Mixed-excitation linear prediction (MELP) is a
United States Department of Defense
The United States Department of Defense (DoD, USDOD or DOD) is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government directly related to national secu ...
speech coding
Speech coding is an 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 da ...
standard used mainly in
military
A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare. It is typically authorized and maintained by a sovereign state, with its members identifiable by their distinct ...
applications and
satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...
communication
Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...
s,
secure voice
Secure voice (alternatively secure speech or ciphony) is a term in cryptography for the encryption of voice communication over a range of communication types such as radio, telephone or IP.
History
The implementation of voice encryption date ...
, and secure radio devices. Its standardization and later development was led and supported by the NSA and NATO. The current "enhanced" version is known as MELPe.
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, and the original MELP related patents have expired by now. 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 Excitation (AMBE), (c)
Enhanced Multiband Excitation (EMBE), (d)
Sinusoid Transform Coder (STC), and (e)
Subband LPC Coder
In signal processing, sub-band coding (SBC) is any form of transform coding that breaks a signal into a number of different frequency bands, typically by using a fast Fourier transform, and encodes each one independently. This decomposition i ...
(SBC). Due to its lower complexity than Waveform Interpolative (WI) coder, the MELP vocoder won the DoD competition and was selected for
MIL-STD
A United States defense standard, often called a military standard, "MIL-STD", "MIL-SPEC", or (informally) "MilSpecs", is used to help achieve standardization objectives by the U.S. Department of Defense.
Standardization is beneficial in achievin ...
-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
Microsoft
Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
),
Compandent, and
AT&T Corporation
AT&T Corporation, originally the American Telephone and Telegraph Company, is the subsidiary of AT&T Inc. that provides voice, video, data, and Internet telecommunications and professional services to businesses, consumers, and government agen ...
, 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, following extensive competition and testing, the 2400 and 1200 bit/s US DoD MELPe was adopted also as
NATO
The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two No ...
standard, known as
STANAG
In NATO, a standardization agreement (STANAG, redundantly: STANAG agreement) defines processes, procedures, terms, and conditions for common military or technical procedures or equipment between the member countries of the alliance. Each NATO st ...
-4591. The NATO testing performance measurements included voice intelligibility, voice quality, speaker recognition, language dependency, speaker dependency, 10 acoustic noise environments, transmission channel under 1% BER, tandem using 16 kbps CVSD vocoder, whispered speech, and real-time implementation. The testing data included Over 36,000 files, or 500 hours of speech under various conditions and languages. As part of NATO testing for new NATO standard, MELPe was tested against other candidates such as
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
's HSX (Harmonic Stochastic eXcitation) and
Turkey
Turkey ( tr, Türkiye ), officially the Republic of Türkiye ( tr, Türkiye Cumhuriyeti, links=no ), is a list of transcontinental countries, transcontinental country located mainly on the Anatolia, Anatolian Peninsula in Western Asia, with ...
's SB-LPC (Split-Band Linear Predictive Coding), as well as the old secure voice standards such as
FS1015 LPC-10e FIPS 137, originally issued as FED-STD-1015, is a secure telephony speech encoding standard for Linear Predictive Coding vocoder developed by the United States Department of Defense and finished on November 28, 1984.
It was based on the earlier STA ...
(2.4 kbit/s),
FS1016
FS-1016 (also called FED-STD-1016) is a deprecated secure telephony speech encoding standard for Code-excited linear prediction (CELP) developed by the United States Department of Defense and finalized February 14, 1991.
Unlike the vocoder used ...
CELP
Code-excited linear prediction (CELP) is a linear predictive speech coding algorithm originally proposed by Manfred R. Schroeder and Bishnu S. Atal in 1985. At the time, it provided significantly better quality than existing low bit-rate algori ...
(4.8 kbit/s) and
CVSD Continuously variable slope delta modulation (CVSD or CVSDM) is a voice coding method. It is a delta modulation with variable step size (i.e., special case of adaptive delta modulation), first proposed by Greefkes and Riemens in 1970.
CVSD encode ...
(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
Code-excited linear prediction (CELP) is a linear predictive speech coding algorithm originally proposed by Manfred R. Schroeder and Bishnu S. Atal in 1985. At the time, it provided significantly better quality than existing low bit-rate algori ...
and
LPC-10e FIPS 137, originally issued as FED-STD-1015, is a secure telephony speech encoding standard for Linear Predictive Coding vocoder developed by the United States Department of Defense and finished on November 28, 1984.
It was based on the earlier STA ...
). The
NATO
The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two No ...
