双声道一次采样
双声道一次采样的结果叫一个audio frame。
The size of a packet is expressed in audio frames.
An audio frame in a PCM stream is a set of samples (the set contains one sample for each channel in the stream) that play or are recorded at the same time (clock tick).
Thus, the size of an audio frame is the sample size multiplied by the number of channels in the stream.
For example, the frame size for a stereo (2-channel) stream with 16-bit samples is four bytes.
对于16bit位深的双声道，一个audio frame为4Bytes。
References:
大学计算机基础 著者： 高敬阳主编. 高敬阳. 出版商：清华大学出版社, Bei jing : Qing hua ta xue chu ban she, 2005. Page 205
https://msdn.microsoft.com/zh-cn/library/windows/hardware/dd316756
