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1. IntroductionInterest 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. 2. Modem and Fax StandardsModems and fax have traditionally been implemented using dedicated chipsets. They are offered as off-the-shelf black boxes. Engineers who use these black boxes in their designs often do not need to understand their internal operational mechanisms. With the increasing popularity of the software implementation of modem and fax, designers need to understand the operational principles and software building blocks of a modem or a fax. Modem and fax technologies are specified by a group of ITU standards. The commonly used standards are summarized in Tables I and II.
V.42 bis defines the data compression procedures using error correction, and V.42 specifies the error-correcting procedures utilizing asynchronous-to-synchoronous conversion.
Note: V.34 fax standard will be available soon. The modem or fax software is composed of two parts: protocols and data pump. The protocols are not computationally intensive, hence are typically handled by a microcontroller. Implementation of the protocols will not be discussed in this article. Data pump is the core of the modem and fax. Generally speaking, major components of a data pump include modulator, demodulator, encoder, decoder, scrambler, descrambler, equalizer, echo canceller, timing recovery, and carrier recovery. As an example, Figures 1 and 2 illustrate the signal processing functions of the V.34 modem. While the V.34 software commercially available from GAO is presented here as a concrete example, the principles involved are common to other modem and fax techniques. Algorithms of the V.34 modem data pump can be categorized into three groups: handshaking, transmitter and receiver. 3. The V.34 handshakingThe V.34 modem handshaking is composed of four phases. Phase 1 is defined in ITU V.8 (a universal automode scheme for modems, faxes and other communication devices). Phase 2 sets the modem parameters and performs line probing and ranging. Phase 3 involves initial training of the equalizer and echo canceller, and Phase 4 is responsible for final training and exchange of final data mode modulation parameters. 4. TransmitterThe transmitter depicted in Fig.1 is based on the structure specified by the ITU V.34 standard. The scrambler randomizes the input binary data to spread it over the entire transmit band. The purpose is to facilitate effective transmission of data over the 3KHz telephone channel. This process also improves the convergence of adaptive equalization and echo cancellation in the receiver. The encoder employs three new techniques. First, multidimensional trellis-coded modulation (MD-TCM) is a technique which combines coding and modulation to attain high reliability with limited transmission frequency bandwidth. The symbol sequences used in the V.34 modulation consist of four-dimensional symbols, with one 4-D symbol being the concatenation of two consecutive 2-D symbols, which are actually points in the quadrature amplitude modulation (QAM) constellation. Trellis is a forward-error correction process that can reduce transmission errors. Second, shell mapping is a process which reduces the average signal energy, thus improving modem performance with the same signal-to-noise ratio. Third, precoding utilizes a nonlinear equalization scheme to reduce the effects of noise enhancement in the equalizer. 5. ReceiverModem designers have much latitude in receiver implementation, since the ITU V.34 standard does not specify the structure of the receiver. As expected, several functions of the receiver are the inverse of transmit functions. In general, the receiver design is more complicated than its transmitter counterpart, as shown in Fig.2. The receiver includes seven major functional blocks: automatic gain control (AGC), echo canceller, equalizer, timing recovery, carrier recovery, demodulator and trellis decoder. AGCs ensure the proper operation of modem receiver algorithms by maintaining the output signal at a constant level. Received signals are typically adjusted by two AGC stages. The first one is set during training and retraining, and the second one is dynamically adjusted as the transmission proceeds. The hybrids in a telephone system perform conversions between two-wire and four-wire facilities. However, such conversions cause changes in impedances in a communication circuit, resulting in near-end and far-end echoes. A near-end canceller and a far-end canceller are thus necessary to cancel these transmission impairments. Although the far-end echo is relatively weak, it is not always easy to cancel; non-linear compensation techniques must be employed if it is frequency translated when passing through modulators and demodulators. Echo cancellers use the received signals and the delayed transmitted signals to estimate the echoes, which are then subtracted from the received signals. The samples received by the modem suffer from intersymbol interference (ISI). In order to enhance transmission performance, a filter is required to estimate the inverse transfer function of the channel and to perform channel equalization. Typically, a linear adaptive transversal filter is used and its coefficients are adjusted by the least mean square (LMS) algorithm. In the absence of a separate clock to synchronize the transmitting and receiving ends, the task of timing recovery is needed to recover a clock at the symbol rate or a multiple of the symbol rate from the modulated waveform. This recovered clock enables the conversion of the received continuous-time signal into a discrete time sequence of data symbols. The strength of the timing information in a signal is dependent on the statistics of the signal, and the pulse shape. In passband systems, the carrier frequency is generated by a timing reference in a transmitter. Coherent demodulation of a passband signal requires exactly the same carrier frequency and phase to perform the demodulation. In addition to decoding points in the QAM constellation, the Viterbi decoder detects transmission errors and corrects them based on a "best guess" scenario. In other words, it is a maximum likelihood decoder. The Viterbi decoder selects, out of all the possible symbol sequences, the one which is the most similar to the transmitted signal sequence according to some metric. The metric is typically the minimum Euclidean distance from the transmitted sequence. The GAO V.34 modem software discussed above complies fully with the ITU standards. It incorporates many state-of-the-art algorithms, and has been tested extensively against modem testing standards such as TSB37A and TSB38. 6. Software ImplementationA general purpose programmable DSP chip, such as a fixed-point or floating-point DSP from Texas Instruments Inc. or Analog Devices Inc., can be used to replace several dedicated functional chips that are required in a traditional design. All signal processing functions are implemented in the DSP software, an approach which facilitates upgrading either by downloading the software through the host processor or by replacing a ROM containing the software. The advantages of the implementation of modem and fax functions in software, are thus apparent. For the OEM, it is flexibility in product design, cost savings in product manufacturing, and extension of product life spans. For the end user, it is simplicity in the use of advanced features as well as affordability. GAO Research & Consulting Ltd. has developed all DSP software required by modem and fax. The software includes V.34 and DSVD modem software which is designed for DSP or microprocessor-based soft modems. The software has three versions: one version is coded in ANSI C and is developed for Pentiums, microprocessors, and floating point DSPs, the other two versions are coded in the DSP assembly languages of the TMS320C54x/C4x /C3x from Texas Instruments Inc. and ADSP21xx/Sharc from Analog Devices Inc. The DSVD software is composed of a speech vocoder, V.34 modem data pump, and control protocols V.70, V.75, and V.76. GAO now offers a complete line of modem and fax software in C for microprocessors such as Pentiums, ARMs, and MIPS, and assembly languages for TMS320C54x and ADSP21xx. 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. The company also offers video
conference software H.324 and a suite of telephony software's such as acoustical/line echo cancellers and DTMF detector.  Figure 1 Transmitter of the V.34 Modem Software from GAO  Figure 2 Receiver of the V.34 Modem Software from GAO |
