Defense Tech Briefs - Variable-Data-Rate Speech Encoder

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Variable-Data-Rate Speech Encoder

Naval Research Laboratory, Washington, DC

Oct 01 2007

Page 1 of 2

This encoder could supplant older encoders that operate at diverse fixed rates.

Avariable-data-rate (VDR) speech encoder has been designed to be interoperable with, and eventually to supplant, the many different voice encoders now used in military communication systems. Because these older systems were designed to utilize specific radio links with fixed and limited channel capacities, these systems utilize many different voice compression algorithms operating at various fixed rates. The incompatibility of these systems is an obstacle to interoperability. Emerging net-centric communication systems promise to provide connectivity to all military users, but compatible encoding will be necessary for interoperability, and encryption will be necessary for secure communications.

The Seven Operating Modes of the VDR voice encoder are characterized by different average data rates. Mode 1, characterized by a fixed rate of 2.4 kb/s, is the same mode as that of the Federal standard MELP encoder for narrow-band speech.

The VDR voice encoder is designed to provide both interoperability and security in net-centric voice communications. The VDR speech encoder can operate at any or all of the various data rates of older military speech encoders. Notably, it can operate over a range of data rates up to 26 kb/s and is backward-compatible with the Multiple Excitation Linear Predictive (MELP) voice encoder, which is a Federal-standard encoder that operates at a data rate of 2.4 kb/s. The VDR speech encoder is interoperable at any and all rates simultaneously. The rate setting can be changed dynamically (that is, during operation) without disrupting operation, even when used with encryption: Hence, without compromising security, the VDR speech encoder can be dynamically adjusted to make efficient use of network bandwidth under changing network traffic conditions.

The heart of the VDR voice encoder is a multirate voice processor in which a single voice algorithm generates multiple data streams at rates from 2.4 kb/s to an average rate of about 23 kb/s for input speech at frequencies from 0 to 4 kHz. The algorithm provides for seven different operating modes (see table). Inclusion of a few more kb/s of data from the 4-to-8-kHz audio frequency band makes it possible to encode wide-band speech comparable in quality to that of standard frequency-modulation (FM) broadcasting.

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