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Theme 1 Telephones and Mobile Phones Unit 4 4.1 Unit 4 Telephones (fixed-line) (Rev 1.0) Telephones (fixed-line) Simple Telephone Set The block diagram of a simple telephone set is shown as below. It consists of a mouthpiece (microphone), an earpiece (speaker), a switch hook, a dialer, and a ringer. The characteristics will be described in later sections. Normally, it doesn’t require any extra electricity (like battery) to make/receive calls. Based on the dialing technique, there are two types of telephone sets: pulse dialing and tone dialing. Normally, all tone dialing phones also support pulse dialing features. The user can make the selection at the side or at the back of the phone (to “T” or “P” positions). Picture of a Simple Telephone Set (Pulse Dialing) Picture of a Simple Telephone Set (Tone Dialing) 1 Theme 1 Telephones and Mobile Phones 4.2 Unit 4 Telephones (fixed-line) (Rev 1.0) Mouthpiece The mouthpiece is also referred to as a microphone. One of the key features for telephone invention was the use of a variable-resistance mouthpiece, which can change the electric current that is proportional to the sound pressure. The basic block diagram is shown in the figure. Block Diagram of a Mouthpiece The diaphragm is sensitive to sound waves; the carbon granules are compressed and released as the sound waves place pressure on the diaphragm. The resistance of the mouthpiece is reduced when the diaphragm moves inward and is increased when the diaphragm moves outward. A simple conceptual graphic explanation is shown in the following diagrams. When the carbon granules are under pressure, the contact surface between the carbon granules is increased and results in resistance reduction. Actually, the carbon granules are of irregular size and shape with many tiny edges on each grain. With only a relatively few edges in contact, the current has only a few paths through the carbon chamber. However, when the carbon granules are under pressure, more edges will be in contact resulting in more current paths. The overall resistance is thus reduced. The carbon mouthpiece does not respond well to a small acoustic signal and thus is good at discriminating against background noise. The low cost and long life nature of a carbon type transmitter are the reasons for its popular use today. 2 Theme 1 Telephones and Mobile Phones 4.3 Unit 4 Telephones (fixed-line) (Rev 1.0) Earpiece In the 19th century, it was discovered that if an electric current passed through a solenoid, it would create a magnetic field like a magnet. This forms the basis for earpiece (speaker) construction. It converts the varying electric current representing the transmitted speech signal to variation in air pressure and restoring the original speech. A typical earpiece consists of many turns of fine wire that is wound on permanently magnetized iron driving the diaphragm which is made of a soft iron material. Under normal conditions, i.e., no signal, the magnetic field of the magnet attracts the diaphragm and causes it to bend inward, as shown in (a). For the positive signal, the current in the coil produces a magnetic field which is opposed to the original field; the overall reduction of the magnetic field will thus make the diaphragm to be less bent, as shown in (b). For the negative signal, the current in the coil produces a magnetic field which is in the same direction as the original field; the overall magnetic field is increased and the diaphragm is bent more towards the magnet, as shown in (c). The varying electrical current, representing speech, flows through the coils and produces a varying magnetic field which causes the diaphragm to vibrate back and forth and reproduce the original speech. 4.4 Switch Hook A switch hook is used to inform the telephone company the status of the telephone. • ON hook (handset is hung on hook) → means that the phone is in idle state, the user has no intention to use the phone. • OFF hook (handset is lifted up) → gives a signal to the telephone company that the handset is lifted up, i.e., the user is going to answer a call or make a call. • When an attempt to make a call is made, a dial tone from the earpiece could be heard after OFF hook if the connection was made and the service was available. 3 Theme 1 Telephones and Mobile Phones 4.5 Unit 4 Telephones (fixed-line) (Rev 1.0) Dialer The purpose of dialing is to specify the telephone number of the called party to the telephone company. There are two types of dialing techniques pulse dialing and tone dialing. Pulse Dialing • • • • • • Dial pulses are produced by the rotary dialer in general. A sequence of pulses represents each digit. Each pulse creates interruption to the current flow in the telephone line. The dial pulse occurs at a rate of from 9 to 11 pulses per second. Dialing time is slow, and the time to complete the dialing depends on the dialed number. Due to the limited signaling capability and less reliability, this technique is seldom used now. Tone Dialing • • • • • Most telephone sets today use dual-tone multi-frequency (DTMF) for dialing or sending signals; each number is represented by two tones (i.e., dual-tone). For example, pressing key “7” generates a 852Hz tone and a 1209Hz tone. These telephone sets are equipped with a push-button touchtone keypad with 12 keys. The extended keypad is normally not constructed. The keypad pattern is normally fixed, i.e., there won’t be a keypad with “7”, “8”, and “9” at the top position of the keypad from a commercial phone. The major advantages of using DTMF are (1) fast dialing time of 0.1sec for any digit is possible, and (2) it can be used in some simple communication services after the call is connected (e.g., banking services, enquiry systems, etc.). 4 Theme 1 Telephones and Mobile Phones 4.6 Unit 4 Telephones (fixed-line) (Rev 1.0) Ringer The ringer is used to indicate that a call is coming and it should stop ringing after the phone is OFF hook. Different countries use different ringing tones. The operation is controlled by the telephone company and not by the telephone set in general. 4.7 Summary In this unit, the basic functional units of a telephone’s (fixed-line) user interface are addressed. These include the mouthpiece, the earpiece, the switch hook, the dialer, and the ringer. Their operating principles are briefly explained. Apart from dialing, the basic functional units and the operating principles of a telephone nearly remain the same as during the last century. The major inventions in telephone are (1) changing the sound into electrical parameter (current/voltage) for transmission, and then (2) changing the electrical parameter (current/voltage) back to reproduce the sound at the receiver side. Key Terms Dialer: A dialer is used to specify the telephone number of the called party to the telephone company. Earpiece: A device which converts the varying electric current representing the transmitted speech signal to variation in air pressure and restoring the original speech. Mouthpiece: A variable-resistance device which generates a reasonable large electric current (or voltage) that is proportional to the sound pressure. It is also referred to as microphone. Pulse Dialing: A sequence of pulses represents each digit, and each pulse creates interruption to the current flow in the telephone line. Ringer: A device to indicate that a call is coming. Switch Hook: A switch hook is used to inform the telephone company the status of the telephone. Tone Dialing: Use dual-tone multi-frequency (DTMF) for dialing or sending signals; each number is represented by two tones (i.e., dual-tone). 5