title: Extracting audio channel from Linear PCM
body: <p>I would like to extract a channel audio from the an LPCM raw file ie extract left and right channel of a stereo LPCM file. The LPCM is 16 bit depth,interleaved, 2 channels,litle endian. From what I gather the order of byte is {LeftChannel,RightChannel,LeftChannel,RightChannel...} and since it is 16 bit depth there will be 2 bytes of sample for each channel right? </p>
  
  <p>So my question is if i want to extract the left channel then I would take the bytes in 0,2,4,6...n*2 address? while the right channel would be 1,3,4,...(n*2+1).</p>
  
  <p>Also after extracting the audio channel, should i set the format of the extracted channel as 16 bit depth ,1 channel?</p>
  
  <p>Thanks in advance</p>
  
  <p>This is the code that I currently use to extract PCM audio from AssetReader.. This code works fine with writing a music file without its channel being extracted so I it might be caused by the format or something...</p>
  
  <pre><code>    NSURL *assetURL = [song valueForProperty:MPMediaItemPropertyAssetURL];
  AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL:assetURL options:nil];
  NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                  [NSNumber numberWithInt:kAudioFormatLinearPCM], AVFormatIDKey, 
                                  [NSNumber numberWithFloat:44100.0], AVSampleRateKey,
                                  [NSNumber numberWithInt:2], AVNumberOfChannelsKey,
                              //  [NSData dataWithBytes:&amp;channelLayout length:sizeof(AudioChannelLayout)], AVChannelLayoutKey,
                                  [NSNumber numberWithInt:16], AVLinearPCMBitDepthKey,
                                  [NSNumber numberWithBool:NO], AVLinearPCMIsNonInterleaved,
                                  [NSNumber numberWithBool:NO],AVLinearPCMIsFloatKey,
                                  [NSNumber numberWithBool:NO], AVLinearPCMIsBigEndianKey,
                                  nil];
  NSError *assetError = nil;
  AVAssetReader *assetReader = [[AVAssetReader assetReaderWithAsset:songAsset
                                                              error:&amp;assetError]
                                retain];
  if (assetError) {
      NSLog (@"error: %@", assetError);
      return;
  }
  
  AVAssetReaderOutput *assetReaderOutput = [[AVAssetReaderAudioMixOutput 
                                             assetReaderAudioMixOutputWithAudioTracks:songAsset.tracks
                                             audioSettings: outputSettings]
                                            retain];
  if (! [assetReader canAddOutput: assetReaderOutput]) {
      NSLog (@"can't add reader output... die!");
      return;
  }
  [assetReader addOutput: assetReaderOutput];
  
  
  NSArray *dirs = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
  NSString *documentsDirectoryPath = [dirs objectAtIndex:0];
  
  //CODE TO SPLIT STEREO
  [self setupAudioWithFormatMono:kAudioFormatLinearPCM];
  NSString *splitExportPath = [[documentsDirectoryPath stringByAppendingPathComponent:@"monoleft.caf"] retain];
  if ([[NSFileManager defaultManager] fileExistsAtPath:splitExportPath]) {
      [[NSFileManager defaultManager] removeItemAtPath:splitExportPath error:nil];
  }
  
  AudioFileID mRecordFile;
  NSURL *splitExportURL = [NSURL fileURLWithPath:splitExportPath];
  
  
  OSStatus status =  AudioFileCreateWithURL(splitExportURL, kAudioFileCAFType, &amp;_streamFormat, kAudioFileFlags_EraseFile,
                                            &amp;mRecordFile);
  
  NSLog(@"status os %d",status);
  
  [assetReader startReading];
  
  CMSampleBufferRef sampBuffer = [assetReaderOutput copyNextSampleBuffer];
  UInt32 countsamp= CMSampleBufferGetNumSamples(sampBuffer);
  NSLog(@"number of samples %d",countsamp);
  
  SInt64 countByteBuf = 0;
  SInt64 countPacketBuf = 0;
  UInt32 numBytesIO = 0;
  UInt32 numPacketsIO = 0;
  NSMutableData * bufferMono = [NSMutableData new];
  while (sampBuffer) {
  
  
      AudioBufferList  audioBufferList;
      CMBlockBufferRef blockBuffer;
      CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(sampBuffer, NULL, &amp;audioBufferList, sizeof(audioBufferList), NULL, NULL, 0, &amp;blockBuffer);
      for (int y=0; y&lt;audioBufferList.mNumberBuffers; y++) {
          AudioBuffer audioBuffer = audioBufferList.mBuffers[y];
          //frames = audioBuffer.mData;
          NSLog(@"the number of channel for buffer number %d is %d",y,audioBuffer.mNumberChannels);
          NSLog(@"The buffer size is %d",audioBuffer.mDataByteSize);
  
  
  
  
  
  
          //Append mono left to buffer data
          for (int i=0; i&lt;audioBuffer.mDataByteSize; i= i+4) {
              [bufferMono appendBytes:(audioBuffer.mData+i) length:2];
          }
  
          //the number of bytes in the mutable data containing mono audio file
          numBytesIO = [bufferMono length];
          numPacketsIO = numBytesIO/2;
          NSLog(@"numpacketsIO %d",numPacketsIO);
          status = AudioFileWritePackets(mRecordFile, NO, numBytesIO, &amp;_packetFormat, countPacketBuf, &amp;numPacketsIO, audioBuffer.mData);
          NSLog(@"status for writebyte %d, packets written %d",status,numPacketsIO);
          if(numPacketsIO != (numBytesIO/2)){
              NSLog(@"Something wrong");
              assert(0);
          }
  
  
          countPacketBuf = countPacketBuf + numPacketsIO;
          [bufferMono setLength:0];
  
  
      }
  
      sampBuffer = [assetReaderOutput copyNextSampleBuffer];
      countsamp= CMSampleBufferGetNumSamples(sampBuffer);
      NSLog(@"number of samples %d",countsamp);
  }
  AudioFileClose(mRecordFile);
  [assetReader cancelReading];
  [self performSelectorOnMainThread:@selector(updateCompletedSizeLabel:)
                         withObject:0
                      waitUntilDone:NO];
  </code></pre>
  
  <p>The output format with audiofileservices is as follows:</p>
  
  <pre><code>        _streamFormat.mFormatFlags = kLinearPCMFormatFlagIsSignedInteger | kLinearPCMFormatFlagIsPacked;
      _streamFormat.mBitsPerChannel = 16;
      _streamFormat.mChannelsPerFrame = 1;
      _streamFormat.mBytesPerPacket = 2;
      _streamFormat.mBytesPerFrame = 2;// (_streamFormat.mBitsPerChannel / 8) * _streamFormat.mChannelsPerFrame;
      _streamFormat.mFramesPerPacket = 1;
      _streamFormat.mSampleRate = 44100.0;
  
      _packetFormat.mStartOffset = 0;
      _packetFormat.mVariableFramesInPacket = 0;
      _packetFormat.mDataByteSize = 2;
  </code></pre>