深入理解Android音视频同步机制（五）如何从零开始写一个音视频同步的播放器

• 深入理解Android音视频同步机制（一）概述
• 深入理解Android音视频同步机制（二）ExoPlayer的avsync逻辑
• 深入理解Android音视频同步机制（三）NuPlayer的avsync逻辑
• 深入理解Android音视频同步机制（四）MediaSync的使用与原理
• 深入理解Android音视频同步机制（五）如何从零开始写一个音视频同步的播放器

前面我们分析了三个播放器的av sync逻辑，可以看到他们都各有不同，那么究竟哪种方法可以达到最好的avsync结果？哪些逻辑是必要的？如果我们想自己从零开始写一个av同步的播放器，都需要做哪些工作？

首先我们测试了几个播放器的音视频同步表现，使用的是syncOne官网的1080p 24fps H264 AAC测试片源，只测试speaker下的结果，测试结果如下

下面我们参考cts中的mediacodec使用示例，尝试着写出一个avsync结果与上面结果接近的播放器

MediaCodecPlayer Demo

demo的地址关注文末公众号查看。

首先来看一下整体流程，如下图所示

总体流程和ExoPlayer基本相似。图中的doSomeWork是核心大loop，同步地调用MediaCodc接口，关键的avsync逻辑在drainOutputBuffer方法中实现。

最朴素简单的同步逻辑

先来总体说一下同步逻辑，然后再详细看看代码
Video部分

1.对实际送显时间的计算

private boolean drainOutputBuffer()

//传入参数为pts，返回的realTimeUs其实就是pts，并没有做什么调整
   long realTimeUs =
       mMediaTimeProvider.getRealTimeUsForMediaTime(info.presentationTimeUs);

   //这里的nowUs代表audio播放的时间
   long nowUs = mMediaTimeProvider.getNowUs(); //audio play time
   //nowUs - realTimeUs代表还有多久该播放这一帧
   long lateUs = nowUs - realTimeUs;
…
    //这里调用的是没有timestamp参数的releaseOutputBuffer方法
    mCodec.releaseOutputBuffer(index, render);

Audio部分

1、current play time的计算

public long getAudioTimeUs()

//这里返回audio的播放时间，就是利用的getPlaybackHeadPosition方法
   int numFramesPlayed = mAudioTrack.getPlaybackHeadPosition();
   return (numFramesPlayed * 1000000L) / mSampleRate;

小结

就是拿video的pts与audioTrack.getPlaybackHeadPosition方法返回的播放时间进行对比，然后直接调用没有timestamp参数的releaseOutputBuffer的方法进行显示

详细的代码如下

private boolean drainOutputBuffer() {
...
   int index = mAvailableOutputBufferIndices.peekFirst().intValue();
   MediaCodec.BufferInfo info = mAvailableOutputBufferInfos.peekFirst();
…
   //传入参数为pts，返回的realTimeUs其实就是pts，并没有做什么调整，参见1.1
   long realTimeUs =
       mMediaTimeProvider.getRealTimeUsForMediaTime(info.presentationTimeUs);

   //这里的nowUs代表audio播放的时间，参见1.1
   long nowUs = mMediaTimeProvider.getNowUs(); //audio play time
   //nowUs - realTimeUs代表还有多久该播放这一帧
   long lateUs = nowUs - realTimeUs;

   if (mAudioTrack != null) {
       ...
   } else {
       // video
       boolean render;

       if (lateUs < -45000) {
           // too early;如果video来早了45ms，则等待下一次loop
           return false;
       } else if (lateUs > 30000) {
           //默认的丢帧门限是30ms
           Log.d(TAG, "video late by " + lateUs + " us.");
           render = false;
       } else {
           render = true;
           mPresentationTimeUs = info.presentationTimeUs;
       }

       //这里调用的是没有timestamp参数的releaseOutputBuffer方法
       mCodec.releaseOutputBuffer(index, render);

       mAvailableOutputBufferIndices.removeFirst();
       mAvailableOutputBufferInfos.removeFirst();
       return true;
   }
}

public long getRealTimeUsForMediaTime(long mediaTimeUs) {
   if (mDeltaTimeUs == -1) {
      //第一次调用的时候会走进这个分支，mDeltaTimeUs代表初始pts的偏差，正常情况下它是0
       long nowUs = getNowUs();
       mDeltaTimeUs = nowUs - mediaTimeUs;
   }
   //所以getRealTimeUsForMediaTime返回的就是pts
   return mDeltaTimeUs + mediaTimeUs;
}

