14.7
In the fax machine, extracted bits from the scanned images are modulated into the voice band of 300 to 3400Hz. 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 14,400 bps. 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 of noise by the use of large signal power and bandwidth and by the use of waveforms that last for longer durations.
Modulation consists of superimposing the user information signal on a carrier signal, which is more adequate to the transmission medium. The carrier can also be used in a voice band of 300 to 3400 Hz for modem and fax applications. Typically, three basic types of modulation are used in fax equipment, namely amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). Quadrature amplitude modulation (QAM) is also used for higher rates of data. In QAM, amplitude and phase modulations are used simultaneously. The QAM system is a very efficient modulation technique in terms of bandwidth usage.
14.7.2
Demodulation
Demodulation is the inverse process of modulation. The modulated signal is transmitted to a receiver at the receiving station. The modulation information is extracted to bits in demodulation. Various modulation schemes used in fax processing are listed below:
• FSK—FSK is used with V.21 at a 300 baud rate. Each baud is of 1 bit.
• PSK—4 and 8 PSK are used in V.27ter demodulations.
• QAM is used in V.29.
• QAM in relation to TCM coding is used in V.17.
• QAM with TCM is also used in V.34. It is more complex than modulations
in V.17.
Various modulation schemes and their principles are explained in the next section.
Baud Rate and Sampling Rate Consideration. In the telephone analog front end, sampling CODEC hardware consists of an analog- to-digital converter (ADC) and digital-to-analog converter (DAC) that sample the signal at 8000 Hz. Narrowband voice processing modules directly operate at 8000 Hz. To process for fax channels, ADC path samples of 8000 Hz are rate converted to arrive at integer multiples of a baud rate that keeps the required analog fax signal bandwidth. In the modulation DAC path, the sample rate converter modifies the samples back to 8000 Hz and delivers on the pulse code modulation (PCM) interface. Depending on the fax module in operation, the fax signal sampling rate has to cater to sampling at multiples of the baud rate (i.e., 300, 1200, 1600, and 2400 baud rates). The original samples at 8000 Hz can work directly for the 1600 baud rate. Catering to the sampling rate with an integer
multiples baud rate helps in better alignment at symbol or bit boundaries. This type of sampling rate conversion is commonly used in multirate signal processing [Vidyanathan (1992)].
