Download 8.2.2

8.2.2: Signals
(electronic signal, electromagnetic signal, analog signal, digital signal, binary pattern,
wave pulse)
In the last section you saw how an electric circuit can be used to do work. However,
it can also be used to send information. Suppose you were in one room and your
friend was in the other. All you had connecting the two of you was an electric circuit
with a light bulb in each room. How could you communicate to each other? If you
both knew Morse code, you could send wave pulses of energy through the circuit to
your friend by opening and closing the circuit. For much of the 1800s, this is how
people communicated with each other over great distances. Samuel Morse patented
a version of the electrical telegraph in 1837, and by the Civil War in 1861, telegraph
lines allowed for communication by Morse code coast to coast across the United
States.
[Graphic: two students on either side of a wall, tapping out morse code on the
switches of a circuit, with light bulbs on both sides; also show Morse code signals
like Figure 17B on p619 of PS:CIA]
How Do Signals Carry Information?
In the example of the Morse code signals in Figure XX, each of the red boxes is a
pulse of energy called a wave pulse that travels through the circuit when the circuit
is closed. Waves are a form of transmitting energy. In a circuit, a wave pulse can be
used to carry information. This information is therefore also a form of energy. The
dots and dashes of Morse code that are sent along telegraph lines are an example of
electronic signals. An electronic signal is information sent as patterns in the
controlled flow of electrons through a circuit. However, there are other ways to send
information. Cell phones and car radios are not connected to any circuits, yet they
still receive information. They do so by receiving electromagnetic signals. An
electromagnetic signal is information sent as patterns of electromagnetic waves
such as light, infrared waves, microwaves, and radio waves. Modern information
technologies use a combination of electronic and electromagnetic signals.
Electronic Signals To understand how electronic signals work, think about
circuits. If a voltage source is connected to a circuit by a wire, electrons will flow
through the wire. Controlling the electron flow—by either altering the voltage or
turning the current on and off—can produce a coded signal. In a telegraph, the
current is simply turned on and off. However, the electronic signal can be turned on
for short (dots) or long (dashes) periods of time, allowing each of the letters of the
alphabet to be written as a combination of dots and dashes. For example, in Morse
code, a = “ −,” b = “− ,” c = “−  − ,” and so on. This works, but it is painstakingly
slow. Suppose you are communicating to your friend in the situation in Figure XX,
but now you can control the strength of the current by varying the voltage, and not
just turn it on or off. Now you can have a telephone, which was patented by
Alexander Graham Bell in 1876. The telephone allows you to talk at one location and
be heard at another. While it may seem hard to believe, both people talking in a
telephone conversation were originally part of the same closed circuit, even if it
spanned the country, and the strength of the electronic current was varied to convey
the sounds of their voices. Television is another kind of electronic signal. In this
case, a changing image is broken down into wave pulses that are sent through wires
to a television, which receives the information and reassembles the video images.
Electromagnetic Signals A whole new way of sending information was
invented by the end of the 1800s—radio. The first radio station was built by
Guglielmo Marconi in 1897. Information was now being sent by electromagnetic
waves—in this case by radio waves. You didn’t need to be connected to the same
electrical circuit; you could send and receive information from anywhere, even at
sea from a boat or (eventually) in the air from a plane. Electromagnetic signals have
another advantage over electronic signals—they travel at the speed of light, which is
much faster than the speed of electrons in a wire. Different frequencies of
electromagnetic signals work best for different applications. Modern mobile phones
communicate using microwaves. If you look around your community, you will spot
many microwave transmitters and receivers on towers and buildings that can
communicate with mobile phones. Optical fibers use visible and infrared light to
carrying great amounts of information; one fiber optic cable can carry 3 million
voice channels or 90 thousand television channels.
[Photo idea: Alexander Graham bell and telephone; airplane pilot on phone]
What are Digital and Analog Signals?
Electronic and electromagnetic signals can carry information from one place to
another in two different ways: as analog signals or as digital signals. An analog
signal is a continuous signal that represents a physical quantity. An example would
be the sound recording of music. It is recorded by continuously changing the voltage
or current in a circuit. A digital signal is the representation of a continuous signal
by a set of measurements taken at discrete time intervals. Both analog and digital
signals have strengths and weaknesses, but the power and flexibility of digital
signals has made them the foundation of modern information technologies.
