细节进行讨论。
转载至:http://blog.youkuaiyun.com/Qidi_Huang/article/details/56834419