= Speech Codec =

The speech codec is a proprietary AMBE variant from [http://www.dvsinc.com/ DVSI]. The official spec only contain a very rough overview and no technical information about the format and how to encode or decode it.

The ETSI lists patents relevant to GMR. (See [http://ip5226084r.etsi.org/IPRDetails.aspx?IPRD_ID=151&IPRD_TYPE_ID=2&MODE=2 here]).

• [http://www.google.com/patents?id=X-EfAAAAEBAJ&zoom=4&pg=PA1#v=onepage&q&f=false US5226084] : Methods for speech quantization and error correction
• [http://www.google.com/patents?id=hv0iAAAAEBAJ&zoom=4&pg=PA1#v=onepage&q&f=false US5715365] : Estimation of excitation parameters
• [http://www.google.com/patents?id=NW8YAAAAEBAJ&zoom=4&pg=PA1#v=onepage&q&f=false US5826222] : Estimation of excitation parameters
• [http://www.google.com/patents?id=eqkjAAAAEBAJ&zoom=4&pg=PA5#v=onepage&q&f=false US5754974] : Spectral magnitude representation for multi-band excitation speech coders
• [http://www.google.com/patents?id=4kMkAAAAEBAJ&zoom=4&pg=PA1#v=onepage&q&f=false US5701390] : Synthesis of MBE-based coded speech using regenerated phase information

This list is not really complete or accurate though. In particular it's missing the patent that has the most in common with the actual codec is (US&EU versions) :

• [http://www.google.com/patents/US6199037B1 US6199037]: Joint quantization of speech subframe voicing metrics and fundamental frequencies
• [http://www.google.com/patents/EP0927988B1 EP0927988B1]: Encoding speech

Osmocom-GMR contains a basic implementation of a voice decoder. Its quality is not as good as the official decoder but it does produce perfectly intelligible voice. You can find the implementation in the src/codec directory of the git.