通过AVAssetReader读取音频样本

Question

如何通过AVAssetReader读取音频样本？我发现了使用AVAssetReader进行复制或混合的示例，但这些循环总是由AVAssetWriter循环控制。是否可以创建一个AVAssetReader并读取它，获取每个样本并将每个音频样本的int32放入一个数组中？

谢谢。

Answer 1

要扩展@amrox的答案，您可以从CMBlockBufferRef获取一个AudioBufferList，例如

CMItemCount numSamplesInBuffer = CMSampleBufferGetNumSamples(buffer);

AudioBufferList audioBufferList;

CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(
      buffer,
      NULL,
      &audioBufferList,
      sizeof(audioBufferList),
      NULL,
      NULL,
      kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
      &buffer
    );

for (int bufferCount=0; bufferCount < audioBufferList.mNumberBuffers; bufferCount++) {
  SInt16* samples = (SInt16 *)audioBufferList.mBuffers[bufferCount].mData;
  for (int i=0; i < numSamplesInBuffer; i++) {
    // amplitude for the sample is samples[i], assuming you have linear pcm to start with
  }
}

//Release the buffer when done with the samples 
//(retained by CMSampleBufferGetAudioBufferListWithRetainedblockBuffer)
CFRelease(buffer); 

Answer 2

AVAssetReader *reader   = [[AVAssetReader alloc] initWithAsset:asset error:&error];
AVAssetTrack  *track    = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
NSDictionary  *settings = @{ AVFormatIDKey : [NSNumber numberWithInt:kAudioFormatLinearPCM] };

AVAssetReaderTrackOutput *readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track
                                                                                    outputSettings:settings];

[reader addOutput:readerOutput]; 
[reader startReading];

CMSampleBufferRef sample = [readerOutput copyNextSampleBuffer];

while ( sample )
{
   sample = [readerOutput copyNextSampleBuffer];

    if ( ! sample )
    {
       continue;
    }

    CMBlockBufferRef buffer = CMSampleBufferGetDataBuffer(sample);

    size_t  lengthAtOffset;
    size_t  totalLength;
    char   *data;

    if ( CMBlockBufferGetDataPointer( buffer, 0, &lengthAtOffset, &totalLength, &data ) != noErr )
    {
        NSLog(@"error!");
        break;
    }

    // do something with data...

    CFRelease(sample);
}

Answer 3

这里的答案不是通用的。当AudioBufferList需要不同的大小时，对CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer的调用可能会失败。当以非交错样本为例时。

正确的方法是调用CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer两次。第一个调用查询AudioBufferList所需的大小，第二个调用实际填充AudioBufferList。

size_t bufferSize = 0;
CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(
    sampleBuffer,
    &bufferSize,
    NULL,
    0,
    NULL,
    NULL,
    0,
    NULL
);

AudioBufferList *bufferList = malloc(bufferSize);
CMBlockBufferRef blockBuffer = NULL;
CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(
    sampleBuffer,
    NULL,
    bufferList,
    bufferSize,
    NULL,
    NULL,
    kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
    &blockBuffer
);

// handle audio here

free(bufferList);
CFRelease(blockBuffer);

在一个真实的例子中，你必须执行错误处理，而且你也不应该对每一帧都进行恶意锁定，而是缓存AudioBufferList。