1.1 BACKGROUND OF THE STUDY
Fig 1: A picture of a modern wireless public address systemA public address system allows you to broadcast information to a large groupof people, whether you are giving a speech or playing live or recorded music.Public address systems typically consist of input sources, preamplifiers,control and monitoring equipment, and loudspeakers. Input sources refer to themicrophones that provides a sound input for the system. These input sources are fedinto the preamplifiers. The pre amplified signals are then passed into the audio poweramplifiers.These amplifiers will amplify the audio signals to an adequate speaker linelevel. In view of the above, one can now say that the public address system is anelectronic amplification system used for communication in public areas.46Microphone is a device that converts sound waves into electrical waves. Sometimes colloquially called a micro mike is an acoustic-to-electric transducer or sensorthat converts sound into an electrical signal. Microphones are used in manyapplications such as telephones, hearing aids, live and recorded audio engineering,in radio and television broadcasting and in computers for recording voice, and fornon-acoustic purposes such as ultrasonic checking. The sensitive transducer elementof a microphone is called its element. Since a wireless microphone is used in thisproject; a wireless microphone is one in which communication is not limited by acable.A transmitter is extremely important equipment and is housed in thebroadcasting station. Its purpose is to produce radio waves for transmission intospace. The important components of a transmitter are microphone, audio amplifiers,oscillator and modulator.It usually sends its signal using a small FM radio transmitter to a nearbyreceiver connected to the sound system, but it can also use infrared light if thetransmitter and receiver are within sign of each other. The transmitter are responsiblefor taking in the signal from the microphone, modulating it, and transmitting it to thereceiver using radio waves.46Fig2:A transmitter circuitThe first transistor (Q1) is the pre-amplifier for the microphone, and you can omitthis circuit if you don’t want to transmit the sound picked up by the mic, for exampleyou can connect your mp3 player directly to C1. The core of this FM transmittercircuit is Q2, a modified Culprits oscillator that the frequency is determined by L1,C4, C6, and the transistor’s internal base-emitter capacitance. The antenna use 1/16wave length to compromise between the efficiency and the size. If you want themicrophone to be less sensitive, we can replace the R1 by a higher resistor, such as10k or 22k, and this might overcome the feedback problem if you use this wirelessmicrophone FM transmitter for a public address systemThe receiver captures the radio waves sent out by the transmitter, demodulatesthe signal, amplifies it to an appropriate level, and sends it out to the audio mixer. Areceiver is an electronic circuit that receives its input from an antenna, useselectronic filters to separate a wanted radio signal from all other signals46picked up by this antenna, amplifies it to a level suitable for furtherprocessing, and finally converts through demodulation and decoding the signal intoa form usable for the consumer, such as sound, and digital data, etc.Amplifier or simply amp is any device that changes, usually increases, theamplitude of a signal. The “signal” is usually voltage or current. Amplifier is adevice for increasing the power of a signal. It does this by taking energy from apower supply and controlling the output to match the input signal shape but withlarger amplitude. Routing the low-frequency parts of the signal to an amplifier cansubstantially improve the clarity of the overall sound reproduction.In this project, the audio amplifier used is capable of delivering 20wattscontinuously. The term “power amplifier” is a relative term with respect to theamount of power delivered to the load and/or sourced by the supply circuit.In general a power amplifier is designated as the last amplifier in atransmission chain (the output stage) and is the amplifier stage that typically requiresmost attention to power efficiency. Power amplifiers have also become lighter,smaller, more powerful and more efficient due to increasing use of Class Aamplifiers, which offer significant weight and space savings as well as increasedefficiency.46Power amplifier circuits (output stages) are classified as A, B, AB and C foranalog designs, and class D and E for switching designs, based upon the conductionangle or angle of flow, ?, of the input signal through the output amplifying device,that is, the portion of the input signal cycle during which the amplifying deviceconducts. The image of the conduction angle is derived from amplifying a sinusoidalsignal. (If the device is always on, ? = 360o.) In this project a class A amplifier isused because it offers low signal distortion.Loudspeaker is an electro-acoustical transducer that converts an electricalsignal to sound. A transducer that turns an electrical signal into sound waves is thefunctional opposite of a microphone.Since a conventional speaker is constructed much like a dynamic microphone, (witha diaphragm, coil and magnet), speakers can actually work “in reverse” asmicrophones. The speaker pushes a medium in accord with the pulsations of anelectrical signal, thus causing sound waves to propagate to where they can then bereceived by the ear. The loudspeaker used in this project is a load of 8 Ohms.1.2 AIMS AND OBJECTIVESPublic speaking is the process of speaking to a group of people in a structured,deliberate manner intended to inform, influence, or entertain the audience. In publicspeaking, as in any form of communication, there are five basic elements, often46expressed as “who is saying what to whom using what medium with whateffects?”(W.Kleitz;2005 pg134).The major objective of this project is to design and construct a public addresssystem with relatively unique operational characteristics that is capable of delivering20 watts of electrical power into an 8 Ohms load (loudspeaker). This will be achievedwith adequate tone controls and mixer stages so as to enable communication orreproduction of speech and recorded music in buildings and institutions.1.3 JUSTIFICATIONOver the ages, science and technology has been developing with newinventions in various fields; including the increase in modifications on existingtechnology all gearing towards improving effectiveness and reliability of equipmentand achieving to a great degree miniaturization and optimal cost.This project is backed by my interest in the area of electronics andcommunication, and having been groomed to a great extent with introductions intothe various fields of electrical and electronics engineering, with the knowledge inprinciples of telecommunications, telecommunication engineering,basic electronics and lab practice. I chose to combine basic electronics and labpractice. I chose to combine all these ideas in embarking on this project to develop46a special system that utilizes all the above stated knowledge in one system; “thepublic address system”.This system makes use of radio communication system, it utilizes radiofrequency to achieve all its set objectives, like data communication in the operationof wireless communication between the transmitter and the receiver.This system is intended to provide a fast and reliable means of communicationin small venues such as school auditoriums, churches, and small bars.1.4 SCOPE OF THE PROJECTThis system is for public address; it has an estimated power rating of 20watts.For power supplies, it uses a 220/12Vac transformer for the receiver, amplifier andloudspeaker units. It has a power On/Off button with volume control.