Download Entire Document in Postscript Format (701K)
FINAL REPORT APRIL 1998
CONTRACT MDA 90497C0298
1. INTRODUCTION
This report documents the work of Texas Instruments and DSPCon to implement the
new 2.4 kb/s Federal Standard Mixed Excitation Linear Predictive (MELP) vocoder in
fixedpoint C and TMS320C50 assembly code. This MELP coder, selected in 1996 based
on an open competition, was jointly developed by TI and Atlanta Signal Processors, Inc.
and provides much higher speech quality than the earlier Standard LPC10 at the same
low data rate. The goal of this project was to develop a version of the MELP coder using
16/32 bit arithmetic typical of a standard Digital Signal Processor (DSP) with equivalent
subjective speech quality to the original floatingpoint coder, and to implement a bitexact
version of this software to run in realtime on the TMS320C50 DSP.
The fixedpoint version of the MELP coder was developed in the C language using a
standard software library to simulate DSP arithmetic. This library, originally developed
for the European Telecommunications Standards Institute (ETSI), defines a minimum
functionality typical of all modern DSP devices and therefore allows straightforward
porting of the MELP coder to a wide range of DSP's. The fixedpoint C MELP coder
was then extensively tested to verify that it met the performance goal of equivalence to
the floatingpoint version, and the resulting processed speech files were archived.
Finally, the TMS320C50 assembly software was developed from the fixedpoint C
simulation, and it was verified that the resulting assembly code implementation produced
output identical (i.e., bitexact) to the C simulation for all of the source files included in
the subjective testing.
In this report, Section 2 gives an overview of the MELP coder, the processing steps
involved in analysis and synthesis, and the transmission format. A description of the
fixedpoint C software and listening test results follows in Section 3 . Finally, Section 4
describes the TMS320C50 assembly code development and results.
