STANAG 4591

Overview

STANAG 4591 is a NATO standardisation agreement that establishes technical specifications and requirements for digital coding of voice using Enhanced Mixed Excitation Linear Prediction (MELPe). STANAG 4591 MELPe is commonly used with MIL-STD-188-110A and MIL-STD-188-110D Par 5.3 compliant HF data modems. By defining a low-rate voice-coding algorithm operating at 2400, 1200, and 600 bits per second, STANAG 4591 facilitates clear and efficient voice transmission over constrained communication channels in military radios and satellite systems.

The standard incorporates a mandatory noise pre-processor to enhance voice clarity in noisy conditions. It also outlines requirements for analog-to-digital conversion, filtering, and other technical processes to maintain high-quality audio. Beyond its military applications, STANAG 4591 supports secure civilian communications, including high-frequency radio systems. By enabling interoperability with other NATO standards and supporting encryption compatibility, STANAG 4591 plays a critical role in fostering coordinated, secure, and effective communication across diverse platforms.

STANAG 4591 Structure

General

The Enhanced Mixed Excitation Linear Prediction coder is based on the traditional Linear Prediction Coder (LPC) parametric model, but also includes five additional features. They are mixed excitation, aperiodic pulses, adaptive spectral enhancement, pulse dispersion, and Fourier magnitude modelling. A MELPe frame interval is 22.5 ms ± 0.01percent in duration which corresponds to 180 voice samples (8000 samples/s).

Analog specification

The MELPe coder requires a nominal analog bandwidth of 100 Hz to 3800 Hz for optimal performance. The A/D conversion process should be at least equivalent to a 16-bit linear conversion, sampling at 8 kHz, with integer values between -32768 and 32767. Additionally, the coder/decoder combination should have unity gain, and the analog input circuit should include filtering or oversampling and filtering to limit signals outside the specified band. The anti-alias filter should attenuate by at least 20 dB at 4 kHz and 40 dB at 8 kHz and above, and input DC offsets should be controlled to prevent them from affecting the coder's operation.

Key Features

• Tri-rate voice coding: MELPe operates at 600 bps, 1200 bps, and 2400 bps.
• Interoperability: Enables seamless communication between different NATO and national communication systems.
• Noise pre-processor: Includes a mandatory noise pre-processor to improve voice quality in noisy environments.
• Analog specifications: Defines requirements for analog-to-digital conversion, filtering, and anti-aliasing.

Benefits

• Improved voice quality: MELPe provides better voice quality than older military standards, especially in noisy environments.
• Increased interoperability: Enables communication between different systems and nations.
• Enhanced security: The design of S4591 allows for integration with external encryption devices, although details of this integration is excluded from S4591.

Applications

• Military communications: Used in military applications, such as satellite communications and secure radio devices.
• NATO operations: Adopted as a NATO standard for secure voice communication.
• Civilian communications: As a widely available standard for low bit-rate voice encoding, MELPe is often used as part of digital voice encryption for civilian HF radio communication.

Related Standards

STANAG 4591 MELPe is commonly used with MIL-STD-188-110A and MIL-STD-188-110D Par 5.3 compliant HF data modems. STANAG 4591 MELPe is also used in the Tactical Secure Voice Cryptographic Interoperability Specification (TSVCIS).

Performance

The table and figure below provide a summary of the voice coder performance using the MIL-STD-188-110A data modem for 2400 & 1200 bps and SHORT interleaver for AWGN & CCIR Poor (CCIR-P) Channels.

The table also indicates the end-to-end latency (for SHORT interleaver), the Perceptual Evaluation Of Speech Quality (PESQ) metric, channel use, speech quality and understandability limit (in SNR) for the two voice coder rates.

MELPe 2400 bps/MS110A 2400 SHORT vs MELPe 2400 bps/MS110A 2400 SHORT CCIR Poor Channel