5.11 Design Example: Software Modem
In this section we design a modem. Low-cost modems generally use specialized chips, but some PCs implement the modem functions in software. Before jumping into the modem design itself, we discuss principles of how to transmit digital data over a telephone line. We will then go through a specification and discuss architecture, module design, and testing.
5.11.1 Theory of Operation and Requirements
The modem will use frequency-shift keying (FSK), a technique used in 1,200-baud modems. Keying alludes to Morse code–style keying. As shown in Figure 5.30, the FSK scheme transmits sinusoidal tones, with 0 and 1 assigned to different frequencies. Sinusoidal tones are much better suited to transmission over analog ...