Mixed Excitation Linear Predictive (MELP) Vocoders

This web site teaches the creation and operation of the MELP and MELPe vocoders, summarizes their most updated information, and provides useful resources and solutions related to MELP (MIL_STD-3005), and the later enhanced version known as MELPe (STANAG-4591) vocoders. Introduction to MELP and MELPe Vocoder Mixed-excitation linear prediction (MELP) is a United States Department of Defense (US DoD) speech coding standard used mainly in military applications and satellite communications, secure voice, and secure radio devices. Its standardization and later development was led and supported by NSA, and NATO. MELPe is the later "enhanced MELP" vocoder. History of MELP and MELPe Vocoders Early MELP Vocoder The initial MELP was invented by Alan McCree around 1995 [1], and standardized in 1997 as MIL-STD-3005.[2] It surpassed other candidate vocoders in the US DoD competition, including:[3] (a) Frequency Selective Harmonic Coder (FSHC), (b) Advanced Multi-Band Excitation (AMBE), (c) Enhanced Multiband Excitation (EMBE), (d) Sinusoid Transform Coder (STC), (e) Subband LPC Coder (SBC), and (f) Waveform Interpolative (WI) Coder. MELPe achived better quality than the first five candidates, and thank to its lower complexity than the WI coder, the MELP vocoder won the DoD competition and was selected for MIL-STD-3005. [4] US MIL-STD-3005, from MELP to MELPe vocoder 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), AT&T Corporation, 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. NATO STANAG-4591 MELPe vocoder In 2002, following extensive competition and testing, the 2400 and 1200 bit/s US DoD MELPe was adopted also as NATO standard, known as STANAG-4591.[5] 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'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 worldwide. 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) 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. The STANAG-4591 MELPe vocoder was tested along with other codecs in different acoustic noise environments, under 1% Bit Error Rate (BER), and CVSD-codec tandem. The Mean Opinion Score (MOS) subjective test results are summarized in the table below. Condition \ Coder 2400 1200 CVSD CELP LPC10e Quiet 3.88 3.47 2.93 3.86 2.78 1% BER 3.86 3.32 2.91 3.80 2.60 Office Noise 3.75 3.29 2.86 3.53 2.68 MCE Noise 3.12 2.68 2.50 3.06 2.07 F15 Fighter 3.86 3.52 3.03 3.62 2.63 Bradely 3.85 3.50 3.00 3.60 2.65 Black Hawk 3.60 3.20 2.97 3.40 1.80 Automobile 3.61 3.15 2.97 3.42 1.81 HMWV 2.41 2.07 2.03 1.94 1.10 12dB Babble 2.64 2.30 2.58 2.71 1.55 6dB Babble 1.74 1.54 2.11 1.98 1.10 Tandem CVSD 2.68 2.22 2.57 2.65 1.74 Table 1. Mean Opinion Score (MOS) for MELPe vocoders and prior military standard vocoders in different conditions (from NATO STANAG-4591 Phase 2 testing) The NATO STANAG-4591 MELPe competition's combined performance index is illustrated in the figure below.