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BRITISH TELECOM MANAGEMENT COLLEGE (BOURNEMOUTH) ****************************************** ** VOICE COMMUNICATION : A FOUNDATION ** ****************************************** COURSE PROGRAMME DAY 1 (Approximately 12.30 - meet at hotel for lunch) 14.00 Course Introduction An Historical View of Communications Introducing the Telephone The Electrical Signal "Digital Rules OK" DAY 2 09.00 The Local Line Network ) ) The Public Switched Exchange Systems ) ) Telephone Network The National Trunk Network ) ) (PSTN) The National Numbering Scheme ) (LUNCH) 14.00 Signalling Systems Exchange Lines Private Circuits Private Branch Exchanges (PBX) (LUNCH) JP3AAJ DAY 3 09.00 PBX's (continued) Key Systems Communications in the UK British Telecom. The Organisation The Future, Convergence ? Course Close - Open Forum (LUNCH) JP3AAJ *************************************** ^ * * * VOICE COMMONICATIONS : A FOUNDATION * * * *************************************** INTRODUCING THE TELEPHONE (C) BRITISH TELECOMMUNICATIONS PLC 1987 BRITISH TELECOMMUNICATION MANAGEMENT COLLEGE JP3AAF Introducing the Telephone What is a telephone? What functions does it perform? Think about your telephone at home. What happens when you receive an incoming call? The telephone instrument alerts you to the fact by sounding a bell or warbler within itself. The familiar "Ring Ring" that we all know and love. This particular function is known under the technical name of "Signalling". Signalling is an essential function of any voice communication network. In this case, the exchange signalling equipment is generating the ringing signal to the required telephone. Without this signal, you would need to lift your telephone handset on a regular basis just to check that nobody is trying to contact you. There will be more on this subject later. The telephones other principle function is in converting sound waves (your voice) into an electrical (analogue) signal. The components which enable a telephone to do this are the microphone and the earpiece. JP2AAJ Refer to Fig.l The Microphone In concept, the microphone has only three components. A diaphram (which doubles as an electrical contact), a layer of carbon granules and another electrical contact. The sound waves coming** from your mouth and throat vibrate the mouthpiece diaphram and in doing so this varies the pressure on the carbon granules between the electrical contacts. ^ Carbon granules posess the property that when compressed their resistance to electrical signals reduces. If we assume that a steady electrical signal is flowing around the circuit (provided by the exchange) known as a current, then the height or size of this current is varied as a direct copy of the pressure variations (and hence the sound waves). JP4AAB ) > Fig. 1 BATTERY ELECTRO MAGNET ^CARBON GRANULES EARPIECE MOUTHPIECE DIAPHRAM DIAPHRAM ELECTRIC CURRENT The Earpiece The earpiece sits at the other end of the telephone circuit. In this case the circuit comprises of two telephone lines connected together in the exchange. The variations in electric current (which dont forget are exact copies of the sound waves generated at the microphone) are now passed through an electromagnet. This electromagnet is the heart of the earpiece, and using the normal rules of magnetism, the poles of the electromagnet attract and repel the earpiece diaphram where the moving air recreates the original sound waves. So these, the earpiece and the microphone, are the most important components of the telephone. The actual components used today may not follow exactly the construction described above, however the principles will be the same. To summarise this section, the telephone is a device for changing Sound Waves into an Electrical Signal and vice versa. JP2AAJ signalling What signals are generated by you a telephone user? 1. You want to make a call (lifting the handset). 2. Dialling the desired telephone number. 3. You have finished with the call (hanging up). These tend to be known as seizure, dialling and cleardown. Seizure and dialling can be known collectively as call set up. In receiving a call, what signals can you receive? 1. Incoming call ringing signal. 2. The person calling you has cleared down. Finally, in addition to the above what signals do you receive from the BT exchange? 