Analog Signals Analog signals are used to provide a continuous recording of
some kind of action. For example, when seismic waves from an earthquake cause
the ground surface to move up and down, a seismograph records that continuous
motion as an analog signal. At any point in time, the seismograph records exactly
where the ground surface is. This is the advantage of analog signals—they provide
the highest resolution of an action by recording it continuously. The disadvantage of
analog signals is that they are hard to record. The seismograph would have to write
out the seismograms on paper in order to record them as analog signals. Other
examples of analog signals are the recordings of music on vinyl records or magnetic
cassette tapes. These have the advantage of being continuous recordings. You could
slow down the records or tapes and you would still hear continuous music. Radios
play music in the form of analog signals that are transmitted as continuous
electromagnetic signals: the music is encoded as a smoothly changing pattern of the
voltage. However, vinyl records scratch easily, magnetic cassette tape can stretch
and wear out over time, and analog radio signals fade and get patchy with increasing
distance from the emitting radio tower. Most importantly, you cannot bring these
analog signals into a computer, so you cannot play them on your mobile phone or
computer and cannot email them through the Internet.
Digital Signals Computers use digital signals. That means that if you want to use
computers or computational systems (such as mobile phones) to store, analyze,
modify, or play back signals, they need to be in the form of digital signals. The digital
signal records a measurement of the numerical value of a signal at a set of
continuous time intervals. As a result, what began as a continuous analog signal is
now represented by a set of successive numbers. These numbers, however, can then
be stored on a computer and used in many different ways. For example, as the
seismic waves of an earthquake cause the ground to move, a seismometer can
record the motion by writing down the numerical value of the ground height
measured by the seismograph at each successive second. You now have a list of
numbers that show the ground motion, second by second. The disadvantage of
digital signals is that you no longer have the value of the signal in between the time
steps of the recording (such as in between one second and the next on the
seismometer). The advantage, however, is that once you have described the signal
as a set of numbers, not only can you store it on a computer, but you can change the
signal by changing the numbers. Music is stored as digital signals on a CD or DVD.
How do you know this? Because a computer can directly read the CD or DVD, and
computers only read digital signals. Digital signals are more reliable than analog
signals. An analog signal will fade and get scratchy as the signal gets weak, whereas
a digital signal will continue to provide an accurate signal.
[Photo idea: graphics of digital and analog signals [show a list of numbers as well];
records and tape cassettes; CDs, DVDs; seismograph on the ground, seismogram on
paper, and a digital seismogram]
What are Binary Signals?
In theory, you could record the numbers within digital signals in many different
ways. In practice, there is just one way—as a binary signal. A binary signal encodes
information as a string of two possible values, usually written as 1’s and 0’s (or “on”
and “off,” “up” or “down,” or “open” or “closed”). This is because binary signals are
how computers record and process information. The hardware of a computer
consists of a vast number of switches that can either be open or closed. This is why
computers can’t work with analog signals. Fundamentally, they are very simple.
They use an alphabet with only two characters: 1 and 0. Everything that gets
processed by a computer—music, photographs, movies—has to be described just in
terms of 1’s and 0’s. This process is very similar to Morse code. Morse code uses just
two characters: dots and dashes. All of the letters of the alphabet are made by
arranging dots and dashes in different combinations. Binary signals work the same
way.
The process is like setting up a secret code. Each 1 or 0 is called a bit. Bits are
arranged into groups of 8 bits called a byte. Each character of the alphabet is
assigned its own byte. Here are the first letters of the alphabet, as both small and
capital letters. Can you identify the pattern? What do you think will be the byte
representations of “f” and “F?”
a=
01100001
A = 01000001
b=
01100010
B = 01000010
c=
01100011
C = 01000011
d=
01100100
D = 01000100
e=
01100101
E = 01000101
The bytes are attached together into very long strings. For example, the word “bead”
would be written as “01100010011001010110000101100100.” Because each
character gets a byte, 1 megabyte of computer storage can store 1 million characters
(“mega” is the prefix meaning “million”). Numbers are also stored with binary bits.
If this seems to you like an exercise in cryptography (the study of secret codes), you
are more correct than you might imagine. The very first programmable digital
electronic computer was the Colossus, which was used by the Allies during WWII to
successfully decipher coded messages being sent by the Nazi German High
Command.
[Photo Idea: computer circuit; Colossus]
How Are Signals Transmitted?
Have you ever made a telephone out of a piece of string and two cans or cups? If so,
you were transmitting an analog signal. When you spoke into the can, your sound
waves caused the bottom of the can to vibrate, which caused the string to vibrate,
which was tightly connected to the other can and caused it to vibrate, which made
new sounds waves that your friend heard. Of course, it couldn’t reach very far.
