AN2113 Freescale Semiconductor / Motorola, AN2113 Datasheet - Page 2

• Manufacturer Part Number: AN2113
• Description: AN2113 Multichannel Voice Coding System on the RTXC Operating System
• Manufacturer: Freescale Semiconductor / Motorola

Project Purpose
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In this project, we explore the issues that arise when third-party software is integrated with an RTOS and
other application software in a C language development environment. This project has better enabled us to
support Motorola customers by providing in-house experience with the following:
The system described in this application note executes multiple channels of the IS-96-A voice coder on a
Motorola DSP56307EVM. Using a variety of software tools and code, we experienced the intricacies that
arise from such an approach. We developed and debugged the application software using the Tasking 2.2r2
DSP563xx Software Development Toolset. C code snippets shown in this document adhere to guidelines
set forth by the 2.2r2 version of the Tasking tool set. Hand-coded assembly code interfaces to the IS-96-A
voice coding library supplied by Signals and Software Limited (SASL). Embedded Power Corporation’s
RTXC is the underlying RTOS. The result of this project is a demo that is shown at Motorola booths in
conferences and other technical events.
Voice coders, often called vocoders, are technically a subset of the entire range of voice coding
technology. The word “vocoder” stands for voice coder/decoder. Voice coders in infrastructure systems
compress the voice data for transmission. Voice coders have evolved from low compression capabilities to
high compression capabilities while retaining quality sound output. With improvements in voice coding
technology, we can now compress voice data to 6.3 kbps or even better with toll quality results. Toll
quality refers to the quality of sound that humans hear with analog speech. Ideally, a person cannot tell the
difference between analog speech and digitized speech coming over the air interface. Through
improvements in compression technology, this is becoming a reality even with high compression ratios.
Voice coding algorithms are evaluated in terms of four metrics:
Speech is separated into three categories: voiced, unvoiced, and mixed. Voiced speech is quasi-periodic in
the time domain and harmonically structured in the frequency domain. Unvoiced speech is random-like.
These categories are studied to determine the most effective method of compression for each. The
characteristics of speech are created through the interaction of the vocal box (speech source) and the vocal
tract that includes the mouth. This interaction creates the spectral envelope. The spectral envelope is
1. TIA/EIA/IS-96-A “Speech Services Option Standard for Wideband Spread Spectrum Digital Cellular Sys-
tem.”
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Project Purpose
Voice Coding
Voice coders/decoders
Multichannel application
C and assembly coding and integration
RTOSs
Subjective quality. Voice coders attempt to achieve toll quality.
Bit rate. The level of compression achieved.
Complexity of the algorithm. Can this algorithm be implemented using today’s technology?
End-to-end delay. If the algorithm adds too much delay to the signal, it becomes unusable in a
real-time voice application.
Freescale Semiconductor, Inc.
For More Information On This Product,
Multichannel Voice Coding System
Go to: www.freescale.com
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