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 worldwide.
Compandent Inc, as a part of MELPe-based projects performed for
NSA
The National Security Agency (NSA) is a national-level intelligence agency of the United States Department of Defense, under the authority of the Director of National Intelligence (DNI). The NSA is responsible for global monitoring, collecti ...
and
NATO
The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two No ...
, 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) made by
Compandent, 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 variation by
Thales Group
Thales Group () is a French multinational company that designs, develops and manufactures electrical systems as well as devices and equipment for the aerospace, defence, transportation and security sectors. The company is headquartered in Paris' ...
(
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
) was added (without extensive competition and testing as performed for the 2400/1200 bit/s MELPe) to the NATO standard STANAG-4591.
300 bit/s MELP
In 2010, MIT Lincoln Labs,
Compandent, BBN, and General Dynamics also developed for DARPA a 300 bit/s MELP device .
[Alan McCree, “A scalable phonetic vocoder framework using joint predictive vector quantization of MELP parameters,” in Proc. IEEE Int. Conf. Acoust., Speech, Signal Processing, 2006, pp. I 705–708, Toulouse, France] Its quality was better than the 600 bit/s MELPe, but its algorithmic delay was longer.
Implementations
The MELPe has been implemented in many applications including secure radio devices, satellite communications, VoIP, and cellphone applications. In such applications, additional expertise is required for combating channel errors, packet loss, and synchronization loss. Such expertise requires the understanding of the MELPe's bits sensitivity to errors. The 2400 bit/s and 1200 bit/s MELPe include synchronization bit, which is useful in serial communications.
Compression level
MELPe is intended for the compression of speech. Given an audio input sampled at 8 kHz, the MELPe codec yields the following compression ratios over a 64 kbit/s μ-Law G.711 datastream, discounting the effects of protocol overhead:
{, class="wikitable"
, -
! 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
Generally, speech coding involves a trade-off of different aspects including bit-rate, speech quality, delay (frame size and lookahead), computational complexity, robustness to different speakers and languages, robustness to different background noises, channel error robustness, and also codec state recovery in the face of packet loss. Since the MELPe's lower rates (600 and 1200 bit/s) are supersets of the 2400 bit/s rate, the algorithm complexity (e.g. in MIPS) is about the same for all rates. The lower rates use increased frames and lookahead, as well as codebook size, therefore they require more memory.
Intellectual property rights
MELPe (and/or its derivatives) is subject to IPR licensing from the following companies,
Texas Instruments
Texas Instruments Incorporated (TI) is an American technology company headquartered in Dallas, Texas, that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globall ...
(2400 bit/s MELP algorithm / source code),
Microsoft
Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
(1200 bit/s transcoder),
Thales Group
Thales Group () is a French multinational company that designs, develops and manufactures electrical systems as well as devices and equipment for the aerospace, defence, transportation and security sectors. The company is headquartered in Paris' ...
(600 bit/s rate),
Compandent, and
AT&T
AT&T Inc. is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's largest telecommunications company by revenue and the third largest provider of mobile tel ...
(Noise Pre-Processor NPP).
See also
*
CVSD Continuously variable slope delta modulation (CVSD or CVSDM) is a voice coding method. It is a delta modulation with variable step size (i.e., special case of adaptive delta modulation), first proposed by Greefkes and Riemens in 1970.
CVSD encode ...
*
LPC-10 FIPS 137, originally issued as FED-STD-1015, is a secure telephony speech encoding standard for Linear Predictive Coding vocoder developed by the United States Department of Defense and finished on November 28, 1984.
It was based on the earlier STA ...
e
*
FS-1015 FIPS 137, originally issued as FED-STD-1015, is a secure telephony speech encoding standard for Linear Predictive Coding vocoder developed by the United States Department of Defense and finished on November 28, 1984.
It was based on the earlier STA ...
*
FS-1016
FS-1016 (also called FED-STD-1016) is a deprecated secure telephony speech encoding standard for Code-excited linear prediction (CELP) developed by the United States Department of Defense and finalized February 14, 1991.
Unlike the vocoder used ...
*
Secure Voice
Secure voice (alternatively secure speech or ciphony) is a term in cryptography for the encryption of voice communication over a range of communication types such as radio, telephone or IP.
History
The implementation of voice encryption date ...
*
Vocoder
A vocoder (, a portmanteau of ''voice'' and ''encoder'') is a category of speech coding that analyzes and synthesizes the human voice signal for audio data compression, multiplexing, voice encryption or voice transformation.
The vocoder was ...
References
Speech codecs