Mixed Excitation Linear Prediction
Encyclopedia
Mixed-excitation linear prediction (MELP) is a United States Department of Defense
speech coding
standard used mainly in military
applications and satellite
communication
s, secure voice
, and secure radio devices. Its standardization and later development was led and supported by NSA, and NATO.
(AMBE), (c) Enhanced Multiband Excitation (EMBE), (d) Sinusoid Transform Coder (STC), and (e) Subband LPC Coder (SBC). Due to its lower complexity than Waveform Interpolative (WI) coder, the MELP vocoder won the DoD competition and was selected for MIL-STD-3005.
), AT&T
and Compandent, which included (a) additional new vocoder at half the rate (i.e. 1200 bit/s), (b) substantially improved encoding (analysis), (c) substantially improved decoding (synthesis), (d) Noise-Preprocessing for removing background noise, (e) transcoding between the 2400 bit/s and 1200 bit/s bitstreams, and (f) new postfilter. This fairly significant development was aimed to create a new coder at half the rate and have it interoperable with the old MELP standard. This enhanced-MELP (also known as MELPe) was adopted as the new MIL-STD-3005 in 2001 in form of annexes and supplements made to the original MIL-STD-3005, enabling the same quality as the old 2400 bit/s MELP's at half the rate. One of the greatest advantages of the new 2400 bit/s MELPe is that it shares the same bit format as MELP, and hence can interoperate with legacy MELP systems, but would deliver better quality at both ends. MELPe provides much better quality than all older military standards, especially in noisy environments such as battlefield and vehicles and aircraft.
-4591. As part of NATO testing for new NATO standard, MELPe was tested against other candidates such as France
's HSX (Harmonic Stochastic eXcitation) and Turkey
's SB-LPC (Split-Band Linear Predictive Coding), as well as the old secure voice standards such as FS1015 LPC-10e (2.4 kbit/s), FS1016 CELP (4.8 kbit/s) and CVSD (16 kbit/s). Subsequently, the MELPe won also the NATO competition, surpassing the quality of all other candidates as well as the quality of all old secure voice standards (CVSD, CELP and LPC-10e). The NATO competition concluded that MELPe substantially improved performance (in terms of speech quality, intelligibility, and noise immunity), while reducing throughput requirements. The NATO testing also included interoperability tests, used over 200 hours of speech data, and was conducted by 3 test laboratories world wide. Compandent Inc, as a part of MELPe-based projects performed for NSA and NATO, provided NSA and NATO with special test-bed platform known as MELCODER device that provided the golden reference for real-time implementation of MELPe. The low-cost FLEXI-232 Data Terminal Equipment (DTE), which are based on the MELCODER golden reference, are very popular and widely used for evaluating and testing MELPe in real-time, various channels & networks, and field conditions.
In 2005, a new 600 bit/s rate MELPe vocoder was added to the NATO standard STANAG-4591 by Thales Group
(France
).
As with any low bitrate codec, reaching high levels of compression will involve a trade-off against distortion, and also codec state recovery in the face of packet loss.
(2400 bit/s MELP algorithm / source code), Microsoft (1200 bit/s transcoder), Compandent Inc. (part of 2400, 1200, 600 bit/s source code), Thales (600 bit/s rate), and AT&T (Noise Pre-Processor NPP).
For US government and NATO applications the IP licensing royalties are waived. For all other applications, licensing is required from all IPR holders.
United States Department of Defense
The United States Department of Defense is the U.S...
speech coding
Speech coding
Speech coding is the application of data compression of digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic data compression algorithms to represent the resulting...
standard used mainly in military
Military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...
applications and satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
communication
Communication
Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
s, secure voice
Secure voice
Secure voice is a term in cryptography for the encryption of voice communication over a range of communication types such as radio, telephone or IP.-History:...
, and secure radio devices. Its standardization and later development was led and supported by NSA, and NATO.
History
The initial MELP was invented by Alan McCree around 1995 while a graduate student at the Center for Signal and Image Processing (CSIP) at Georgia Tech. The technology was subsequently licensed to Atlanta Signal Processos Inc. (ASPI) (later acquired by Polycomm), for commercial development. Subsequently, ASPI teamed with Texas Instruments to create the ASPI/GT/TI 2400 bit/s MELP. That initial speech coder was standardized in 1997 and was known as MIL-STD-3005. It surpassed other candidate vocoders in the US DoD competition, including: (a) Frequency Selective Harmonic Coder (FSHC), (b) Advanced Multi-Band ExcitationAdvanced Multi-Band Excitation
Advanced Multi-Band Excitation is a proprietary speech coding standard developed by Digital Voice Systems, Inc.- Overview :AMBE is a codebook-based vocoder that operates at bitrates of between 2 and 9.6 kbit/s, and at a sampling rate of 8 kHz in 20-ms frames...
