TIA/EIA-125-A:2000

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Recommended Minimum Performance Standard for Digital Cellular Spread Spectrum Speech Service Option 1

Published By	Publication Date	Number of Pages
TIA	2000	NA

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Category: TIA

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Description

This document specifies the procedures, which may be used to
ensure that implementations of IS-96-C compatible
variable-rate speech codecs meet recommended minimum performance
requirements. This speech codec is the Service Option 1 described
in IS-96-C. The Service Option 1 speech codec is used to digitally
encode the speech signal for transmission at a variable data rate
of 8550, 4000, 2000, or 800 bps.

Unlike some speech coding standards, IS-96-C 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 on 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 trying to ensure proper interoperation with other
manufacturers' equipment.

The issue of interoperability may 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 both the speech decoder
function and the encoder rate determination function to ensure that
its implementation closely follows that of the IS-96-C
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-96-C 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.

Additional information