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Although we take it completely for granted, the telephone you have in your house is one of the most amazing devices ever created. If you want to talk to someone, all that you have to do is pick up the phone and dial a few digits. Instantly you are connected to that person and you can have a two-way conversation with him or her. The telephone network extends worldwide, so you can reach nearly anyone on the planet. When you compare that to the state of the world just 100 years ago, when it might have taken up to several weeks to get a one-way written message to someone, you realize just how amazing the telephone is!
In this edition of How Stuff Works we will look at the telephone device that you have in your house, as well as the telephone network it connects to. You can also explore a number of links to get into things in incredible detail!
The Telephone
Surprisingly, a telephone is one of the simplest devices you have in your house. It is so simple because
the telephone connection to your house has not changed in nearly a century. If you
have an antique phone from the 1920s, you could connect it to the wall jack in your house and it
would work fine!
The very simplest working telephone would look like this inside:
As you can see, it only contains 3 parts and they are all simple:
The only problem with the phone shown above is that when you talk you will hear your voice through the speaker. Most people find that annoying, so any real phone contains a device called a duplex coil or something functionally equivalent to block the sound of your own voice from reaching your ear. A modern telephone also includes a bell so it can ring and a touch-tone keypad and frequency generator. A "real" phone looks like this:
Still, it's pretty simple! In a modern phone there is an electronic microphone, amplifier and circuit to replace the carbon granules and loading coil. The mechanical bell is often replaced by a speaker and a circuit to generate a pleasant ringing tone. But a normal $6.95 telephone that you buy at Wal-Mart remains one of the simplest devices ever.
Creating Your Own Telephone Network
Not only is a telephone a simple device, but the connection between you and the phone company is
even simpler. In fact, you can easily create your own intercom system using two telephones,
a 9-volt battery (or some other simple power supply) and a 300-ohm resistor that you can get for
a dollar at Radio Shack. You can wire it up like this:
Your connection to the phone company consists of two copper wires. Usually they are red and green. The green wire is common, and the red wire supplies your phone with 6 to 12 volts DC at about 30 milliamps. If you think about a simple carbon granule microphone, all it is doing is modulating that current (letting more or less current through depending on how the sound waves compress and relax the granules), and the speaker at the other end "plays" that modulated signal. That's all there is to it!
The easiest way to wire up a private intercom like this is to go to a hardware or discount store and buy a 100-foot phone cord. Cut it, strip the wires and hook in the battery and resistor as shown. [Most cheap phone cords contain only two wires, but if the one you buy happens to have 4, then use the center two.] When two people pick up the phones together, they can talk to each other just fine. This sort of arrangement will work at lengths up to several miles apart.
The only thing your little intercom cannot do is ring the phone to tell the person at the other end to pick up. The "ring" signal is a 90-volt AC wave at 20 hertz. [Fun fact - the hand crank on old-fashion telephones was used to generate this AC wave and ring the bell at the other end!]. The "Telephone Stuff" link in the link section below has a discussion of how to generate the ring signal.
Calling Someone on the Telephone Network
If you go back to the days of the manual switchboard, it is easy to understand how the larger phone system
works. In the days of the manual switchboard, there was a pair of copper wires running from every house
to a central office in the middle of town. The switchboard operator sat in front of a board with one jack
for every pair of wires entering the office. Above each jack was a small light.
A large battery supplied current through a resistor to each wire
pair (in the same way you saw in the previous section). When someone picked up the handset on his or her
telephone, the hook switch would complete the circuit and let current flow through wires between the
house and the office. This would light the light bulb above that person's jack on the switchboard.
The operator would connect his/her handset into that jack and ask who the person would like to talk to.
The operator would then send a ring signal to the receiving party and wait for the party to pick
up the phone. Once the receiving party picked up, the operator would connect the two people together
in exactly the same way the simple intercom above was connected! It is that simple!
In a modern phone system the operator has been replaced by an electronic switch. When you pick up the phone, the switch senses the completion of your loop and it plays a dial tone sound so you know the switch and your phone is working. The dial tone sound is simply a combination of 350 hertz tone and a 440 hertz tone and it sounds like this:
[For more information on tones, see the HSW article entitled How a Guitar Works.] You then dial the number using a touch-tone keypad. The different dialing sounds are made of pairs of tones, as shown here:
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A typical number that you dial sounds like this:
If the number is busy, you hear a busy signal that is made up of a 480 hertz and a 620 hertz tone, with a cycle of 1/2 second on and 1/2 second off, like this:
Telephone Bandwidth
In order to allow more long-distance calls to be transmitted, the frequencies transmitted are
limited to a bandwidth of about 3,000 hertz. All the frequencies in your voice below
400 hertz and above 3,400 hertz are eliminated. That's why someone's voice on a phone has a
distinctive sound. Compare these two voices:
You can prove that this sort of filtering actually happens by using the following sound files:
Call up someone you know and play the 1,000 hertz sound file on your computer. The person will be able to hear the tone clearly. The person will also be able to hear the 2,000 and 3,000 hertz tones. However, the person will have trouble hearing the 4,000 hertz tone, and will not hear the 5,000 or 6,000 hertz tones at all! That's because the phone company clips them off completely.
The Telephone Network
The telephone network starts in your house. A pair of copper wires runs from a box at the road to
a box (often called an "entrance bridge") at your house. From there the pair of wires is connected
to each phone jack in your house (usually using red and green wires).
If your house has two phone lines, then two separate pairs of copper wire
run from the road to your house. The second pair is usually colored yellow and black inside your house.
Along the road runs a thick cable packed with 100 or more copper pairs. Depending on where you are located, this thick cable will run directly to the phone company's switch in your area, or it will run to a box about the size of a refrigerator that acts as a digital concentrator. The concentrator digitizes your voice at a sample rate of 8,000 samples per second and 8-bit resolution (See the HSW article entitled How CDs Work for information on digitizing sounds). It then combines your voice with hundreds of others and sends them all down a single wire (usually a coax cable or a fiber-optic cable) to the phone company office. Either way, your line connects into a line card at the switch so you can hear the dial tone when you pick up your phone.
If you are calling someone connected to the same office, then the switch simply creates a loop between your phone and the phone of the person you called. If it's a long-distance call, then your voice is digitized and combined with millions of other voices on the long-distance network. Your voice normally travels over a fiber-optic line to the office of the receiving party, but it may also be transmitted by satellite or by microwave towers.
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How Stuff Works (http://www.howstuffworks.com) is a production of BYG Publishing, Inc. http://www.bygpub.com - questions@bygpub.com P.O. Box 40492 - Raleigh, NC 27629 |
© 1998 BYG Publishing, Inc. All rights reserved.
