ignal 100-4000 Hz Figure 2.7 Putting three signals on one wire using FDM Chapter Two Next, we want to recover Gretchen's speech signal. This is a two-part job. First, a bandpass filter (BPF) is applied, with start frequency 8,000 Hz and stop frequency 12,000 Hz. This filter allows only the frequencies between 8,000 Hz and 12,000 Hz to pass through, cutting out every other frequency. In the case at hand, this means that only Gretchen's speech signal gets through the filter. This is shown in Figure 2.8. We still have one task left. Gretchen's speech signal has been moved from 100-4,000 Hz to 8,100-12,000 Hz. We want to bring it back to the original 100-4000 Hz. This is done by applying a mixer (operating at 8,000 Hz), which returns Gretchen's voice signal to its original frequency components. Monica's signal is recovered on a single wire in much the same way as Gretchen's, and, rather than use many words, I'll simply refer you to Figure 2.8. Figure 2.8 Getting three speech signals back from one wire in FDM Carl's speech signal 0-4000 Hz Carl, Gretchen and Monica's speech Gretchen's on one wire Mixer speech signal 8000-12000 Hz 8000 Hz Monica's Mixer speech signal 16000-20000 Hz 16000 Hz Along Came TDM TDM, short for time-division multiplexing, is the second commonly used technique to let several people's speech share a single wire. TDM works like this. Let's say we've again got Carl, Gretchen, and Monica, who all want to make their calls from Fort Collins to Boulder. Carl, Gretchen, and Monica's speech sounds are first turned into an electrical signal on a wire by their phones, as explained in Chapter 1. Then, their electrical speech signals are turned into digital signals, again as explained in Chapter 1. The digitized, electricized versions of the speech signal are the incoming signals that will share a wire. Figure 2.9 shows these incoming signals. These signals, coming along the wire, then meet "the big switch," as shown in Figure 2.9. The big switch makes contact with each of the three incoming signals, touchi