Written by Robin Johnson and Paul Heney
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.
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.
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 wave the 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,
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.
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