public long getNowUs() {
   //如果是video only的流，则返回系统时间，否则返回audio播放的时间
   if (mAudioTrackState == null) {
       return System.currentTimeMillis() * 1000;
   }

   return mAudioTrackState.getAudioTimeUs();
}

public long getAudioTimeUs() {
   //这里返回audio的播放时间，就是利用的getPlaybackHeadPosition方法
   int numFramesPlayed = mAudioTrack.getPlaybackHeadPosition();

   return (numFramesPlayed * 1000000L) / mSampleRate;
}

测试一下这个最简单avsync逻辑的结果 ： -173ms，果然惨不忍睹

第一步改造

改造audio time的获取，加上latency

public long getAudioTimeUs() {
  int numFramesPlayed = mAudioTrack.getPlaybackHeadPosition();
  if (getLatencyMethod != null) {
      try {
          latencyUs = (Integer) getLatencyMethod.invoke(mAudioTrack, (Object[]) null) * 1000L /2;
          latencyUs = Math.max(latencyUs, 0);
      } catch (Exception e){
          getLatencyMethod = null;
      }
  }
  return (numFramesPlayed * 1000000L) / mSampleRate - latencyUs;
}

此时的测试结果是：-128ms， 好了一些，但还不够

第二步改造

考虑到getLatency方法是一个被google自己吐槽的接口，我们再来下一步改造，用上getTimeStamp方法，如下

public long getAudioTimeUs() {
   long systemClockUs = System.nanoTime() / 1000;
   int numFramesPlayed = mAudioTrack.getPlaybackHeadPosition();
   if (systemClockUs - lastTimestampSampleTimeUs >= MIN_TIMESTAMP_SAMPLE_INTERVAL_US) {
       audioTimestampSet = mAudioTrack.getTimestamp(audioTimestamp);
       if (getLatencyMethod != null) {
           try {
               latencyUs = (Integer) getLatencyMethod.invoke(mAudioTrack, (Object[]) null) * 1000L / 2;
               latencyUs = Math.max(latencyUs, 0);
           } catch (Exception e) {
               getLatencyMethod = null;
           }
       }
       lastTimestampSampleTimeUs = systemClockUs;
   }

   if (audioTimestampSet) {
       // Calculate the speed-adjusted position using the timestamp (which may be in the future).
       long elapsedSinceTimestampUs = System.nanoTime() / 1000 - (audioTimestamp.nanoTime / 1000);
       long elapsedSinceTimestampFrames = elapsedSinceTimestampUs * mSampleRate / 1000000L;
       long elapsedFrames = audioTimestamp.framePosition + elapsedSinceTimestampFrames;
       long durationUs = (elapsedFrames * 1000000L) / mSampleRate;
       return durationUs;
   } else {
       long durationUs = (numFramesPlayed * 1000000L) / mSampleRate - latencyUs;
       //durationUs = Math.max(durationUs, 0);
       return durationUs;
   }
}

当我们对比getTimeStamp和getPosition返回的结果，就能发现两者的精度差距

1.getPosition, 最后的nowUs代表audio播放的duration
12-06 16:11:47.695 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 166667,realTimeUs is 46667,nowUs is 40000
12-06 16:11:47.696 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 208333,realTimeUs is 88333,nowUs is 40000
12-06 16:11:47.700 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 362666,realTimeUs is 242666,nowUs is 40000
12-06 16:11:47.706 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 208333,realTimeUs is 88333,nowUs is 40000
12-06 16:11:47.707 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 384000,realTimeUs is 264000,nowUs is 40000
12-06 16:11:47.714 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 208333,realTimeUs is 88333,nowUs is 80000
12-06 16:11:47.716 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 250000,realTimeUs is 130000,nowUs is 80000
12-06 16:11:47.720 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 405333,realTimeUs is 285333,nowUs is 80000
12-06 16:11:47.726 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 250000,realTimeUs is 130000,nowUs is 80000
12-06 16:11:47.728 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 426666,realTimeUs is 306666,nowUs is 80000
12-06 16:11:47.734 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 250000,realTimeUs is 130000,nowUs is 80000
12-06 16:11:47.736 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 448000,realTimeUs is 328000,nowUs is 120000
12-06 16:11:47.742 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 250000,realTimeUs is 130000,nowUs is 120000
12-06 16:11:47.743 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 291667,realTimeUs is 171667,nowUs is 120000
12-06 16:11:47.746 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 469333,realTimeUs is 349333,nowUs is 120000
12-06 16:11:47.753 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 291667,realTimeUs is 171667,nowUs is 120000
12-06 16:11:47.756 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 490666,realTimeUs is 370666,nowUs is 120000
12-06 16:11:47.764 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 291667,realTimeUs is 171667,nowUs is 160000
12-06 16:11:47.764 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 333333,realTimeUs is 213333,nowUs is 160000
12-06 16:11:47.767 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 512000,realTimeUs is 392000,nowUs is 160000
12-06 16:11:47.774 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 333333,realTimeUs is 213333,nowUs is 160000
12-06 16:11:47.776 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 533333,realTimeUs is 413333,nowUs is 160000
12-06 16:11:47.782 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 333333,realTimeUs is 213333,nowUs is 160000
12-06 16:11:47.783 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 554666,realTimeUs is 434666,nowUs is 160000
12-06 16:11:47.790 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 333333,realTimeUs is 213333,nowUs is 160000
12-06 16:11:47.791 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 576000,realTimeUs is 456000,nowUs is 160000
12-06 16:11:47.798 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 333333,realTimeUs is 213333,nowUs is 160000
12-06 16:11:47.806 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 597333,realTimeUs is 477333,nowUs is 200000
12-06 16:11:47.813 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 333333,realTimeUs is 213333,nowUs is 200000
12-06 16:11:47.814 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 375000,realTimeUs is 255000,nowUs is 200000
12-06 16:11:47.817 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 618666,realTimeUs is 498666,nowUs is 200000
12-06 16:11:47.825 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 375000,realTimeUs is 255000,nowUs is 200000
12-06 16:11:47.827 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 640000,realTimeUs is 520000,nowUs is 200000
12-06 16:11:47.836 20194-20657/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 375000,realTimeUs is 255000,nowUs is 200000
12-06 16:11:47.840 20194-20657/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 661333,realTimeUs is 541333,nowUs is 200000

