When
you make a long-distance phone call, your voice is transformed
from sound waves to electrical current to light waves to radio
waves and back to sound waves again as it travels to its destination.
Millions
of far-flung families were unable to celebrate together during
the recent holiday season. Yet, they still wished each other
well and caught up on family news via the telephone, placing
millions of long-distance calls over billions of miles of circuits.
Americans dial 1-plus-area-code-plus-number so often, we don't
even think about it. But a closer look will reveal that during
these cross-country journeys, the human voice is transformed
many times.
For
example, eight-year-old Junior Smith of Small Town, Georgia,
has just put aside his SuperNintendo for a minute to say hello
to his grandmother in San Francisco. The grandmother, Belle,
is driving in her red convertible on the way to an all-day rock
concert. Mom dials Belle's car phone, and after turning down
the volume on her Guns N' Roses tape, Belle answers. Mom hands
the phone to Junior. "Hi, Granny," he says. (He calls
her Granny because she hates it.)
Then
a really amazing thing happens. At that moment, seemingly instantaneously,
on the other side of the continent, Belle hears Junior's voice
say "Hi, Granny." She doesn't just hear the words
"Hi" and "Granny" recreated by a machine;
she hears the unique sound and expressive tone of her grandson's
voice, as if he were sitting beside her in the car. How did
the words get to her?
The
Physics of the Spoken Word
When
Junior spoke, he made his vocal cords vibrate by expelling air
through them. As they vibrated, the cords produced sound waves
in the air.
If
you drop a pebble in a pond, it creates a circular wave that
ripples through the water. Similarly, a sound wave is a pressure
wave that ripples through the air (or other substance), spreading
out in all directions from the source and exerting a force on
objects in its path.
The motion
of Junior's vocal cords set up an alternating pattern of compression
and expansion in the surrounding air. After being kicked by
the cords, the moving molecules of air kicked neighboring molecules
into motion, so that the pattern of movement was communicated
through the air, domino-style. This pattern of movement was
the sound wave.
The
sound wave's frequency (that is, the rate at which the pattern
of compression and expansion repeated itself) was determined
by how quickly Junior's vocal cords vibrated
back and forth.Frequency affects the pitch of a sound.As Junior
spoke, muscles in his throat contracted or relaxed to change
the tension in his vocal cords, which changed their rate of
vibration, which changed the pitch of his voice. At the same
time, Junior changed the shape of his throat and mouth to form
the sounds we call words.
To
transmit and recreate the sound waves Junior produced, the telephone
system used a series of transducers, devices which convert one
type of signal into another. On its way from Georgia to San
Francisco, the sound of Junior's voice took the form of electric
current, light waves, and radio waves. All these signals traveled
at fantastic speed, moving through wires, optical fibers, and
air. A variety of receivers and transmitters picked up and passed
on the signals as they traveled.
Translating
Signals
As
Junior spoke into the telephone, a microphone in the handset
responded to the sound waves of his voice and delivered an equivalent
electric signal. The microphone in Junior's telephone is a carbon
button mike, which contains a packet of carbon granules through
which an electric current flows. The amount of current depends
on how tightly the granules are packed. A thin diaphragm linked
to the packet compresses and expands the granules and varies
the flow of the current.
Like
puffs of wind blowing against a sail, the sound waves from Junior's
voice set the diaphragm in motion, which, in turn, pressed on
the carbon granules. As the granules were pressed together and
released, the current in the microphone changed in response
to the sound wave. This changing pattern of electrical current
was the signal transmitted through the wires by the telephone.
Physically,
the current consisted of the movement of electrons in the metal
telephone wire. So, at this point in the call, the pattern of
the motion of Junior's vocal cords, converted into the motion
of the air, and then converted again into the motion of the
diaphragm, had resulted in a corresponding pattern in the motion
of electrons in a
telephone wire.
Going
Digital
Initially,
the electrical signal was an analog signal. That is, the pattern
of rising and falling electrical current corresponded directly
with the rising and falling pressure of the sound wavethe electrical
signal was analogous to the sound wave. However, soon after
leaving Junior's house, the electrical signal reached a nearby
collection point where it was converted from an analog signal
to a digital signal.
The
analog signal mimicked the original sound pattern. The digital
signal, on the other hand, consisted of a series of discrete
electrical pulses that, taken together, described the analog
pattern in binary code. Binary code, the mathematical language
used in computer programming, uses a binary or "base two"number
system. Just as Morse code uses various combinations of long
and short beeps to represent letters, this system uses only
two digits, 1 and 0, to represent all numerical values.
The
digital conversion is made because digital networks have many
advantages over analog systems. For one thing, the distinct
pulses can be transmitted more accurately than the infinitely
varying analog signal. In addition, the binary code "shorthand"
lets digital networks carry more information at one time than
analog.
To
make the conversion, the analog-to-digital converter in Junior's
neighborhood sampled the incoming analog signal eight thousand
times a second. For each sampled value, the converter generated
a series of electrical pulses that represented the value in
binary code, with high signals represented as 1s and low signals
represented as 0's.
Moving
onto the Information Highway
After
being digitized, Junior's message was routed to the local ex-change
office, or what the telephone company calls the central office
(CO). At the CO, digital switches routed Junior's signal on-to
the most efficient path through the network of switches in cities
between Georgia and San Francisco.