Modern information technologies turn original signals, such as your speaking voice,
into electronic signals that can be transmitted great distances. This is what happens
inside a telephone or mobile phone. When you speak into the phone, the sound
waves of your voice are converted into an electronic signal that can then be
transmitted by electric currents or electromagnetic waves, and then converted back
into sound waves at the speaker of another phone. There are many different
technological ways that this can be done.
For analog telephones, the sound waves are directly converted into electronic wave
pulses that represent the sound waves by varying in amplitude and voltage. For
mobile phones, the sound waves are converted into digital electronic signals that
are encoded as binary signals. One the signal is in electronic form, it can be sent to
another location though a combination of electronic signals, when direct wires are
available, or as electromagnetic waves, either through the air or through fiber optic
cables. Other technologies such as radio and television work the same way. The
information is either encoded as analog or digital signals at one location and then
transmitted to another location through many different ways, where it is converted
back into the original signal.
Any kind of information can be encoded in one location and sent to another,
including two-dimensional images, but this has to be done with digital signals. For
example, a picture of the famous painting by Leonardo daVinci, the Mona Lisa, can’t
be sent directly over an electrical wire. However, you can break it up into small
pieces called pixels and send the information for each pixel, one by one. A pixel is
any one of the small dots that together form the picture on a television screen,
computer monitor, or digital photograph. Each pixel is represented by a certain
number of bytes that describe the color and brightness of the pixel. The photograph
is broken down into a long string of 1’s and 0’s that describe every pixel, written out
one after the other in binary code. That string of 1’s and 0’s can then be stored on a
computer, emailed to a friend, or sent to a printer where the binary code is turned
back into an image and printed. You have to be careful in choosing how many pixels
are used to represent the image. For the Mona Lisa, the left image has pixels that are
small enough that it looks like the original painting. For the middle and right images,
the pixels are too large, and you can tell that it is a digital version of the painting.
You can even send three-dimensional representations of objects digitally, and a 3-D
printer can then recreate them in a new location. For example, the sculpture of the
gargoyle on the left of this photo was converted into binary code information using a
3-D digital scanner. A representation of the scan is shown on the computer screen.
This information can then be sent to a 3-D printer anywhere around the world,
where a replica of the sculpture can be reconstructed (as shown on the right) using
a polymer material like plastic. You can even send digital signals to a 3-D printer for
all the pieces of a house (if you have enough plastic!), like the one shown here. The
pieces can then be snapped together to make the house!
There are three primary means of transmitting analog and digital signals from one
location to another: as electric signals through a wire, as electromagnetic signals
through open space, and as electromagnetic signals through fiber optic cables.
Though electric wires are the oldest method, they are still the most common way to
transmit information. They can carry both analog and digital signals. There are over
15 million miles of telephone and telegraph wires used in the U.S. alone, enough to
circle the earth 600 times. In addition, there are a growing number of television
cable wires that send television signals as digital signals. For places that are not
connected by wires, electromagnetic waves can carry analog and digital
information. This information can be relayed anywhere around the earth by
satellites out in space. Most radio signals are sent as electromagnetic radio waves.
However, there are also growing numbers of digital radio stations. Some television
signals are still broadcast as analog high-frequency radio waves, though television
signals are increasingly sent as digital signals, either electronically through cable
wires or electromagnetically from satellites. Fiber optic cables only transmit
electromagnetic waves. The cables are typically made of silica glass, which allows
the light to propagate through them and also keeps the light trapped within them.
[Photo idea: show two kids communicating with a string and cans or cups; pixelated
mona lisas; 3D printer; cable tv cables; fiber optic cables]
Assessment
1. a. Communicate Follow the pattern of the binary code representations of
letters to write out the rest of the alphabet. Write out a five-letter word in
binary code. Swap your coded word with a classmate and then decode their
word.
b. Extrapolate How many characters could be represented with a Gigabyte
of computer storage?
c. CCC (Nature of Science) Describe how the ability to vary voltage in an
electric circuit changed how people communicated.
2. a. Evaluate What problem could arise if the time interval in a digital signal is
too large?
b. CCC (Connections to Engineering and Technology) Why might fiber
optic cables be advantageous for transmitting information under water?
Study Guide
2.2 Signals
How Is Information Sent As Signals? The energy of information is transmitted as
wave pulses that can take the form of electronic signals or electromagnetic signals.
What Are Digital and Analog Signals? Analog signals are continuous functions that
represent physical quantities; digital signals represent these physical quantities
with a discrete set of measurements at regular time intervals.
What Are Binary Signals? Binary signals are discrete signals that can have only
two possible values, usually represented as 1’s and 0’s.
How Are Signals Transmitted? Signals are transmitted as either electric currents
through wires or as electromagnetic waves through open space or through fiber
optic cables.