2.getTimeStamp，同样的，最后的nowUs代表audio播放的duration，其实这其中的一大部分功劳也是因为系统时间参与了计算
12-06 16:17:22.122 30072-30257/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 333333,realTimeUs is 213333,nowUs is 161312
12-06 16:17:22.124 30072-30257/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 554666,realTimeUs is 434666,nowUs is 163250
12-06 16:17:22.131 30072-30257/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 333333,realTimeUs is 213333,nowUs is 170125
12-06 16:17:22.131 30072-30257/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 375000,realTimeUs is 255000,nowUs is 170562
12-06 16:17:22.133 30072-30257/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 576000,realTimeUs is 456000,nowUs is 172666
12-06 16:17:22.141 30072-30257/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 375000,realTimeUs is 255000,nowUs is 180604
12-06 16:17:22.142 30072-30257/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 597333,realTimeUs is 477333,nowUs is 181666
12-06 16:17:22.150 30072-30257/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 375000,realTimeUs is 255000,nowUs is 189937
12-06 16:17:22.153 30072-30257/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 618666,realTimeUs is 498666,nowUs is 192145
12-06 16:17:22.159 30072-30257/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 375000,realTimeUs is 255000,nowUs is 198458
12-06 16:17:22.160 30072-30257/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 640000,realTimeUs is 520000,nowUs is 199687
12-06 16:17:22.166 30072-30257/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 375000,realTimeUs is 255000,nowUs is 205520
12-06 16:17:22.167 30072-30257/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 661333,realTimeUs is 541333,nowUs is 206812
12-06 16:17:22.173 30072-30257/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 375000,realTimeUs is 255000,nowUs is 212895
12-06 16:17:22.174 30072-30257/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 416667,realTimeUs is 296667,nowUs is 213104
12-06 16:17:22.176 30072-30257/com.example.zhanghui.avplayer D/avsync: audio: presentationUs is 682666,realTimeUs is 562666,nowUs is 215125
12-06 16:17:22.182 30072-30257/com.example.zhanghui.avplayer D/avsync: video: presentationUs is 416667,realTimeUs is 296667,nowUs is 221479

此时的测试结果是：-87.5ms， 好了一些，已经达到和ijkPlayer差不多的水平了，是否还能更好呢？

第三步改造

基于vsync调整送显时间

audio部分似乎已经没什么可以改的了，下面我们来看看video部分加上基于vsync的调整后会怎么样，前面我们看到在最朴素的逻辑中，使用的是不带timestamp参数的releaseOutputBuffer方法，实际上，这时在framework中会把pts作为timestamp传进去，如下

status_t MediaCodec::onReleaseOutputBuffer(const sp &msg) {
...
    BufferInfo *info = &mPortBuffers[kPortIndexOutput].editItemAt(index);
...
    if (render && info->mData != NULL && info->mData->size() != 0) {
        info->mNotify->setInt32("render", true);

        int64_t mediaTimeUs = -1;
        info->mData->meta()->findInt64("timeUs", &mediaTimeUs);

        int64_t renderTimeNs = 0;
        if (!msg->findInt64("timestampNs", &renderTimeNs)) {
            // use media timestamp if client did not request a specific render timestamp
            ALOGV("using buffer PTS of %lld", (long long)mediaTimeUs);
            renderTimeNs = mediaTimeUs * 1000;
        }
...

