Transmitting digital data with discrete analog signals - Curt M. White

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C. Transmitting digital data with discrete analog signals

The technique of converting digital data to an analog signal is also an example of modulation. But in this type of modulation, the analog signal takes on a discrete number of signal levels. It could be as simple as two signal levels (such as the first technique shown in the next paragraph) or something more complex as 256 levels as is used with digital television signals. The receiver then looks specifically for these unique signal levels. Thus, even though they are fundamentally analog signals, they operate with a discrete number of levels, much like a digital signal from the previous section. So to avoid confusion, we’ll label them discrete analog signals. Let’s examine a number of these discrete modulation techniques beginning with the simpler techniques (shift keying) and ending with the more complex techniques used for systems such as digital television signals—quadrature amplitude modulation.

1. Amplitude Shift Keying

The simplest modulation technique is amplitude shift keying. As shown in Figure 2-15, a data value of 1 and a data value of 0 are represented by two different amplitudes of a signal. For example, the higher amplitude could represent a 1, while the lower amplitude (or zero amplitude) could represent a 0. Note that during each bit period, the amplitude of the signal is constant.

Amplitude shift keying is not restricted to two possible amplitude levels. For example, we could create an amplitude shift keying technique that incorporates four different amplitude levels, as shown in Figure 2-16. Each of the four different amplitude levels would represent 2 bits. You might recall that when counting in binary, 2 bits yield four possible combinations: 00, 01, 10, and 11. Thus, every time the signal changes (every time the amplitude changes), 2 bits are transmitted. As a result, the data rate (bps) is twice the baud rate. This is the opposite of a Manchester code in which the data rate is one-half the baud rate. A system that transmits 2 bits per signal change is more efficient than one that requires two signal changes for every bit.

Amplitude shift keying has a weakness: It is susceptible to sudden noise impulses such as the static charges created by a lightning storm. When a signal is disrupted by a large static discharge, the signal experiences significant increases in amplitude. For this reason, and because it is difficult to accurately distinguish among more than just a few amplitude levels, amplitude shift keying is one of the least efficient encoding techniques and is not used on systems that require a high data transmission rate. When transmitting data over standard telephone lines, amplitude shift keying typically does not exceed 1200 bps.

2. Frequency Shift Keying

Frequency shift keying uses two different frequency ranges to represent data values of 0 and 1, as shown in Figure 2-17. For example, the lower frequency signal might represent a 1, while the higher frequency signal might represent a 0. During each bit period, the frequency of the signal is constant. 

Unlike amplitude shift keying, frequency shift keying does not have a problem with sudden noise spikes that can cause loss of data. Nonetheless, frequency shift keying is not perfect. It is subject to intermodulation distortion, a phenomenon that occurs when the frequencies of two or more signals mix together and create new frequencies. Thus, like amplitude shift keying, frequency shift keying is not used on systems that require a high data rate.

3. Phase Shift Keying

A third modulation technique is phase shift keying. Phase shift keying represents 0s and 1s by different changes in the phase of a waveform. For example, a 0 could be no phase change, while a 1 could be a phase change of 180 degrees, as shown in Figure 2-18.

Phase changes are not affected by amplitude changes, nor are they affected by intermodulation distortions. Thus, phase shift keying is less susceptible to noise and can be used at higher frequencies. Phase shift keying is so accurate that the signal transmitter can increase efficiency by introducing multiple phase-shift angles. For example, quadrature phase shift keying incorporates four different phase angles, each of which represents 2 bits: a 45-degree phase shift represents a data value of 11, a 135-degree phase shift represents 10, a 225-degree phase shift represents 01, and a 315-degree phase shift represents 00. Figure 2-19 shows a simplified drawing of these four different phase shifts. Because each phase shift represents 2 bits, quadrature phase shift keying has double the efficiency of simple phase shift keying. With this encoding technique, one signal change equals 2 bits of information; that is, 1 baud equals 2 bps.

The efficiency of this technique can be increased even further by combining 12 different phase-shift angles with two different amplitudes. Figure 2-20(a) (known as a constellation diagram) shows 12 different phase-shift angles with 12 arcs radiating from a central point. Two different amplitudes are applied on each of four angles. Figure 2-20(b) shows a phase shift with two different amplitudes. Thus, eight phase angles have a single amplitude, and four phase angles have double amplitudes, resulting in 16 different combinations. This encoding technique is an example from a family of encoding techniques termed quadrature amplitude modulation, which is commonly employed in contemporary modems and uses each signal change to represent 4 bits (4 bits yield 16 combinations). Therefore, the bps of the data transmitted using quadrature amplitude modulation is four times the baud rate. For example, a system using a signal with a baud rate of 2400 achieves a data transfer rate of 9600 bps (4 × 2400). Interestingly, it is techniques like this that enable us to access the Internet via DSL and watch digital television broadcasts.