基于audio unit的回音消除
获取麦克风输入：
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];
播放pcm音频数据：
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];