简单起见，我们直接复用exoplayer中基于vsync调整的逻辑，做如下修改

这个方法以前是直接返回pts，现在我们让他返回经过vsync调整后的release时间
public long getRealTimeUsForMediaTime(long mediaTimeUs) {
   if (mDeltaTimeUs == -1) {
       long nowUs = getNowUs();
       mDeltaTimeUs = nowUs - mediaTimeUs; //-32000
   }
   long earlyUs = mDeltaTimeUs + mediaTimeUs - getNowUs();
   long unadjustedFrameReleaseTimeNs = System.nanoTime() + (earlyUs * 1000);
   long adjustedReleaseTimeNs = frameReleaseTimeHelper.adjustReleaseTime(
           mediaTimeUs, unadjustedFrameReleaseTimeNs);
   return adjustedReleaseTimeNs / 1000;
}

相应的，在drainOutputBuffer中也要改一下
private boolean drainOutputBuffer() {
   ...

   long realTimeUs =
       mMediaTimeProvider.getRealTimeUsForMediaTime(info.presentationTimeUs); //返回调整后的releaseTime

   long nowUs = mMediaTimeProvider.getNowUs(); //audio play time

   long lateUs = System.nanoTime()/1000 - realTimeUs;

   if (mAudioTrack != null) {
       ...
       return true;
   } else {
       // video
       ….

       //mCodec.releaseOutputBuffer(index, render);
       mCodec.releaseOutputBuffer(index, realTimeUs*1000);
       mAvailableOutputBufferIndices.removeFirst();
       mAvailableOutputBufferInfos.removeFirst();
       return true;
   }
}

此时的测试结果是-76ms，又好了一些，但是已经不甚明显了，能不能再做进一步的优化呢？

第四步改造

提前两倍vsync时间调用releaseOutputBuffer

我们想到之前在NuPlayer和MediaSync中都有提前两倍vsync duration时间调用releaseOutputBuffer方法的逻辑，加上试试看，如下

private boolean drainOutputBuffer() {
  ...
   long lateUs = System.nanoTime()/1000 - realTimeUs;

   if (mAudioTrack != null) {
      …
   } else {
       // video
       boolean render;
       //这里改为如果video比预期release时间来的早了2*vsyncduration时间以上，则跳过并且进入下次循环，否则予以显示
       long twiceVsyncDurationUs = 2 * mMediaTimeProvider.getVsyncDurationNs()/1000;
       if (lateUs < -twiceVsyncDurationUs) {
           // too early;
           return false;
       } else if (lateUs > 30000) {
           Log.d(TAG, "video late by " + lateUs + " us.");
           render = false;
       } else {
           render = true;
           mPresentationTimeUs = info.presentationTimeUs;
       }

       //mCodec.releaseOutputBuffer(index, render);
       mCodec.releaseOutputBuffer(index, realTimeUs*1000);
...
   }
}

此时的测试结果是-66ms，又好了一点点，但是同样已经不甚明显了

第五步改造

再做一些微调
比如在解码audio时也做了一轮vsync调整，这显然是多余的，去掉它

private boolean drainOutputBuffer() {
   
   if (mAudioTrack != null) {
   } else {
       // video
       boolean render;
       long twiceVsyncDurationUs = 2 * mMediaTimeProvider.getVsyncDurationNs()/1000;

       long realTimeUs =
               mMediaTimeProvider.getRealTimeUsForMediaTime(info.presentationTimeUs); 
       long nowUs = mMediaTimeProvider.getNowUs(); //audio play time
       long lateUs = System.nanoTime()/1000 - realTimeUs;
     ...
       mCodec.releaseOutputBuffer(index, realTimeUs*1000);
       mAvailableOutputBufferIndices.removeFirst();
       mAvailableOutputBufferInfos.removeFirst();
       return true;
   }
}

此时测试的结果是-56ms，看似是和avsync无关的调整，却也产生了一定的效果，也从侧面证明了根据vsync调整releaseTime是一个很耗时的操作

总结一下我们每一步所做的事情和avsync优化的结果，如此，我们可以更好地理解avsync逻辑中各个环节的必要性和所产生的影响了

关注公众号，回复“avsync”，查看源码地址，掌握更多多媒体领域知识与资讯

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