TIA/EIA-736

Description / Abstract: This document specifies the procedures which may be used to ensure that implementations of IS-733 compatible variable-rate speech codecs meet recommended minimum performance requirements. This speech codec is the Service Option 17 described in IS-733. The Service Option 17 speech codec is used to digitally encode the speech signal for transmission at a variable data rate of 266, 124,54 or 20 bits for each 20 ms frame.

Unlike some speech coding standards, IS-733 does not specify a bit-exact description of the speech coding system. The speech-coding algorithm is described in functional form, leaving exact implementation details of the algorithm to the designer. It is, therefore, not possible to test compatibility with the standard by inputting certain test vectors to the speech codec and examining the output for exact replication of a reference vector. This document describes a series of tests that may be used to test conformance to the specification. These tests do not ensure that the codec operates satisfactorily under all possible input signals. The manufacturer shall ensure that its implementation operates in a consistent manner. These requirements test for minimum performance levels. The manufacturer should provide the highest performance possible.

Testing the codec is based on two classes of procedures: objective and subjective tests. Objective tests are based on actual measurements of the speech codec function. Subjective tests are based on listening tests to judge overall speech quality. The purpose of the testing is not only to ensure adequate performance between one manufacturer's encoder and decoder but also that this level of performance is maintained with operation between any pairing of manufacturers' encoders and decoders. This interoperability issue is a serious one. Any variation in implementing the exact standard shall be avoided if it cannot be ensured that minimum performance levels are met when inter-operating with all other manufacturers' equipment meeting the standard. This standard provides a means for measuring performance levels while ensuring proper interoperation with other manufacturers' equipment.

The issue of interoperability may only be definitively answered by testing all combinations of encoderldecoder pairings. With the number of equipment manufacturers expected to supply equipment, this becomes prohibitive task. The approach taken in this standard is to define an objective test on both the speech decoder function and the encoder rate determination function to ensure that its implementation closely follows that of the IS-733 specification. This approach is designed to reduce performance variation exhibited by various implementations of the decoder and rate determination functions. Because the complexity of the decoder is not large, constraining the performance closely is not onerous. If all implementations of the decoder function provide essentially similar results, then interoperation is more easily ensured with various manufacturers' encoder implementations.

The objective and subjective tests rely upon the use of a master codec. This is a floating-point implementation of IS-733 written in the C programming language. The master codec is described more fully in 3.4. This software is used as part of the interoperability testing.

By convention in this document, the Courier font is used to indicate C language and other software constructs, such as file and variable names.