Unable to stream audio from bluetooth device

Question

The following code is what I use to stream audio data.

func prepareStreamRecording() throws -> OSStatus {
    try AVAudioSession.sharedInstance().setPreferredIOBufferDuration(10)

    // Describe the RemoteIO unit
    var audioComponentDescription = AudioComponentDescription()
    audioComponentDescription.componentType = kAudioUnitType_Output;
    audioComponentDescription.componentSubType = kAudioUnitSubType_RemoteIO;
    audioComponentDescription.componentManufacturer = kAudioUnitManufacturer_Apple;
    audioComponentDescription.componentFlags = 0;
    audioComponentDescription.componentFlagsMask = 0;

    // Get the RemoteIO unit
    let remoteIOComponent = AudioComponentFindNext(nil, &audioComponentDescription)
    var status = AudioComponentInstanceNew(remoteIOComponent!, &remoteIOUnit)

    if (status != noErr) {
        return status
    }

    let bus1 : AudioUnitElement = 1
    var oneFlag : UInt32 = 1

    // Configure the RemoteIO unit for input
    status = AudioUnitSetProperty(remoteIOUnit!,
                                  kAudioOutputUnitProperty_EnableIO,
                                  kAudioUnitScope_Input,
                                  bus1,
                                  &oneFlag,
                                  UInt32(MemoryLayout<UInt32>.size));
    if (status != noErr) {
        return status
    }

    // Set format for mic input (bus 1) on RemoteIO's output scope
    var asbd = AudioStreamBasicDescription()
    asbd.mSampleRate = Double(16000)
    asbd.mFormatID = kAudioFormatLinearPCM
    asbd.mFormatFlags = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked
    asbd.mBytesPerPacket = 2
    asbd.mFramesPerPacket = 1
    asbd.mBytesPerFrame = 2
    asbd.mChannelsPerFrame = 1
    asbd.mBitsPerChannel = 16
    status = AudioUnitSetProperty(remoteIOUnit!,
                                  kAudioUnitProperty_StreamFormat,
                                  kAudioUnitScope_Output,
                                  bus1,
                                  &asbd,
                                  UInt32(MemoryLayout<AudioStreamBasicDescription>.size))

    if (status != noErr) {
        return status
    }

    // Set the recording callback
    var callbackStruct = AURenderCallbackStruct()
    callbackStruct.inputProc = recordingCallback
    callbackStruct.inputProcRefCon = nil
    status = AudioUnitSetProperty(remoteIOUnit!,
                                  kAudioOutputUnitProperty_SetInputCallback,
                                  kAudioUnitScope_Global,
                                  bus1,
                                  &callbackStruct,
                                  UInt32(MemoryLayout<AURenderCallbackStruct>.size));
    if (status != noErr) {
        return status
    }
    // Initialize the RemoteIO unit
    return AudioUnitInitialize(remoteIOUnit!)
}

func startStreamRecording(handler: ((_ data: Data) -> Void)?) -> OSStatus {
    streamHandler = handler;
    if(remoteIOUnit == nil) {
        return -1
    }
    return AudioOutputUnitStart(remoteIOUnit!)
}

The audio data will be received through this callback

func recordingCallback(inRefCon:UnsafeMutableRawPointer, ioActionFlags:UnsafeMutablePointer<AudioUnitRenderActionFlags>, inTimeStamp:UnsafePointer<AudioTimeStamp>, inBusNumber:UInt32, inNumberFrames:UInt32, ioData:UnsafeMutablePointer<AudioBufferList>?) -> OSStatus {
    var status = noErr
    let channelCount : UInt32 = 1
    var bufferList = AudioBufferList()
    bufferList.mNumberBuffers = channelCount
    let buffers = UnsafeMutableBufferPointer<AudioBuffer>(start: &bufferList.mBuffers,
                                                          count: Int(bufferList.mNumberBuffers))
    buffers[0].mNumberChannels = 1
    buffers[0].mDataByteSize = inNumberFrames * 2
    buffers[0].mData = nil

    // get the recorded samples
    status = AudioUnitRender(AudioManager.shared.remoteIOUnit!,
                             ioActionFlags,
                             inTimeStamp,
                             inBusNumber,
                             inNumberFrames,
                             UnsafeMutablePointer<AudioBufferList>(&bufferList))
    if (status != noErr) {
        return status;
    }

    let data = Data(bytes:  buffers[0].mData!, count: Int(buffers[0].mDataByteSize))

    NSLog("recorded data length is \(data.count)")
    NSLog("Recorded data part is \(data.subdata(in: 0..<50).hexadecimal())")

    AudioManager.shared.streamHandler?(data)
    return noErr
}

The code works when recording with the phone mic. However, when connected to a bluetooth mic, the recorded data content is always 00000000000000000000000000....

Please be noted that I did not write the piece of code. I got it from a sample app made by Google about using their Cloud Speech API.

Answer 1

I have fixed this by changing the value of preferred IO buffer duration of Audio Session. Currently I am setting it to 0.01.

try AVAudioSession.sharedInstance().setPreferredIOBufferDuration(0.01)