Digital Modulation

A very important function of modern electronics is the transport of information. Examples of information are text, photos, videos, voice, music, etc. Signals containing information may be sent through the air utilizing radio waves. Another common medium for sending signals is metal wire, utilizing electricity. A third type is fiber optics, which utilize light.

Radio waves of constant frequency and amplitude (strength) cannot transport information (other than the most elementary information that can be conveyed by the mere detection of the radio wave). Likewise, if we apply constant voltage and current to a copper wire, we cannot transport information. We must change the carrier (radio wave, light or electrical current) in some systematic way in order to add information to the carrier. This process is called MODULATION.

There are two broad categories of modulation:

  • Analog
  • Digital

Analog is the older method of the two and is not used as much today. Two examples of analog modulation still currently used are AM (amplitude modulation) and FM (frequency modulation) radio. Formerly, broadcast TV utilized analog modulation of radio waves, but in 2009 TV broadcasting was converted to digital modulation. Other examples of signals employing digital modulation include cell phones, cable TV service and networking of computing systems (Internet, etc.).

In analog modulation, the information is applied by a method that results in a continuous variation of magnitude. As an example, suppose we have an electronic device that has a knob for adjusting the volume of a tone produced by a speaker. As the knob is rotated, the volume rises in a continuously variable way until maximum volume is reached. This would be considered an analog circuit in electronics.

As the name implies, digital modulation is accomplished by sending a signal containing numbers. Lets take our example of the electronic device that produces a tone. But in this case, we change the volume control to one that has discrete steps. As we turn the knob, it clicks into position at the next step, which results in a specific amount of increase or decrease in volume. It is not possible to to adjust the volume to any level we want, only to a set of specific levels. This kind of behavior in a circuit is typical of digital electronics.

Lets take a look first at the analog method. The figure below is an animation of a signal wave along with its modulation of radio waves in AM (pink waves) and FM (blue waves). Notice that the wave moves up and down in a continuously variable way.

In AM, amplitude modulation, the amplitude (height) of the radio waves are modulated to match the pattern of the signal wave. In this example the signal wave represents a pure tone of sound audible to the human ear. In FM, frequency modulation, it is the frequency of the radio waves that are modulated to match the pattern of the signal wave. This is more difficult to visualize. Notice that when the signal wave dips down, the blue radio waves of FM are stretched out. That is, their frequency is reduced.

modulation

One of our projects will be to assemble an AM radio. When we do that, we will be learning more about analog Amplitude Modulation. Right now we are working on the Infrared Module for the Line Following Robot. The electronics in that module are similar to those of an IR remote control and TV, which employ digital modulation. Therefore, we should learn something now about Digital Modulation techniques used in electronics.

A computer, as the name implies, is a device that performs mathematical operations. Our modern computers are capable of doing so many things, it is easy to forget that basically, a computer is a device that processes numbers. The number system used by computers is BINARY. Yes, it is true that your computer can have a calculator that handles numbers in the decimal system. However, we must keep in mind that at the base level, the computer actually works with binary numbers, just series of zeros and ones (100111100011110001000111000111).

Engineers and scientists working on the first electronic computers learned quickly that the easiest way to design digital electronics was by utilizing a binary number system. Take the example of the transistor, the electronic component that revolutionized the electronics industry and made it possible for millions of people to own computers. The easiest way to make a transistor handle numerical data is by either turning it on or off. That is, we can set it to one of two states (on or off). That matches very well with the binary number system that contains only two digits, 0 and 1. If the transistor is off, we can say it is storing the digit 0 and if it is on, we can say it is storing the digit 1.

Using binary, we need a system of sending signals containing a series of zeros and ones. We could do that in a wire by simply pulsing an electrical current. Suppose we pick a base time unit of one millisecond just for fun. As the clock runs then, every millisecond of time, we can apply no current to represent the digit zero and we can apply current to represent the digit one.

The figure below depicts an electrical signal in binary format. As time moves forward, at each unit of time, the voltage is either zero or five, as indicated by the red line. Zero volts represents the digit 0 in binary and five volts represents the digit 1. How can we translate this into a signal modulated on a radio wave?fig1

The figure below demonstrates one method of modulating a radio wave (the red line) with our digital data (the blue dotted line). In this case we are just turning the radio waves on and off. If they are off for a unit of time, then that represents the digit zero and if they are on it represents the digit one. This kind of digital modulation is called Binary Amplitude-Shift Keying (ASK). It is a special type of binary ASK called On-Off Keying (OOK). In OOK, for the digit 0 the radio waves are simply turned off (i.e., zero amplitude).

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The figure below depicts a second type of digital modulation called Frequency-Shift Keying (FSK). Notice here that to represent a digit of 1, the frequency of the radio wave is increased (more waves per unit time).

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The figure below shows the third type of basic digital modulation known as Phase-Shift Keying (PSK). This may be the most difficult to understand. Notice that the height of the waves (amplitude) is constant, so it can't be ASK. Even though there seems to be something strange happening to the waves, there is actually not a change in frequency, so this can't be FSK.

The PHASE of a wave is shifted by moving the wave to the left or right. Notice that the pattern of the waves seems to be interrupted. Starting from the left, when the shift comes to signal the first one digit, notice that the wave does not continue to rise, but reverses suddenly and goes down. This is the phase shift. The phase shift signals a change in state. If the current state is a zero, then with a phase shift, the next digit will be a one and vise versa.

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We can combine the three types of digital modulation in various combinations to create a transmission system capable of transmitting more information in a specific amount of time. Furthermore, we can fine tune each kind of modulation to carry more information. For example, it is possible to shift the phase of a wave in various amounts, which provides a means of modulating more information in the same amount of time.

Another method of increasing the speed of communication is by Multiplexing. A multiplexing method adds many sub carriers at slightly different frequencies in a group called a channel. Suppose we create a channel with 56 sub carriers. Then we can transmit 56 times more information in the same time period. By combining various methods, it has been possible to create communication speeds fast enough to support such things as streaming video on a cell phone.