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1999 Edition, April 1999

Complete Document

Project 25 Vocoder Reference Test

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Description / Abstract:

This document specifies one method that may be employed to test that implementations of TIA\EIA-l02. BABA compatible speech codecs meet recommended minimum performance requirements. This speech codec is the Improved Multi-Band Excitation (IMBETM ) system described in [2]. The IMBE speech codec is used to digitally encode the speech signal at 4400 bps or decode the speech signal back to analog audio. Forward error correcting codes (FEC) are added to this to increase the signaling rate to 7200 bps. Most of the tests in this document are for the speech codec, and some of the tests verify the conformance of the FEC. All of the tests use an interface operating with the combined FEC/codec signal at 7200 bps.

The need for this procedure arises from the fact that implementations of speech coders do not have to be bit exact to achieve compatibility. Since individual designers implement algorithms in different manners, it is not possible to create a standard set of test vectors to assess conformance to a standard. This document describes a series of tests which are used to test conformance to specific aspects of the specification. These test do not necessarily ensure that the speech codecs operate satisfactorily under all possible input signals, and it is still a requirement of the manufacturer to ensure that his implementation operates in a consistent manner. The purpose of these tests is to test for minimum performance levels and the manufacturer should attempt to provide the highest performance possible.

The testing of the codecs described in this document is based upon quantitative tests with specific pass/fail criteria. The major purpose of these tests is not to assess the performance between one manufacturer's encoder and decoder, but to assess the Performance between any pairing of manufacturer's encoders and decoders. This interoperability issue is of a critical nature to end users. The overall purpose of this standard is to provide a means for measuring performance levels while trying to ensure proper inter-operability with other manufacturer's equipment.

The issue of inter-operation can only be definitively answered by testing all combinations of encoder/decoder pairings. With the number of equipment manufacturers expected to supply equipment this may well be a prohibitive task. This standard takes the approach of defining a reference speech codec and then each manufacturer tests his implementation relative to the reference. Specific parameters identified by the developer of the speech codec are then compared against the reference. The speech codec is said to be in conformance once the objective performance requirements have been satisfied.

This document requires use of proprietary test equipment which may be purchased from Digital Voice Systems Incorporated. An alternative method of testing using Mean Opinion Scores (MOS) is described in [4].