1. Tones (Dial tone, engaged tone, ringing tone, NU tone etc) 2. Meter pulses. The other componenets of the telephone control these signal arrangements, i.e. bell or buzzer, dial or keypad, switchook contacts etc. JP2AAJ For the purpose of this handout we have obviously simplified the description. Telephones come in a wide range of styles, varieties and capabilities. They all however, work to the same basic principles and these are the important points to understand. JP2AAJ t **************************************** * * * VOICE COMMaNICATIONS : A FOUNDATION * * * THE ELECTRICAL SIGNAL (c) BRITISH TELECOMMUNICATIONS PLC 1987 BRITISH TELECOMMUNICATION MANAGEMENT COLLEGE JP3AAF Voice communication is a way of exchanging information between people. The mouth, throat and vocal cords combine to produce sound waves which travel through the air to other people. Our ears contain a diaphram (eardrum) which responds to the sound waves and our brains produce the result. Although you cannot see them, you can understand what is actually happening with air waves by dropping a stone into the middle of a pool of water. By doing this, the waves can be seen rippling away from the point that the stone entered the water, equally in all directions. Only when waves hit a movable object does the object begin to rock. Exactly the same thing is happening between our mouths and our ears when we speak. But what happens when the distance between two people increases? You raise your voice (or shout). Are there any problems associated with this means of communication? a) Over long distances, it does not matter how loud you shout, the person at the far end will not hear you. The air gradually attenuates (or reduces) sound waves the further JP2AAJ they have to travel, similar to the way that if the lake is big enough, the water ripples from a small stone entering its centre will not reach the shore. b) If everyone tried to hold a conversation in the same way (by shouting), we would all interfere with each other. Imagine the City of London!! Hence, we need a better medium than air to carry sound waves across long distances, whilst maintaining secrecy between conversations. JP4AAB \hE "ANALOGUE" SIG^^aL + 2- AMPUTUDE (VOLTS) TIME (SECONDS) Using an electrical signal A telephone is a device which allows two way voice communication by converting sound waves (ripples in the air) into electrical signals. We will discuss how it does this in a later session. The electrical signal can then be passed through an electrical circuit (a pair of wires) to the distant end, where another telephone recreates the original sound waves, ^ This technique will allow communication over any distance and with secrecy between conversations. But what does a Voice look like? Remember that when I whistled I produced a tone like a musical note. When we picked up the whistle with a microphone and displayed the resulting waveform (another word for signal) on an oscilloscope screen we obtained a picture like that in figure opposite. JP4AAB Definition of terms Amplitude. This is a signals maximum variation about its central value. Think of it as the height or volumn of the signal. It is measured in a unit called Volts. Frequency. ^ This is defined as the number of repetitions of the basic wave shape in one second or the number of cycles per second. It is measured in a unit called Hertz, which is given the abbreveation Hz. Remember the whistle. The pitch of the note determined the number of waves on the screen. The higher the note, the higher the frequency, more waves on the screen etc. But what happened when we tried to look, with the same equipment at a normal speech statement? ^ It was impossible to form any steady picture. This is because speech (the voice) is a very subtle combination of lots of the basic wave shapes, which are all varying continually in amplitude and frequency. JP4AAB Normal speech is unpredictable, and very complicated to analyse. Just think of it as a clever combination of whistles. Our voices (vocal cords, throats, mouths etc. controlled by the brain) are very sophisticated devices. Voice communication signals are sometimes referred to as Analogue signals. The term Analogue means "a signal which varies continually in amplitude and frequency". This is very smilar to the description of speech that we arrived at earlier. This is not surprising as voice sound waves are produced by the throat as analogue signals. Therefore voice electrical signals which are analogue are a direct copy of the sound they represent. JP2AAJ Finally, I want to define the term "Bandwidth". Bandwidth Bandwidth is the range of frequencies that can be carried by a transmission system (ie a telephone line). From our workshop exercise you should remember that the width (bandwidth) of the human ear to signals, depending upon age, is approximately 20 Hz to 16,000 Hz. Other typical bandwidths are;- a) Understandable speech via a telephone line. [300 3,400 Hz.] * b) A Hi-Fi music signal. [0 - 16,000 Hz] c) A Colour Television picture signal (Yes, this is also an analogue signal).