VoIP Voice and Fax Signal Processing

14.1 FAX MACHINE OVERVIEW

At a high level, the facsimile terminal device (fax) consists of a paper input device (scanner) paper output device (printer); accessories like telephone keypad, display, handset, and controller [McConnell et al. (1999), URL (Maine), URL (ATIS)]; and a fax modem as shown in Fig. 14.2. The fax modem is connected to the phone line of the PSTN. Once a call is established between two fax machines on the PSTN line, the PSTN infrastructure is transparent without keeping any distinction between voice or fax calls. A functional representation of a PSTN-based fax call is shown in Fig. 14.2.

Figure 14.2. Functional representations of G3 fax equipment (G3FE) internal modules.

A fax machine scans a page and electrically breaks up a document into picture elements or pixels or pels. The most common type of scanning method used in fax machines is the flat bed type, and in this method, a document is moved across an optical unit one line at a time in the vertical direction and then the optical device scans the line in the horizontal direction. The size of each picture element or pixel is different based on the resolution of the image being scanned. A smaller pixel size generates a higher resolution of the document, but it increases the amount of picture data sent over the phone lines. These pixels generate a binary bit pattern. The scanned picture elements are coded using ...