How to decode AAC audio buffer to PCM buffer in iOS?

Question

I am trying to decode AAC audio for PCM audio in iOS, what is the best way to do this? Any sample code would be very helpful ... Are there any simple APIs for this ??

Answer 1

I have some sample code to do this.

First you must configure ASBD (AudioStreamBasicDescription) and create a converter:

- (void)setupAudioConverter{ AudioStreamBasicDescription outFormat; memset(&outFormat, 0, sizeof(outFormat)); outFormat.mSampleRate = 44100; outFormat.mFormatID = kAudioFormatLinearPCM; outFormat.mFormatFlags = kLinearPCMFormatFlagIsSignedInteger; outFormat.mBytesPerPacket = 2; outFormat.mFramesPerPacket = 1; outFormat.mBytesPerFrame = 2; outFormat.mChannelsPerFrame = 1; outFormat.mBitsPerChannel = 16; outFormat.mReserved = 0; AudioStreamBasicDescription inFormat; memset(&inFormat, 0, sizeof(inFormat)); inFormat.mSampleRate = 44100; inFormat.mFormatID = kAudioFormatMPEG4AAC; inFormat.mFormatFlags = kMPEG4Object_AAC_LC; inFormat.mBytesPerPacket = 0; inFormat.mFramesPerPacket = 1024; inFormat.mBytesPerFrame = 0; inFormat.mChannelsPerFrame = 1; inFormat.mBitsPerChannel = 0; inFormat.mReserved = 0; OSStatus status = AudioConverterNew(&inFormat, &outFormat, &_audioConverter); if (status != 0) { printf("setup converter error, status: %i\n", (int)status); } } 

After that, you should execute the callback function for the audio converter:

 struct PassthroughUserData { UInt32 mChannels; UInt32 mDataSize; const void* mData; AudioStreamPacketDescription mPacket; }; OSStatus inInputDataProc(AudioConverterRef aAudioConverter, UInt32* aNumDataPackets /* in/out */, AudioBufferList* aData /* in/out */, AudioStreamPacketDescription** aPacketDesc, void* aUserData) { PassthroughUserData* userData = (PassthroughUserData*)aUserData; if (!userData->mDataSize) { *aNumDataPackets = 0; return kNoMoreDataErr; } if (aPacketDesc) { userData->mPacket.mStartOffset = 0; userData->mPacket.mVariableFramesInPacket = 0; userData->mPacket.mDataByteSize = userData->mDataSize; *aPacketDesc = &userData->mPacket; } aData->mBuffers[0].mNumberChannels = userData->mChannels; aData->mBuffers[0].mDataByteSize = userData->mDataSize; aData->mBuffers[0].mData = const_cast<void*>(userData->mData); // No more data to provide following this run. userData->mDataSize = 0; return noErr; } 

And the frame decoding method:

 - (void)decodeAudioFrame:(NSData *)frame withPts:(NSInteger)pts{ if(!_audioConverter){ [self setupAudioConverter]; } PassthroughUserData userData = { 1, (UInt32)frame.length, [frame bytes]}; NSMutableData *decodedData = [NSMutableData new]; const uint32_t MAX_AUDIO_FRAMES = 128; const uint32_t maxDecodedSamples = MAX_AUDIO_FRAMES * 1; do{ uint8_t *buffer = (uint8_t *)malloc(maxDecodedSamples * sizeof(short int)); AudioBufferList decBuffer; decBuffer.mNumberBuffers = 1; decBuffer.mBuffers[0].mNumberChannels = 1; decBuffer.mBuffers[0].mDataByteSize = maxDecodedSamples * sizeof(short int); decBuffer.mBuffers[0].mData = buffer; UInt32 numFrames = MAX_AUDIO_FRAMES; AudioStreamPacketDescription outPacketDescription; memset(&outPacketDescription, 0, sizeof(AudioStreamPacketDescription)); outPacketDescription.mDataByteSize = MAX_AUDIO_FRAMES; outPacketDescription.mStartOffset = 0; outPacketDescription.mVariableFramesInPacket = 0; OSStatus rv = AudioConverterFillComplexBuffer(_audioConverter, inInputDataProc, &userData, &numFrames /* in/out */, &decBuffer, &outPacketDescription); if (rv && rv != kNoMoreDataErr) { NSLog(@"Error decoding audio stream: %d\n", rv); break; } if (numFrames) { [decodedData appendBytes:decBuffer.mBuffers[0].mData length:decBuffer.mBuffers[0].mDataByteSize]; } if (rv == kNoMoreDataErr) { break; } }while (true); //void *pData = (void *)[decodedData bytes]; //audioRenderer->Render(&pData, decodedData.length, pts); } 

Answer 2

You need to use Core Audio. Locate Core Audio Overview in the Apple documentation.