How Does The Telephone Work?

Now, let's get to what is inside the telephone. Remember that these components described are commonly found in the older traditional telephones. We are providing this information to lend an historical perspective. Most of today's telephone systems substitute electronic components to emulate these functions.

1. The transmitter. ( Figure 1) The transmitter is the ear of the telephone in that it "hears" the voice of the person speaking into it. The transmitter is a miniature carbon pile rheostat. A rheostat is a device that controls an electric current by varying the resistance in the circuit, similar to the action of a dimmer switch control. The variations in sound pressure from the voice vibrating against the diaphragm change the compression of the carbon granules. This varies the resistance of the transmitter. The transmitter has two contacts that are insulated from each other. Current can only flow through the carbon granules. As sound pressure from the voice presses against the diaphragm, the carbon is more closely compressed within the chamber. Compressing the carbon granules lowers the resistance of the transmitter resulting in more current flow through the transmitter circuit. When the pressure on the diaphragm is released, it momentarily snaps out farther than its original position. The carbon is under less pressure than normal and the resistance of the transmitter is momentarily greater. The current flow decreases.

   Figure 1: The Transmitter

   The diaphragm of a transmitter is made of lightweight phosphor bronze, duraluminum or a similar material. Either an extra inner cone of the same material strengthens the center or it is corrugated to act as a stabilizer. The flexible outer edge is securely clamped in the transmitter housing. This design enables the diaphragm to move in and out at the center like a piston. Since the diaphragm is sensitive to sound waves, the carbon granules are compressed and released as the corresponding pressure from the sound wave's changes.

   The telephone transmitters in use today are, in principle, like the ones invented more than 100 years ago by Thomas Edison. Many modern electronic telephones use real microphones connected to related speech processing equipment to vary the line current. Small microchips allow economy and space saving, enabling inexpensive, high quality "throwaway" telephones. The output now generated by microchip-based telephones must emulate the same variations created by the carbon granule type of transmitter.

   What is known as the basic 500 set, a single-line telephone like the one that was in use in most homes, has dictated the industry's electrical standard for the telephone instrument and all related signal processing equipment.

   All types of 2- and 4-wire circuits are still designed around that 500 set.

2. The receiver { Figure 2). The receiver is the "mouth" of the telephone in that it speaks into the ear of the person using the telephone. It also contains a diaphragm whose movement is caused by the strengthening and weakening of the field created by the magnet within the receiver. The receiver converts the varying electrical current representing the transmitted speech signal to variations in air pressure perceived as sound by the human ear. An electromagnetic receiver consists of coils of many turns of fine wire wound on permanently magnetized soft iron cores that drive an armature. The armature is a diaphragm made of a soft iron material.

   Figure 2: The Receiver

   When someone speaks a word into a transmitter, the current flow in the circuit is alternately increased and decreased as the moving electrode moves in and out of the carbon chamber. A requirement for an electromagnetic receiver is a permanent magnet to provide a constant bias field for the varying electromagnetic field to work against. Otherwise, both positive and negative currents would push the armature in the same direction. The varying electrical current representing speech flows through coils and produces a varying electromagnetic field. It alternately aids and opposes the permanent magnetic field; thus, it alternately increases and decreases the total magnetic field acting on the diaphragm. This causes the diaphragm to vibrate in step with the varying current and moves the air to reproduce the original speech that caused the current changes. Other types of receivers operate similarly, except that the armature is a separate part and is connected to a conical non-magnetic diaphragm. The rocking action of the armature causes the aluminum diaphragm to vibrate to reproduce the original speech. In some telephones this receiver is created with the use of microprocessors.

   The electromagnetic receiver was a central element of Alexander Graham Bell's original telephone patent.

   Part of the design of the telephone handset that enables you to hear your own voice while talking is called side tone or side noise. The reason for this is to give you some feedback that the telephone is working. Too much side tone causes an echo.

3. The ringer. There is a wide variety in types of ringers. Telephones run on DC (direct current) where electrons flow in one direction. The bell or ringer operates on AC (alternating current), which means that electrons are moving in two different directions to activate the bell. This AC sent on the local loop (telephone line) is called ring generator (90 to 105 volts AC at 20-Hz). Minus 48 volts DC is always on the line, which is used to operate the telephone after is answered.

