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A simple phone call

The phone is one of the simplest devices in your home. It is so simple that the phone connection in your home has not changed for nearly a century. If you have an antique phone from the 1920s, you can connect it to a socket on the wall of your home and it will still work!

The simplest internal structure of a working phone is shown in the following figure:


As you can see, there are only three parts inside, and they are all very simple:

A switch that decides to connect and disconnect from the network is generally referred to as a spring switch. When you pick up the receiver, it starts connecting to the network.
A speaker, this is a regular 50 cent, 8-ohm small speaker.
In the past, the construction of a telephone amplifier was very simple, consisting of two thin metal plates sandwiching carbon particles. The sound waves when you speak constantly apply pressure and reduce pressure on the carbon particles, changing their impedance and thereby regulating the current flowing through the amplifier.
It's that simple! You can make this simple phone call by quickly tapping the spring switch, and all phone switch converters will still recognize this' pulse dialing '. If you pick up the microphone and quickly tap the spring switch four times, the telephone company's switch converter will know that you have dialed the number "4".

The real phone

The only problem with the simplest telephone implementation is that when speaking, one can still hear their own voice through the speaker. Most people find this kind of echo annoying, so any "real" phone includes a device called a double-layer coil or other equivalent device to block your own voice from entering your ear. Modern telephones also include telephone rings that can make the phone sound, button style keyboards, and frequency generators. The 'real' phone is shown in the following picture:


Modern telephones also include telephone rings that can make the phone sound, button style keyboards, and frequency generators
Nevertheless, it is still quite simple. In modern telephones, there is an electronic amplifier and a circuit that replaces carbon particles and load coils. The original mechanical ringtones are usually replaced by speakers and circuits that can produce pleasant tones. However, this $6.95 telephone is still one of the simplest devices in history.

Telephone network: wires and cables

Your house is the starting point of the telephone network. By using a pair of copper wires, the roadside telephone adapter can be connected to your home telephone adapter (usually referred to as a bridge). Then, these wires are connected from the telephone adapter to the earpiece jack in your home (usually using red and green wires). If you have two telephone lines at home, there will be two independent pairs of copper wires running from the roadside to your home. In general, the second pair of copper wires at home are yellow and black, respectively. (For a description of the phone adapter and wires you saw on the roadside, please refer to what the small box installed by the telephone company in our residential area is for.)

Ordinary telephone adapter
The ordinary telephone adapter set up by the telephone company you saw on the roadside.
There will be a bundle of cables along the highway, including 100 or more pairs of copper wires. Depending on the location where you place this large bundle of cables, it will directly reach the switch converter of the telephone company in your area or enter a refrigerator sized box, similar to a digital hub.

Telephone Network: Digitization and Transmission

The hub will digitize your voice at a sampling rate of 8000 samples per second and an 8-bit resolution (for information on digitized sound, please refer to the principles of analog and digital recording). Then mix your voice with the voices of many others, and send them all along a wire (usually a coaxial cable or fiber optic cable) to the telephone company's office. Anyway, connect your phone line to the line card of the switch converter so that you can hear a dial tone when you pick up the phone.

If you want to call other people in the same office, the switch converter only forms a loop between your phone and the called person's phone. If it is a long-distance call, your voice will be digitized and mixed with millions of other voices in the long-distance network. In general, your voice will be transmitted to the recipient's office through fiber optic telephone lines, but it may also be transmitted through artificial satellites or microwave transmission towers. (For more detailed descriptions, please refer to the working principle of long-distance calls.)

