TIA TIA-136-220

Abstract

This document specifies the procedures to be employed to test that implementations of speech codecs compatible with that specified in specified in TIA/EIA-136-420 meet recommended minimum performance requirements. This speech codec is the Vector Sum Excited Linear Predictive coding (VSELP) system described in TIA/EIA-136-420 TDMA <em>Cellular</em>/<em>PCS - Radio Interface - VSELP</em>. The VSELP speech codec is used to digitally encode the speech signal for transmission at a data rate of 7950 bps. <br>Unlike some speech coding standards, TIA/EIA-136-420 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 then 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 which are used to test conformance to the specification. These tests do not necessarily ensure that the codec operates 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. <br>The testing of the codec is based upon two classes of procedures: objective tests, which are based on actual measurements on the speech codec function; and subjective tests, which are based on listening tests to judge overall speech quality. One of the purposes 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 inter-operability issue is a serious one. Any variation in implementing the exact standard must be avoided if it cannot be ensured that minimum performance levels are met when inter-operating with all other manufacturer's equipment meeting the standard. The purpose of this standard is to provide a means for measuring performance levels while trying to ensure proper inter operation with other manufacturer's equipment. <br>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 becomes a prohibitive task. The approach taken in this standard is to define an objective test on the speech decoder function to ensure that its implementation closely follows that of the TIA/EIA-136-420 specification. This is to reduce as much as possible the amount of performance variation exhibited by various implementations of the decoder function. 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 inter-operation is more easily assured with various manufacturers' encoder implementations. <br>The objective and subjective tests rely upon the use of a master codec. This is a floating point implementation of the speech coding algorithm written in the C programming language. The master codec is described more fully in &#167;3.5. This software is used as part of the inter-operability testing. <br>By convention in this document, the Courier font is used to indicate C language and other software constructs such as file and variable names. The diacritical mark, &#167;, is used to indicate section numbers.