For
Junior's call, that path included copper cable, optical fiber
cables, and cellular radio links. If Junior's call had taken
another path to California, it could have traveled via microwave
radio links as well. If Junior were making an international
call, the path might have also included satellite transmissions
or submarine cables.
First,
Junior's call was routed onto a high-capacity telephone cable,
which carries multiple conversations simultaneously on one line.
A device known as a time-division multiplexer routed the signals
from multiple calls onto the cable and inserted routing signals
that allowed the mingled calls to be separated by a de-multiplexer
once they reached their destination.
Seeing
the Light
For
Junior's call, the next destination was a fiber-optic conversion
station. Here, the digital electrical signal that represented
Junior's message was converted into pulses of light produced
by a light-emitting diode (LED).
Fiber-optic
cables are smaller, lighter, more economical, and more noise
resistant than their copper counterparts, and they carry more
information more quickly. By using different frequencies of
light to transmit different calls, more than twenty calls can
be sent simultaneously through a single fiber one-tenth of a
millimeter in
diameter.
Like
an electrical signal, a light signal owes its existence to moving
electrons. Light, like microwaves, radio waves, and X-rays,
is a type of electromagnetic wave. Such waves are produced when
electrically charged particles, such as electrons, jump about
and radiate energy. The type of electromagnetic wave produced
depends on the amount of energy per jump. In the case of an
LED, the incoming electrical pulses cause electrons to jump
into a higher energy state, then fall back to their starting
level, giving off the extra energy in the form of visible light
or infrared rays.
Once
produced, Junior's light pulses traveled through the hair-thin
glass fibers at about 123,000 miles per second (two-thirds the
speed of light in a vacuum) to another conversion station on
the opposite side of the country. There, the light pulses were
decoded back into electronic digital signals.
Making
Waves
The
electronic signal made its way through telephone cables to the
central office serving Granny. Granny's CO routed the call to
a digital switching center operated by her cellular telephone
company. This central switch, or base station, regularly routes
calls via land-based telephone cables to the cluster of high-frequency
radio towers that serve Granny's area. Towers are located two
to ten miles apart, and each tower transmits calls only to its
immediate area, or cell.
When
it received Junior's call, the base station sent a signal to
all the towers to let them know it was looking for Granny's
car phone, kind of an electronic All-Points-Bulletin. Each tower
then broadcast this signal to its area over a special set-up
channel, asking in essence, "Hello, are you there?"Granny's
car phone, constantly listening for such a call to come in over
the set-up channel, sent back an "I'm here" signal
to the tower transmitting the strongest signal the tower closest
to Granny's car. With that response, the base station knew to
send the incoming call via that radio tower.
Once
it reached the appropriate tower, the digital electrical signal
representing Junior's message was converted back to an analog
signal, then converted into high-frequency radio waves.
Remember
the light waves that were produced when an electron jumped around?
Radio waves are also types of electromagnetic waves, which are
produced when electrons move and radiate energy. Any changing
electrical current gives off electromagnetic waves, but antennas
and transmitters are designed to control and direct the waves
that are produced. By feeding an electrical signal of the right
frequency to an antenna, engineers can generate and transmit
a radio frequency signal that moves away from the antenna into
the air, or even into space.
In
the radio tower, a radio frequency generator produced a regular
pattern of radio waves. This pattern, the carrier signal, was
then modulated by Junior's incoming message signal. In other
words, Junior's signal was superimposed onto the carrier signal.
The composite signal was amplified and broadcast by the antenna
tower
toward Granny's car.
Back
to Sound
As
Granny moved down the freeway, the radio signals were intercepted
by her antenna, where they set electrons in motion, to produce
the electrical signals identical to the ones that Junior's voice
produced in the telephone back in Georgia. These signals ran
into the receiver located in Granny's telephone and traveled
through a coil of wire in the phone's earpiece. The wire was
wrapped around a small magnet to form an electromagnet.
Any
electric current generates a magnetic field, which varies in
strength as the current varies. If iron is placed inside a current-carrying
coil of wire, the resulting magnetic field is even stronger,
and the combination is called an electromagnet.
The
magnetic force produced by the electromagnet in Granny's phone
changed in strength as the incoming signal changed. The changing
force acted on a nearby metal armature, causing it to rock back
and forth. The armature was attached to a diaphragm that moved
the air and generated the sound waves that traveled out of the
phone and into Granny's ear.
These
sound waves mimicked the ones Junior set in motion in Georgia
an instant earlier. They set Granny's middle-ear structures,
inner-ear fluids, and finally her inner-ear membrane and hair
cells into motion, which produced an electrical impulse in her
brain, so that she heard the words "Hi, Granny" in
Junior's voice.
On
the cross-country trek, the pattern of motion of Junior's vocal
cords had been transformed into sound waves in the air, the
mechanical vibrations of a diaphragm, the movement of electrons
in metal cable, pulses of light, radiated radio waves, a varying
magnetic field, and finally back to sound waves just like the
ones his own
vocal cords produced. All in a heartbeat.
And
Granny thinks that SuperNintendo is complicated!
Originally
published in Exploring Magazine, Vol.16, No. 4 by The
Exploratorium, San Francisco, 1994. Written by Robin Johnson
and Paul Heney. Duplicated with permission of the authors and
the Exploratorium.
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