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POExtracting audio channel from Linear PCM
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copied!<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:&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:&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, &_streamFormat, kAudioFileFlags_EraseFile, &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, &audioBufferList, sizeof(audioBufferList), NULL, NULL, 0, &blockBuffer); for (int y=0; y<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<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, &_packetFormat, countPacketBuf, &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>

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