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SpeechCodec » History » Version 4

sylvain, 02/19/2016 10:50 PM

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h1. Speech Codec
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The speech codec is a proprietary AMBE variant from "DVSI":http://www.dvsinc.com/. The official spec only contain a very rough overview and no technical information about the format and how to encode or decode it.
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h2. Patents
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The ETSI lists patents relevant to GMR. (See "here":http://ip5226084r.etsi.org/IPRDetails.aspx?IPRD_ID=151&IPRD_TYPE_ID=2&MODE=2).
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* "US5226084":http://www.google.com/patents?id=X-EfAAAAEBAJ&zoom=4&pg=PA1#v=onepage&q&f=false : Methods for speech quantization and error correction
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* "US5715365":http://www.google.com/patents?id=hv0iAAAAEBAJ&zoom=4&pg=PA1#v=onepage&q&f=false : Estimation of excitation parameters
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* "US5826222":http://www.google.com/patents?id=NW8YAAAAEBAJ&zoom=4&pg=PA1#v=onepage&q&f=false : Estimation of excitation parameters
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* "US5754974":http://www.google.com/patents?id=eqkjAAAAEBAJ&zoom=4&pg=PA5#v=onepage&q&f=false : Spectral magnitude representation for multi-band excitation speech coders
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* "US5701390":http://www.google.com/patents?id=4kMkAAAAEBAJ&zoom=4&pg=PA1#v=onepage&q&f=false : Synthesis of MBE-based coded speech using regenerated phase information
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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) :
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* "US6199037":http://www.google.com/patents/US6199037B1: Joint quantization of speech subframe voicing metrics and fundamental frequencies
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* "EP0927988B1":http://www.google.com/patents/EP0927988B1: Encoding speech
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h2. Implementation
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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.
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