Interest in modem and fax software for DSP's and native signal processing has increased substantially. This is due to the fact that DSP's often offer the best cost performance ratios among different design alternatives for embedded communications applications, and that native signal processing (NSP) on a host PC potentially provides an economical solution by eliminating some hardware components. The utilization of software modules in this approach enables flexibility in equipment design and elimination of some hardware components, both of which result in cost reduction. It is a challenging task for an engineer to develop algorithms and software modules which have both good performances and minimal requirements on memories and MIPS. Understanding these signal processing functions and their requirements is crucial for the success of a DSP-based or NSP product design. This article presents a tutorial on major signal processing functions required by modem and fax.
DSP performance depends on the particular design of DSP algorithms and software modules. Engineers must aim at minimizing MIPS as well as memory requirements. In addition, telecom products must conform to the appropriate international and industry standards. This article summarizes the basic functions involved in the DSP-based software implementation of modem and fax. In order to make the discussion more concrete, products developed by and commercially available from GAO Research & Consulting Ltd. are presented as examples. GAO now offers a complete line of modem and fax software in C for microprocessors such as Pentiums, ARMs, MIPS, and assembly languages for fixed-point DSP's TMS320C54x and ADSP21xx or floating-point DSP's TMS320C3x/C4x and SHARC. The software covers all of the most-commonly used modem standards such as DSVD, V.34, V.34Q, V.32bis/32, V.22bis/22, and V.21 and fax standards such as V.17, V.29, V.27ter, and V.21 Channel 2.
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