(AMBE), (c) Enhanced Multiband Excitation (EMBE), (d) Sinusoid Transform Coder (STC), and (e) Subband LPC Coder (SBC). Due to its lower complexity than Waveform Interpolative (WI) coder, the MELP vocoder won the DoD competition and was selected for MIL-STD-3005.
MIL-STD-3005
Between 1998 and 2001, a new MELP-based vocoder was created at half the rate (i.e. 1200 bit/s) and substantial enhancements were added to the MIL-STD-3005 by SignalCom (later acquired by MicrosoftMicrosoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
), AT&T
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
and Compandent, which included (a) additional new vocoder at half the rate (i.e. 1200 bit/s), (b) substantially improved encoding (analysis), (c) substantially improved decoding (synthesis), (d) Noise-Preprocessing for removing background noise, (e) transcoding between the 2400 bit/s and 1200 bit/s bitstreams, and (f) new postfilter. This fairly significant development was aimed to create a new coder at half the rate and have it interoperable with the old MELP standard. This enhanced-MELP (also known as MELPe) was adopted as the new MIL-STD-3005 in 2001 in form of annexes and supplements made to the original MIL-STD-3005, enabling the same quality as the old 2400 bit/s MELP's at half the rate. One of the greatest advantages of the new 2400 bit/s MELPe is that it shares the same bit format as MELP, and hence can interoperate with legacy MELP systems, but would deliver better quality at both ends. MELPe provides much better quality than all older military standards, especially in noisy environments such as battlefield and vehicles and aircraft.
STANAG-4591 (NATO)
In 2002, the US DoD MELPe was adopted also as NATO standard, known as STANAGSTANAG
STANAG is the NATO abbreviation for Standardization Agreement, which sets up processes, procedures, terms, and conditions for common military or technical procedures or equipment between the member countries of the alliance. Each NATO state ratifies a STANAG and implements it within their own...
-4591. As part of NATO testing for new NATO standard, MELPe was tested against other candidates such as France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
's HSX (Harmonic Stochastic eXcitation) and Turkey
Turkey
Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...
's SB-LPC (Split-Band Linear Predictive Coding), as well as the old secure voice standards such as FS1015 LPC-10e (2.4 kbit/s), FS1016 CELP (4.8 kbit/s) and CVSD (16 kbit/s). Subsequently, the MELPe won also the NATO competition, surpassing the quality of all other candidates as well as the quality of all old secure voice standards (CVSD, CELP and LPC-10e). The NATO competition concluded that MELPe substantially improved performance (in terms of speech quality, intelligibility, and noise immunity), while reducing throughput requirements. The NATO testing also included interoperability tests, used over 200 hours of speech data, and was conducted by 3 test laboratories world wide. Compandent Inc, as a part of MELPe-based projects performed for NSA and NATO, provided NSA and NATO with special test-bed platform known as MELCODER device that provided the golden reference for real-time implementation of MELPe. The low-cost FLEXI-232 Data Terminal Equipment (DTE), which are based on the MELCODER golden reference, are very popular and widely used for evaluating and testing MELPe in real-time, various channels & networks, and field conditions.
In 2005, a new 600 bit/s rate MELPe vocoder was added to the NATO standard STANAG-4591 by Thales Group
Thales Group
The Thales Group is a French electronics company delivering information systems and services for the aerospace, defense, transportation and security markets...
(France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
).
Compression Level
MELPe is intended for the compression of speech. Given an 8 kHz audio input, the MELPe codec yields the following compression ratios over a 64 kbit/s μ-Law G.711 datastream, discounting the effects of protocol overhead:| Bitrate | Compression ratio over G.711 | Payload size | Payload interval |
|---|---|---|---|
| 2400 bit/s | 26.7 X | 54 bits | 22.5 ms |
| 1200 bit/s | 53.3 X | 81 bits | 67.5 ms |
| 600 bit/s | 106.7 X | 54 bits | 90 ms |
As with any low bitrate codec, reaching high levels of compression will involve a trade-off against distortion, and also codec state recovery in the face of packet loss.
Intellectual Property Rights
Note that MELPe (and/or its derivatives) is subject to IPR licensing from the following companies, Texas InstrumentsTexas Instruments
Texas Instruments Inc. , widely known as TI, is an American company based in Dallas, Texas, United States, which develops and commercializes semiconductor and computer technology...
(2400 bit/s MELP algorithm / source code), Microsoft (1200 bit/s transcoder), Compandent Inc. (part of 2400, 1200, 600 bit/s source code), Thales (600 bit/s rate), and AT&T (Noise Pre-Processor NPP).
For US government and NATO applications the IP licensing royalties are waived. For all other applications, licensing is required from all IPR holders.