Reading audio samples via AVAssetReader

Question

How do you read audio samples via AVAssetReader? I've found examples of duplicating or mixing using AVAssetReader, but those loops are always controlled by the AVAssetWriter loop. Is it possible just to create an AVAssetReader and read through it, getting each sample and throwing the int32 of each audio sample into an array?

Thanks.

Answer 1

To expand on @amrox's answer, you can get an AudioBufferList from the CMBlockBufferRef, e.g.

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);
}