How to decode AAC compressed frames to PCM using AudioConverterFillComplexBuffer iOS

Question

I want to implement SIP calls in my application, and first problem, that I need to solve, is converting audio from compressed AAC format with ADTS header to linear PCM.

My input data is an NSArray of ADTS frames with different framesize. Each frame is typeof NSMutableData. Each frame is of the same format and sample rate, only difference is framesize.

I tried to implement sample code, suggested by Igor Rotaru for this issue, but can't make it work.

Now my code looks like this. First of all, I configure the AudioConverter:

- (void)configureAudioConverter {
    AudioStreamBasicDescription inFormat;
    memset(&inFormat, 0, sizeof(inFormat));
    inputFormat.mBitsPerChannel = 0;
    inputFormat.mBytesPerFrame = 0;
    inputFormat.mBytesPerPacket = 0;
    inputFormat.mChannelsPerFrame = 1;
    inputFormat.mFormatFlags = kMPEG4Object_AAC_LC;
    inputFormat.mFormatID = kAudioFormatMPEG4AAC;
    inputFormat.mFramesPerPacket = 1024;
    inputFormat.mReserved = 0;
    inputFormat.mSampleRate = 22050;

    AudioStreamBasicDescription outputFormat;
    memset(&outputFormat, 0, sizeof(outputFormat));
    outputFormat.mSampleRate       = inputFormat.mSampleRate;
    outputFormat.mFormatID         = kAudioFormatLinearPCM;
    outputFormat.mFormatFlags      = kLinearPCMFormatFlagIsSignedInteger;
    outputFormat.mBytesPerPacket   = 2;
    outputFormat.mFramesPerPacket  = 1;
    outputFormat.mBytesPerFrame    = 2;
    outputFormat.mChannelsPerFrame = 1;
    outputFormat.mBitsPerChannel   = 16;
    outputFormat.mReserved         = 0;

    AudioClassDescription *description = [self
                                      getAudioClassDescriptionWithType:kAudioFormatMPEG4AAC
                                      fromManufacturer:kAppleSoftwareAudioCodecManufacturer];

    OSStatus status =  AudioConverterNewSpecific(&inputFormat, &outputFormat, 1, description, &_audioConverter);

    if (status != 0) {
        printf("setup converter error, status: %i\n", (int)status);
    }
}

After that I wrote the callback function:

struct MyUserData {
    UInt32 mChannels;
    UInt32 mDataSize;
    const void* mData;
    AudioStreamPacketDescription mPacket;
};

OSStatus inInputDataProc(AudioConverterRef inAudioConverter,
                         UInt32 *ioNumberDataPackets,
                         AudioBufferList *ioData,
                         AudioStreamPacketDescription **outDataPacketDescription,
                         void *inUserData)
{
    struct MyUserData* userData = (struct MyUserData*)(inUserData);

    if (!userData->mDataSize) {
        *ioNumberDataPackets = 0;
        return kNoMoreDataError;
    }

    if (outDataPacketDescription) {
        userData->mPacket.mStartOffset = 0;
        userData->mPacket.mVariableFramesInPacket = 0;
        userData->mPacket.mDataByteSize = userData->mDataSize;
        *outDataPacketDescription = &userData->mPacket;
    }

    ioData->mBuffers[0].mNumberChannels = userData->mChannels;
    ioData->mBuffers[0].mDataByteSize = userData->mDataSize;
    ioData->mBuffers[0].mData = (void *)userData->mData;

    // No more data to provide following this run.
    userData->mDataSize = 0;

    return noErr;
}

And my function for decoding frames looks like this:

- (void)startDecodingAudio {
    if (!_converterConfigured){
        return;
    }

    while (true){
        if ([self hasFramesToDecode]){
            struct MyUserData userData = {1, (UInt32)_decoderBuffer[_currPosInDecoderBuf].length, _decoderBuffer[_currPosInDecoderBuf].bytes};

            uint8_t *buffer = (uint8_t *)malloc(128 * sizeof(short int));
            AudioBufferList decBuffer;
            decBuffer.mNumberBuffers = 1;
            decBuffer.mBuffers[0].mNumberChannels = 1;
            decBuffer.mBuffers[0].mDataByteSize = 128 * sizeof(short int);
            decBuffer.mBuffers[0].mData = buffer;

            UInt32 numFrames = 128;

            AudioStreamPacketDescription outPacketDescription;
            memset(&outPacketDescription, 0, sizeof(AudioStreamPacketDescription));
            outPacketDescription.mDataByteSize = 128;
            outPacketDescription.mStartOffset = 0;
            outPacketDescription.mVariableFramesInPacket = 0;

            OSStatus status = AudioConverterFillComplexBuffer(_audioConverter,
                                                              inInputDataProc,
                                                              &userData,
                                                              &numFrames,
                                                              &decBuffer,
                                                              &outPacketDescription);

            NSError *error = nil;

            if (status == kNoMoreDataError) {
                NSLog(@"%u bytes decoded", (unsigned int)decBuffer.mBuffers[0].mDataByteSize);
                [_decodedData appendData:[NSData dataWithBytes:decBuffer.mBuffers[0].mData length:decBuffer.mBuffers[0].mDataByteSize]];
                _currPosInDecoderBuf += 1;
            } else {
                error = [NSError errorWithDomain:NSOSStatusErrorDomain code:status userInfo:nil];
            }
        } else {
            break;
        }
    }
}

Each time, AudioConverterFillComplexBuffer returns status 1852797029 which is, according to Apple API, kAudioCodecIllegalOperationError. If somebody succeded in converting with such formats, please, share some examples, or advice.

Answer 1

Finally, I decoded my bytes with StreamingKit library (original reposiory can be found here).