 to Use? Choice can be a wonderful thing, but it can also be overwhelming. I've just given you seven possible choices for your baseband modulator, but if you want to build a commu- nication system, you'll have to choose just one of them. As with all things, what you choose depends on what you want. There are six things most people want from their baseband modulator. Ultimately, you'll have to decide which modulator is best suited for your communication needs. (1) No DC component: In some communication systems, very low-frequency components (that is, frequencies around 0 Hz) don't make it to the receiver. In these cases, we want to use modulators that output waveforms with NO fre- quency component right at 0 Hz (Figure 5.10). Such modulators are said to have no DC component. Modulator Channel Receiver Avoid sending a signal Frequencies around OHz with a component at OHz don't make it Figure 5.10 Modulators with no DC components It's easy to tell if a modulator creates a waveform with a component at 0 Hz. Just look at what happens if you send a sequence of 0101010101. If the average is 0, then there's no DC component; if the average is not 0, then there is a DC component. For example, unipolar RZ has a DC component, NRZ-L does not. (2) Self-Clocking: In most communication systems, we want to help the receiver out. One way to do this is to make it easy for the receiver to create a clock signal with dura- T b Tb tion T (Figure 5.11). The modulator can help the receiver create a clock signal by Figure 5.11 sending a waveform that always transitions Clock signal with duration (for example, +V to 0 or +V to -V) once Tb wanted at the receiver every bit duration . Such a modulator helps the receiver create a clock and people (and receivers) appreciate it. For example, the Manchester code helps with self-clocking, but the NRZ-L doesn't (consider what happens if we send all l's-there are no transitions). (3) Error Detection: Receivers appreciate it if you can help them detect an error in transmission. Modulators can help re