Modulation and Demodulation

Most people can fully appreciate the concept of the speed of a dialed circuit in terms of bits per second. However, another term, baud, often is used to describe the speed of a modem. In fact, some people say things like "That modem runs at 33 kilo baud per second," really meaning 33 kilobits per second (kbps), thinking that "bits per second" and "baud per second" are the same thing. But the two terms are not synonymous, as you will read shortly.

Modems create an analog signal when sending data. As mentioned earlier, analog electrical signals can be analyzed in terms of frequency, amplitude, and phase. So, modem standards define that particular values for these three parts of the signal imply a 1 or a 0. To appreciate what that means, consider the two parts of Figure 15-6.


Amplitude Modulation


Frequency Modulation Time

Phase Modulation


AM + FM Combined 00 01 10 11

The figure depicts some very simple ways that a modem could be used to create an analog signal that can be interpreted by the receiver as a set of binary digits. In Graph A, a low amplitude means a binary 0, and a high amplitude means a binary 1. All the sending modem has to do is modulate (change) the amplitude of the signal to imply a 1 or a 0. For instance, if the modem was running at 28 kbps, then every 1/28,000th of a second, it would make the amplitude of the signal low or high, to encode a binary 0 or 1.

The process of changing, or modulating, the amplitude is called amplitude modulation. Modulation, as defined by, is "the variation of a property of an electromagnetic wave or signal, such as its amplitude, frequency, or phase," which is exactly what amplitude modulation does, specifically for the amplitude.

Graph B in Figure 15-6 depicts frequency modulation. In this simple example, the higher frequency (the part with the curved lines closer together) means 0, and the lower frequency means 1. Notice that the amplitude stays the same in that case, so this modem standard simply changes the frequency to imply a 1 or a 0. So, if the modems are running at 28 kbps, then every 1/28,000th of a second, the modem would make the frequency high or low to encode a binary 0 or 1.

Graph C in Figure 15-6 depicts phase modulation. Phase modulation changes the phase of the signal—instead of the signal following its normal pattern of rising to the highest positive voltage, gradually lowering to the lowest voltage and back again, the signal changes directions—which changes the phase. Modems can modulate the phase to imply a binary 0 or 1 as well.

Finally, Graph D in Figure 15-6 shows a combination of frequency modulation and amplitude modulation. With this final scheme, each signal represents 2 bits. For instance, a

Figure 15-6 Amplitude, Frequency, and Phase Modulation


High High Low r\ Low "

Low High Low wi/iwv

0 110

Phase Shifts

low-amplitude and low-frequency signal might mean 00, whereas a low-amplitude but high-frequency signal might mean 01. Table 15-2 lists the four combinations possible with this example combined modulation scheme.

Table 15-2 Combinations of Bits with FM and AM Together



Used for This Binary Code













The modulation scheme in Graph D of Figure 15-6 provides a good context from which to understand the term baud. To achieve higher bit rates, modems tend to use modulation techniques that encode more than 1 bit in the signal, as in this example. For instance, to achieve 28 kbps with this last modulation scheme, the modems would need to change (sender) or sample (receiver) the analog signal only every 1/14,000th of a second, because each sample represents 2 bits.

The term baud refers to a single encoded energy signal that can represent 1 or more bits. In this final example, a baud happens to represent 2 bits. Baud is not an acronym; it is taken from the name of the inventor (Baudot) of one of the first modulation schemes that implied more than 1 bit. So, the modem running at 28,000 bps, with a modulation scheme that sends/ receives 2 bits per baud, is running at 14,000 baud per second.

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