Mixed-excitation linear prediction (MELP) is a

United States Department of Defense The United States Department of Defense (DoD, USDOD, or DOD) is an United States federal executive departments, executive department of the federal government of the United States, U.S. federal government charged with coordinating and superv ...

speech coding Speech coding is an application of data compression to 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. Militaries are typically authorized and maintained by a sovereign state, with their members identifiable by a d ...

applications and

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

communication Communication is commonly defined as the transmission of information. Its precise definition is disputed and there are disagreements about whether Intention, unintentional or failed transmissions are included and whether communication not onl ...

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 Voice over IP, IP. History The implementation of voice en ...

, 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 Multi-Band Excitation (MBE) is a series of proprietary speech coding standards developed by Digital Voice Systems, Inc. (DVSI). Overview In 1967 Osamu Fujimura (MIT) showed basic advantages of the multi-band representation of speech ("An App ...

(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 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 United States Department of Defense. Standardization is beneficial i ...

-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 corporation and technology company, technology conglomerate headquartered in Redmond, Washington. Founded in 1975, the company became influential in the History of personal computers#The ear ...

), Compandent, and

AT&T Corporation AT&T Corporation, an abbreviation for its former name, the American Telephone and Telegraph Company, was an American telecommunications company that provided voice, video, data, and Internet telecommunications and professional services to busi ...

, 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 ; , OTAN), also called the North Atlantic Alliance, is an intergovernmental organization, intergovernmental Transnationalism, transnational military alliance of 32 Member states of NATO, member s ...

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 kbit/s 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 located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...

's HSX (Harmonic Stochastic eXcitation) and

Turkey Turkey, officially the Republic of Türkiye, is a country mainly located in Anatolia in West Asia, with a relatively small part called East Thrace in Southeast Europe. It borders the Black Sea to the north; Georgia (country), Georgia, Armen ...

's SB-LPC (Split-Band Linear Predictive Coding), as well as the old secure voice standards such as

FS1015 FIPS 137, originally issued as FED-STD-1015, is a Secure telephone, secure telephony speech encoding standard for Linear predictive coding, Linear Predictive Coding vocoder developed by the United States Department of Defense and finished on Novemb ...

LPC-10e (2.4 kbit/s), FS1016

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 Speech coding, voice coding method. It is a delta modulation with variable step size (i.e., special case of adaptive DPCM, adaptive delta modulation), first proposed by Greefkes and ...

(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,

and LPC-10e). The

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 an 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, collection, and proces ...

and

, 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 S.A., Trade name, trading as Thales Group (), is a French multinational corporation, multinational aerospace and defence industry, defence corporation specializing in electronics. It designs, develops and manufactures a wide variety of aer ...

(

) 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 multinational semiconductor company headquartered in Dallas, Texas. It is one of the top 10 semiconductor companies worldwide based on sales volume. The company's focus is on developing analog ...

(2400 bit/s MELP algorithm / source code),

(1200 bit/s transcoder),

(600 bit/s rate), Compandent, and

AT&T AT&T Inc., an abbreviation for its predecessor's former name, the American Telephone and Telegraph Company, is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the w ...

(Noise Pre-Processor NPP).

References

Speech codecs