From: https://communications.support/threads/2494-DVSI-Boards-for-P25-regeneration Based on this list here: http://wiki.multimedia.cx/index.php?title=IMBE US#6199037 Joint quantization of speech subframe voicing metrics and fundamental frequencies Filing date: Dec 4, 1997 Issue date: Mar 6, 2001 Expiry: (20 years after filing) Dec 4, 2017 US#5870405 Digital transmission of acoustic signals over a noisy communication channel Filing date: Mar 4, 1996 Issue date: Feb 9, 1999 Expiry: (20 years after filing) Mar 4, 2016 US#5754974 Spectral magnitude representation for multi-band excitation speech coders Filing date: Feb 22, 1995 Issue date: May 19, 1998 Expiry: (the longer of 17 years from issue date or 20 years from filing date) May 19, 2015 US#5664051 Method and apparatus for phase synthesis for speech processing Filing date: Jun 23, 1994 Issue date: Sep 2, 1997 Expiry: (the longer of 17 years from issue date or 20 years from filing date) Sept 2, 2014 US#5630011 Quantization of harmonic amplitudes representing speech Filing date: Dec 16, 1994 Issue date: May 13, 1997 Expiry: (the longer of 17 years from issue date or 20 years from filing date) Dec 16, 2014 US#5517511 Digital transmission of acoustic signals over a noisy communication channel (mentioned in 1997 ambe1000 data sheet) Filing date: Nov 30, 1992 Issue date: May 14, 1996 Expiry: (the longer of 17 years from issue date or 20 years from filing date) May 14, 2013 US#5491772 Methods for speech transmission (mentioned in 1997 ambe1000 data sheet) Filing date: May 3, 1995 Issue date: Feb 13, 1996 Expiry: (the longer of 17 years from issue date or 20 years from filing date) May 3, 2015 US#5247579 Methods for speech transmission (mentioned in 1997 ambe1000 data sheet) Filing date: Dec 3, 1991 Issue date: Sep 21, 1993 Expiry: (the longer of 17 years from issue date or 20 years from filing date) Dec 3, 2011 US#5226108 Processing a speech signal with estimated pitch Filing date: Sep 20, 1990 Issue date: Jul 6, 1993 Expiry: (the longer of 17 years from issue date or 20 years from filing date) Sept 20, 2010 US#5226084 Methods for speech quantization and error correction Filing date: Dec 5, 1990 Issue date: Jul 6, 1993 Expiry: (the longer of 17 years from issue date or 20 years from filing date) Dec 5, 2010 US#5216747 Voiced/unvoiced estimation of an acoustic signal Filing date: Nov 21, 1991 Issue date: Jun 1, 1993 Expiry: (the longer of 17 years from issue date or 20 years from filing date) Nov 21, 2011 US#5081681 Method and apparatus for phase synthesis for speech processing Filing date: Nov 30, 1989 Issue date: Jan 14, 1992 Expiry: (the longer of 17 years from issue date or 20 years from filing date) Nov 30, 2009 US#6912496 Preprocessing modules for quality enhancement of MBE coders and decoders for signals having transmission path characteristics Filing date: Oct 26, 2000 Issue date: Jun 28, 2005 Expiry: (the longer of 17 years from issue date or 20 years from filing date) June 28, 2022 US#6199037 Joint quantization Of speech subframe voicing metrics and fundamental frequencies Filing date: Dec 4, 1997 Issue date: Mar 6, 2001 Expiry: (the longer of 17 years from issue date or 20 years from filing date) Mar 6, 2018 US#8359197 Half-rate Vocoder (For P25 - Phase 2) Filing date: Apr 1, 2003 Issue Date: Jan 22, 2013 Expiry: (the longer of 17 years from issue date or 20 years from filing date) Jan 22, 2030 I tend to think that since in 1993 DVSI was already marketing IMBE and it entered service with at least Motorola's iDEN system in 1994, that most of the later patents may relate to AMBE but not to IMBE.