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The DDVPC has recently released a set of Recommendations for the Implementation of MELP

MELP at 2.4Kbps

The Mixed Excitation Linear Prediction (MELP) voice coder model is based on the traditional LPC vocoder. However, the synthesizer has the following additional abilities that allow MELP to mimic more of the characteristics of natural human speech:

1. Mixed pulse and noise excitation
2. Periodic or aperiodic impulses
3. Adaptive spectral enhancement
4. Pulse dispersion filter
5. Fourier magnitude modeling

The coder employs Fourier magnitude coding of the prediction residual to improve the speech quality, as well as vector quantization techniques to efficiently encode the LPC and Fourier information.^[1] Some of the other characteristics of MELP are described below:

• Frame Size: 22.5ms
• Sampling Rate: 8kHz
• Analyzer
  • High Pass Filter: 4^thorder Chebychev type II
    • Cutoff Frequency: 60Hz
    • Stopband Rejection: 30dB
  • Bandpass Voicing Analysis: 6^th order Butterworth filters, 5 frequency bands
    • 0-500Hz
    • 500-1000Hz
    • 1000-2000Hz
    • 2000-3000Hz
    • 3000-4000Hz
  • Linear Prediction Analysis: 10^th Order
  • Error Protection: Unused coder parameters during unvoiced mode are replaced with forward error correction
    • Three Hamming (7,4) - corrects single bit errors
    • One Hamming (8,4) - detects double bit errors
• Synthesizer
  • Mixed Excitation Generation - The sum of the filtered pulse and noise excitations
    • Pulse - Inverse Discrete Fourier Transform of one pitch period in length
    • Noise - Uniform Random Number Generator, RMS:1000, Range: -1732 to 1732
    • Pulse Filter: Sum of bandpass filter coefficients for voiced frequency bands
    • Noise Filter: Sum of bandpass filter coefficients for unvoiced frequency bands
  • Adaptive Spectral Enhancement Filter: 10^th order pole/zero with 1^st order tilt compensation
  • Linear Prediction Synthesis: Direct form filter, Coefficients correspond to interpolated LSF's
  • Pulse Dispersion: 65^th order FIR filter derived from spectrally flattened triangle pulse

[1] "Analog to Digital Conversion of Voice by 2,400 Bit/second Mixed Excitation Linear Prediction (MELP)," Federal Information Processing Standards Publication (FIPS PUB) Draft, June 12, 1997

Performance Measures - Evaluation results of the 2.4k bps MELP algorithm were compared to other DoD Voice Processors in a poster session presented at the IEEE International Conference on Acoustics, Speech, and Signal Processing in Munich Germany, 1997 (ICASSP '97) entitled: A Comparison of the New 2400 BPS MELP Federal Standard with Other Standard Coders. More information concerning the test and evaluation process used for selection of the new 2400bps standard, including papers and test results presented during a special session at ICASSP 1996 in Atlanta, GA. may be found here: New FS 2400bps Test Results.

Sound Samples - The accompanying table entries are links to sound samples of the 2.4kbps MELP algorithm. These are 8KHz sampled, 16 bit linear PCM files in WAV format. To learn more about these and other test conditions used by the DDVPC for narrowband digital voice visit the DDVPC TEC webpage.

Source Code - ARCON Corporation, as part of it's technical support of the DDVPC, has provided an ftp site for the housing and distribution of narrowband digital voice compression algorithms. There are three different implementations of MELP available. Each is available as a compressed tar file and a self-extracting PC executable. A copy of the included "README" file is available to view before download for the non-realtime implementations.

1. A non-realtime floating point version - MELP1.2 README file.
2. A non-realtime fixed point version - MELP FXP README file.
3. A realtime 'C50 DSP version version - MELP 'C50 README file.

A Final report has been prepared documenting the MELP implementation in fixed-point C and TMS320C50 code. The introduction is posted here and the entire report is available in Postscript format. Fixed Point MELP Final Report

Simply point and click on one of the hyperlinks below to begin the ftp transfer.

Compressed tar of MELP 1.2 Source Code (70KB)

Self-Extracting MELP 1.2 Source Code (97KB)

Compressed tar of MELP Fixed Point Source Code (147KB)

Self-Extracting MELP Fixed Point Source Code (195KB)

Compressed tar of MELP 'C50 DSP Source Code (575KB)

Self-Extracting MELP 'C50 DSP Source Code (711KB)

Please note the size of the files and the filename extensions (.exe and .gz). Make sure your browser is setup to save *.exe and *.gz files to disk and be aware of the time it will take to download.

Algorithmic Delay - The 2.4kbps MELP algorithm, implemented with minimal buffering, has a speech throughput delay of 45.5 msecs.

 This website is sponsored by ARCON Corporation for the DDVPC and as such must conform to DoD policies concerning websites. Please read this disclaimer for more information.

Comments concerning design and content of these pages  should be sent to info@arcon.com. This page was last updated on 09-Jan-2002.