VoIP Voice and Fax Signal Processing

14.7 FAX MODULATIONS AND DEMODULATIONS

In the fax machine, extracted bits from the scanned images are modulated into the voice band of 300 to 3400 Hz. Several modulation techniques are mainly decided by the data rate requirements and support for interoperation. The popular group-3 modulations are ITU-T-V.21, V.27ter, V.29, and V.17 supporting up to 14,400bps. Recently, V.34-based fax machines have also been launched for 33,600-bps fax transmission. The fax modem modules are together known by the name “fax data pump,” “fax modem data pump,” or simply “data pump.” This section describes the basic modulation techniques and the functional blocks of facsimile modem modules of V.21, V.27ter, V.29, V.17, and V.34 [ITU-T-V.21 (1988), ITU-T-V.27ter (1988), ITU-T-V.29 (1988), ITU-T-V.17 (1991), ITU-T-V.34 (1998)]. Refer to Berg (2000), and documents [URL (SPRA080), URL (SPRA073), URL (ADI Vol-2)] for more details on fax modules, coding, formulation on modulation and demodulation.

14.7.1 Modulation

Modulation plays a key role in any communication system. It accepts a bit stream as input and converts it to an electrical waveform for transmission. It can be used effectively to minimize the effects of channel noise, match frequency spectrum of the transmitted signal with channel characteristics, multiplex many signals, and overcome some equipment limitations. The most important parameters of the modulators are amplitude, frequency, phase, and bandwidth used. The modulator minimizes the effects ...