基于audio unit的回音消除

    XBEchoCancellation *echo = [XBEchoCancellation shared];
    echo.bl_input = ^(AudioBufferList *bufferList) {
    	AudioBuffer buffer = bufferList->mBuffers[0];
        // buffer即从麦克风获取到的数据，默认已经消除了回音
    };
    [echo startInput];

    XBEchoCancellation *echo = [XBEchoCancellation shared];
    echo.bl_output = ^(AudioBufferList *bufferList, UInt32 inNumberFrames) {
    	AudioBuffer buffer = bufferList->mBuffers[0];
        // 这里把要传给发声设备的pcm数据赋给buffer
    };
    [echo startOutput];