Audio bit depth
In digital audio using pulse-code modulation (PCM), bit depth is the number of bits of information in each sample, and it directly corresponds to the resolution of each sample.
Examples of bit depth include Compact Disc Digital Audio, which uses 16 bits per sample, and DVD-Audio and Blu-ray Disc which can support up to 24 bits per sample.
Audio bit depth一般为16位，即采样时用16位来表示一个音频采样点.
Audio bit depth越大说明，量化的颗粒度越细.
Bit rate and file size
Bit depth affects** bit rate and file size**. Bit rate refers to the amount of data, specifically bits, transmitted or received per second.
立体声16位44.1KHz声音的比特率：
44100*16*2=1411200bps
Example
当Audio bit depth为4时，可表示数字范围为2的4次方,即16个数字.
从图中可以看到从-8到7为16个数字，当对音频进行采样时，即从这16个数字中按照一定的策略进行量化.
Perferences:
https://en.wikipedia.org/wiki/Audio_bit_depth
http://wenda.so.com/q/1375015015065360