Create your own telephone network

The telephone is a simple device, making the connection between you and the telephone company even simpler. In fact, you can build your own internal communication system using the following materials: two telephones, a 9-volt battery (or some other simple power source), and a 300 ohm resistor that can be purchased for just one dollar from a chain store. Wiring can be done in the following way:

Your connection with the telephone company relies on two copper wires. They are usually red and green respectively. The green line is a public line, and the red line provides approximately 30mA, 6V~12V DC voltage for telephones. If you are curious about a simple carbon particle amplifier, we can tell you that its function is to modulate the current (determining how much current passes through the particles based on the degree of compression or relaxation of sound waves), while the speaker "plays" the modulated signal at the other end. That's all

Manual power generation!
Do you know the hand crank on those old-fashioned phones? It is used for hand cranking electricity to generate ringtone signals and AC radio waves, causing the ringtone on the other side of the phone to ring!
The simplest way to wire a private internal communication system similar to the one mentioned above is to buy a 30.5-meter-long telephone line from a hardware store or discount store. Cut it off, peel off the plastic skin of the wire, and wrap it around the battery and resistor, as shown in the above picture. (Most cheap phone lines only contain two wires, but if you buy exactly four wires, please use the middle two.) When two people pick up the phone at the same time, they can talk to each other. The effective distance of this circuit can reach several kilometers at most.

The only thing your small internal communication network cannot do is make the phone sound to notify the person on the other end to pick up the phone. The "ringtone" signal is a 20 hertz, 90 volt alternating current wave.

Working mode of manual wiring switchboard

If we go back to the era of manual switchboard, it is easy to understand the working principle of large-scale telephone systems. In the era of manual switchboard, every household had a pair of copper wires connected to the telephone headquarters located in the town center. The switchboard operator sits in front of a panel with sockets, and each pair of wires entering the headquarters will have a socket.

There is a small light on each socket. A large current provided by the battery flows through the resistor to each pair of wires (in the same way as you saw in the previous section). When someone picks up their phone receiver, the spring switch will close the circuit, and the current will flow to the wire between the home and the telephone headquarters. This will light up the light bulb on the socket representing his home on the wiring switchboard. The operator will connect his headphones to the socket and ask him who he wants to call. Next, the operator will send a ringtone signal to the recipient and wait for them to answer the call. Once the recipient picks up the phone, the operator connects both parties in exactly the same way as the internal communication described on the previous page! It's that simple!

Dial tone

In modern telephone systems, operators have been replaced by electronic switches. When you pick up the phone, the switch senses the circuit is closed and emits a dial tone, so you know that both the switch and the phone are working properly. The dial tone is just a mixture of 350 Hz and 440 Hz tones, which sounds like:

Click here to listen to the dial tone
(For more pitch information, please refer to the working principle of acoustic guitar)

Then, you can use the button style keypad to dial. Different dial tones are composed of different tone pairs, as shown in the table below:


1209 Hz
1336 Hz
1477 Hz
697 Hz
one
two
three
770 Hz
four
five
six
852 Hz
seven
eight
nine
941 Hz
*
0
#
The sound of your dialing is like this:

Click here to listen to the button sound
If the other party is busy on the phone, you will hear a busy tone, which consists of 480 Hz and 620 Hz tones. It rings for half a second and stops for half a second, repeating this cycle. It sounds like this:

Click here to listen to busy tones
Telephone bandwidth

In order to transmit more long-distance calls, the frequency bandwidth for transmission is limited to around 3000 hertz. All sounds below 400 Hz and above 3400 Hz in your voice have been filtered out. That's also why some people's voices sound a bit special on the phone. Compare the following two sounds:

Click here to listen to normal sound.
Click here to listen to the voice of the same person on the phone.
You can find that the phone does filter out some frequencies of sound through the following sound files:

1000 Hz pitch
2000 Hz pitch
3000 Hz pitch
4000Hz pitch
5000 Hz pitch
6000 Hz pitch
Call someone you know and play a 1000 Hz sound file on your computer. He should be able to hear this tone clearly. He can still hear tones of 2000 and 3000 hertz. However, he will have difficulty listening to 4000 hertz tones, and he simply cannot hear 5000 or 6000 hertz tones! That's because the phone company has completely deleted them.

Regarding phone calls