, the waveforms in NRZ-L are more "different," SO it takes more added noise to create an error. There you have it-the six things people look for in baseband modulators. The final choice is based on which of these criteria is most important to you. Example 5.1 Name a baseband modulation scheme which provides both zero DC component and low bandwidth. Solution: Let's start by taking a look at NRZ-M. In NRZ-M, a 1 is sent as a +V for the bit duration and a 0 is sent as a -V for the bit duration. On average, when an equal number of O's and 1's are sent, an equal number of +V's and -V's are sent. As a result, the average value of the signal sent is 0. The NRZ-M has a zero DC component. In NRZ-M, the signal sent (when 1 or 0 is input) is constant for the entire duration T. As a result, this signal has a small bandwidth as discussed earlier. Hence, NRZ-M meets both our requirements: it has no DC component and it employs a small bandwidth. 5.2.2 Bandpass Modulators A bandpass modulator takes incoming bits and outputs a waveform centered around frequency W C . This is the modulator you want to use when your communication channel will provide safe passage to frequencies around WC (that is, when the channel lets frequencies around WC get to the receiver with little distortion). Basically, these modulators are pretty straightforward. The modulator creates the waveform s(t) = Acos(at+0) (5.1) Getting It from Here to There: Modulators and Demodulators where W is a frequency at or near Wc. The modulator stores the information bits in either the amplitude (A), the frequency (w), or the phase (0). For example, bit 1 may be sent as + Acos(at+0) and bit 0 may be sent as - In this case the information is in the amplitude-the receiver checks to see if it has a +A or -A to figure out what bit was sent. Now, let's talk in more detail about how bandpass modulators work. The first bandpass modulators we'll look at are called ASK modulators, short for amplitude shift-keying modulators. This refers to the modulators that, given the input bi