Mixed Excitation Linear Prediction

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Mixed Excitation Linear Prediction is a United States Department of Defense speech coding standard used mainly in military applications and satellite communications, secure voice, and secure radio devices. Its development was led and supported by NSA, and NATO. The 2400 bit/s MELP was created by Texas Instruments, and first 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 Multiband Excitation (AMBE), (c) Enhanced Multiband Excitation (EMBE), (d) Sinusoid Transform Coder (STC), and (e) Subband LPC Coder (SBC). Due to its lower complexity^{[citation needed]} than Waveform Interpolative (WI) coder, the MELP vocoder won the DoD competition and was selected for 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), Compandent, and AT&T, 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. 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. The significant breakthrough of the 1200 bit/s MELPe enables 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.

In 2002, the US DoD MELPe was adopted also as NATO standard, known as STANAG-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, as a part or 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 (France).

[edit] Intellectual Property Rights

Note that MELPe (and/or its derivatives) is subject to IPR licensing from the following companies, Texas Instruments (2400 bit/s rate), Microsoft (1200 bit/s transcoder), Compandent Inc. (2400,1200,600 bit/s source code, see copyright registration number: TX0006833875 and many more), Thales (600 bit/s rate), and AT&T (Noise Pre-Processor NPP). In order to simplify the MELPe licensing, Compandent tried to form a one stop shopping for the MELPe IPR, but conflict of interest between other IPR holders did not enable that.

Although for US government and for NATO applications the IP licensing royalties are waived (the MELPe development was led & supported by NSA & NATO), for all other applications IPR licensing of MELPe (and/or its derivatives) is needed from all IPR holders. TI usually waives its licensing fee whenever TI's DSP is used. Contacts for IPR licensing may be found via web search, or by contacting Compandent Inc..