   There is a good analogy for understanding these electrical signals. Envision a garden hose. The hose represents the wire. The water is the current. The water pressure is the voltage (electrical pressure). Stepping on the hose with your foot is equivalent to resistance on an electrical circuit.

4. Microprocessors. The microprocessors in electronic telephones may replace any of the above internal components and may also add additional capabilities and functions to the telephone, such as speed dialing, etc.

Many telephones look the same, but there is wide variation in the capabilities and prices. You can buy a throwaway single-line telephone for less than ten dollars or a multi-line multi-featured telephone to work with a business telephone system for six hundred dollars. As with many manufactured items, there is variation in the quality of the components that is reflected in the price. The price also tends to be higher on the proprietary telephones, which work with a specific manufacturer's system, even though they may look the same as those you, buy in your neighborhood telephone store.

Parts Of The Telephone

First, we will point out the things you can see, and next we will get to what is inside. Here is a diagram of a typical telephone with its external parts labeled.

Figure 1: Parts of the Telephone

1. The telephone handset, also called the receiver. In fact, it includes both the receiver enabling you to hear and the transmitter through which you speak. It may also have a volume control or a bar that you can depress to mute either the receiving or transmitting capability. (Buttons on the telephone may also control these functions.)

   Handsets come in different shapes and sizes and are usually made to work with a telephone from a specific manufacturer.

   The handset may be directly wired (also called hardwired) to the telephone cord, which in turn is directly wired into the telephone instrument, or there may be a plastic modular connector at one or both ends of the cord.

   People who spend the entire day on the telephone such as customer service representatives, stock brokerage traders and switchboard attendants often use a headset instead of a handset. On many telephones, you must leave the handset attached and "off the hook" while using a headset, which is cumbersome.

2. The handset cord. Also known as the curly cord. This often gets very twisted which can break or damage the wires inside causing interference (static or "noise" on the line). Holding it up and letting the handset dangle at the end, enabling the cord to unwind, can straighten it out. As mentioned above, most handsets are connected to the telephone with a small plastic modular connector that plugs into a jack opening on the telephone. Some handsets are hardwired into the telephone and cannot be unplugged.

3. The mounting cord. A straight cord (cable), usually gray or a translucent gray called silver satin. Typical lengths are 6 feet, 9 feet, 13 feet and 25 feet. This cord sometimes has a modular connector at each end, one plugging into a jack opening on the telephone and the other plugging into a jack opening in the wall. In some cases the mounting cord is wired directly to the telephone or the wall or to both and cannot be unplugged.

4. The dial pad. Also called the keypad, touch-tone pad, touch-tone buttons or DTMF pad ("DTMF" stands for dual tone multi-frequency, referring to the touch-tone signals). Most telephones use the DTMF method for sending a telephone number to the telecommunications service provider. The local telephone company central office and the business telephone system (PBX) must have the capability to process these tones. The telephone is equipped with the dial pad having 12 buttons that represent the numbers 0 through 9 and the symbols * and #. Pressing one of the buttons causes an electronic circuit to generate two tones. There is a low-frequency tone for each row and a high-frequency tone for each column. Pressing button number 5, for example, generates a 770-Hz tone and a 1,336-Hz tone. By using this dual tone method, only seven tones produce 12 unique combinations. The frequencies and the dial pad layout have been internationally standardized, but the tolerances for variations in frequencies may vary in different countries.

   The touchtone signals are used not only to dial telephone numbers, but also to interact with Voice Processing systems such as Voice Mail, Automated Attendant. (for Sales press 1, for Service press 2, etc.) and Interactive Voice Response.

   Some telephones may have a round rotary dial, but this is becoming less common. The signals sent out by a rotary dial telephone are called dial pulse.

   Rotary dial telephones are still in use. In general, their dial pulses are not recognized by voice processing systems. Some of the newer voice processing systems use voice recognition (Say "Yes" for the Sales Department.)

5. The feature buttons, also known as feature keys or function keys. These can serve a variety of functions. They enable different outside lines and extensions to be answered. They may activate telephone system functions such as call transfer, call conferencing, call forwarding, etc. They can also be used to speed dial frequently called numbers. Every telephone system manufacturer treats these feature buttons differently, so what you learn about one system may not apply to another. Some feature buttons are flexible, meaning that they can be programmed for a variety of functions. Some are fixed, meaning they can provide only a specific function. Some systems have soft keys meaning that the same button performs different functions at different times.

