直接上干货：
1. MELP and MELPe Vocoder (codec)：https://www.compandent.com/about-melpe/
支持 2400 bit/s ，1200 bit/s，600 bit/s，300 bit/s，需要向compandent购买
2. HVXC@MPEG4 Audio：HVXC(Harmonic Vector Excitation Coding)
https://www.cnblogs.com/gaozehua/archive/2013/05/19/3087427.html
固定比特率有：2kbps和4kbps。可变比特率有：1.2~1.7kbps，需要向MPEG购买
3.AMBE https://www.dvsinc.com/software/technology.shtml#ambelr
AMBE-LR: 1.6 - 1.8 kbps,需要向dvsinc购买
4. Codec 2 http://www.rowetel.com/?page_id=452
开源免费 Codec 2 is an open source speech codec designed for communications quality speech between 700 and 3200 bit/s
https://github.com/drowe67/codec2
5. SELP 清华大学崔慧娟老师
0.3k~4.8k bps
https://xueshu.baidu.com/usercenter/paper/show?paperid=2e90f79c330c0c95197462c18a179ac8
6.HELP 北京工业大学 鲍长春老师
内容不详