通過AVAssetReader讀取音頻樣本

Question

你如何通過AVAssetReader讀取音頻樣本？ 我找到了使用AVAssetReader進行復制或混合的示例，但這些循環始終由AVAssetWriter循環控制。 是否可以創建一個AVAssetReader並讀取它，獲取每個樣本並將每個音頻樣本的int32拋入一個數組？

謝謝。

Answer 1

要擴展@ amrox的答案，您可以從CMBlockBufferRef獲取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

這里的答案不是通用的。 當AudioBufferList大小不同時，對CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer的調用可能會失敗。 以非交錯樣本為例。

正確的方法是兩次調用CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer 。 第一個調用查詢AudioBufferList所需的大小，第二個調用實際填充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);

在現實世界的示例中，您必須執行錯誤處理，並且您不應該每幀都使用malloc，而是緩存AudioBufferList 。