说明：  使用mediastreamer2的g711common.h. unsigned char s16_to_alaw(int pcm_val). 将16bit的原始样本encode 成为8bit的a-law码.
(use mediastreamer2 the g711common.h. Unsign ed char s16_to_alaw (int pcm_val). to the original 16bit samples The encode into a 8 bit-law code.)