6. The display, also known as the LCD (liquid crystal display). Not all telephones have displays, although most newer ones do. All business telephone system manufacturers provide them, but the display telephones may cost more. In most cases they are worth the investment since operating the telephone on a business telephone system without the benefit of the information provided in the display can be cumbersome. Different systems provide different information in the display. Some show the date and time when the telephone is not in use. Some provide instruction prompts to the person attempting to use system functions. Most show the name or extension number of the person calling you, if the call is coming from someone else within your office. Some show the name or telephone number of the person calling you from another location (known as Caller ID or ANI - automatic number identification - if this information is being delivered to your PBX over your outside lines.). A few systems enable you to leave a pre-selected message so that when someone calls your telephone from within your office, his display will read that you are "out to lunch" or "in a meeting." Other systems enable a secretary to send a silent message to the boss while the boss is on another call, although this is not common. As with the feature buttons, the important thing to remember is that telephones and telephone systems from different manufacturers use the display differently. No two are exactly the same.

7. Lights (also called lamps or LEDs - light emitting diodes) On some systems there may not be a light, but an LCD (liquid crystal display) indicator instead. Most people find the lights easier to see than the LCD. The purpose of the light or LCD is to indicate the status of a call in progress on one of the outside lines or extensions. The light may be red, green, white or amber and more than one color may be lit at the same time. This differs considerably depending upon the manufacturer and model of the system. A light flashing on and off slowly may indicate a new incoming call and is sometimes accompanied by an audible ring. If the same extension appears on more than one telephone it may simply flash at some telephones and audibly ring on others. A steady light usually indicates that the line is in use on either your telephone or another telephone that picks up the same line. A rhythmically flickering light may indicate that a call is on hold. Some telephones have a light next to the button of the extension number that remains lit even when the telephone is not in use.

8. The switchhook. This refers to those two little plastic buttons that press down on a conventional telephone when you hang up the receiver. When you hang up you are actually breaking an electrical circuit that connected you to the person at the other end while you were talking. On some telephones, the switchhook may be a single bar that depresses when you hang up. Other telephones have a magnetic switchhook inside the telephone, directly under the receiver when it is hung up, that cannot be seen from the outside. In very old movies, we often see someone frantically tapping the switchhook trying to get help as the intruder is banging on the front door. This method was once used to reach an "operator" or switchboard attendant. Instead you would now dial 0. In telephone systems introduced in the 1960's and 1970's, the switchhook was used as a means of activating the system functions such as call transfer. If you held the switchhook down for a second too long, you'd disconnect the call! These functions are now accomplished more easily with feature buttons.

9. Speaker. Most multi-line and a few single-line telephones are equipped with some type of speaker. A speakerphone enables the person using the telephone to have hands-free conversation with another person at a distant location without lifting the handset. Some speakers are one way only. This may be called a monitor rather than a speakerphone, which is two-way. The monitor enables the person using the telephone to dial out or wait on hold without lifting the handset. They can hear what is on the open line, but cannot speak back to the caller without picking up the handset. Another capability, also called monitoring, enables others in the room to hear both sides of a telephone call in progress, while the person in the room who is speaking uses the handset and the caller at the other end does not sense that he is on a speaker. This is sometimes used for training staff members on how to handle particular types of calls. Speakers may also be used for internal intercom communication only, where someone in on the same premises can call you and his voice will be projected over the speaker. Some systems will allow you to answer back hands-free while others will not. Not surprisingly, this feature is known as hands free answer back!

10. Message waiting indicator. If the system is working with a Voice Mail system, this lamp or LCD indicator lets you know that you have a message waiting in your Voice Mailbox. There may also be a message on the display of the telephone such as MW for "message waiting." Any of these may also indicate a message waiting at the reception desk or message desk if there is no Voice Mail. Although this use is less common most telephone systems enable a receptionist to manually activate a message-waiting indicator on any telephone in the system. In some systems, the message waiting indicator is a button that, when pressed, will automatically connect you to the Voice Mail system or receptionist to retrieve your messages.

11. Base of the telephone; telephone housing. This is generally a molded plastic casing designed to house a specific type of telephone.

12. Faceplate or Face Layout. Most telephones that work with business telephone systems enable you to print a layout of the front (face) of the telephone including the extension numbers and system features that correspond to each button. This printed layout may slip into place over the buttons and under a clear plastic cover that is often called the faceplate.