1.1 BACKGROUND OF THE STUDY
Fig 1: A picture of a modern wireless public address system
A public address system allows you to broadcast information to a large group of 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 the microphones that provides a sound input for the system. These input sources are fed into the preamplifiers. The pre amplified signals are then passed into the audio power amplifiers.
These amplifiers will amplify the audio signals to an adequate speaker line level. In view of the above, one can now say that the public address system is an electronic amplification system used for communication in public areas.
Microphone is a device that converts sound waves into electrical waves. Some times colloquially called a micro mike is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, live and recorded audio engineering, in radio and television broadcasting and in computers for recording voice, and for non-acoustic purposes such as ultrasonic checking. The sensitive transducer element of a microphone is called its element. Since a wireless microphone is used in this project; a wireless microphone is one in which communication is not limited by a cable.
A transmitter is extremely important equipment and is housed in the broadcasting station. Its purpose is to produce radio waves for transmission into space. 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 nearby receiver connected to the sound system, but it can also use infrared light if the transmitter and receiver are within sign of each other. The transmitter are responsible for taking in the signal from the microphone, modulating it, and transmitting it to the receiver using radio waves.
Fig2:A transmitter circuit
The first transistor (Q1) is the pre-amplifier for the microphone, and you can omit this circuit if you don’t want to transmit the sound picked up by the mic, for example you can connect your mp3 player directly to C1. The core of this FM transmitter circuit 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 1/16 wave length to compromise between the efficiency and the size. If you want the microphone to be less sensitive, we can replace the R1 by a higher resistor, such as 10k or 22k, and this might overcome the feedback problem if you use this wireless microphone FM transmitter for a public address system
The receiver captures the radio waves sent out by the transmitter, demodulates the signal, amplifies it to an appropriate level, and sends it out to the audio mixer. A receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, such as sound, and digital data, etc.
Amplifier or simply amp is any device that changes, usually increases, the amplitude of a signal. The “signal” is usually voltage or current. Amplifier is a device for increasing the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with larger amplitude. Routing the low-frequency parts of the signal to an amplifier can substantially improve the clarity of the overall sound reproduction.
In this project, the audio amplifier used is capable of delivering 20watts continuously. The term “power amplifier” is a relative term with respect to the amount 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 a transmission chain (the output stage) and is the amplifier stage that typically requires most attention to power efficiency. Power amplifiers have also become lighter, smaller, more powerful and more efficient due to increasing use of Class A amplifiers, which offer significant weight and space savings as well as increased efficiency.
Power amplifier circuits (output stages) are classified as A, B, AB and C for analog designs, and class D and E for switching designs, based upon the conduction angle or angle of flow, È, of the input signal through the output amplifying device, that is, the portion of the input signal cycle during which the amplifying device conducts. The image of the conduction angle is derived from amplifying a sinusoidal signal. (If the device is always on, È = 360o.) In this project a class A amplifier is used because it offers low signal distortion.
Loudspeaker is an electro-acoustical transducer that converts an electrical signal to sound. A transducer that turns an electrical signal into sound waves is the functional opposite of a microphone. Since a conventional speaker is constructed much like a dynamic microphone, (with a diaphragm, coil and magnet), speakers can actually work “in reverse” as microphones. The speaker pushes a medium in accord with the pulsations of an electrical signal, thus causing sound waves to propagate to where they can then be received by the ear. The loudspeaker used in this project is a load of 8 Ohms.
1.2 AIMS AND OBJECTIVES
Public 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 public speaking, as in any form of communication, there are five basic elements, often expressed as “who is saying what to whom using what medium with what effects?”(W.Kleitz;2005 pg134).
The major objective of this project is to design and construct a public address system with relatively unique operational characteristics that is capable of delivering 20 watts of electrical power into an 8 Ohms load (loudspeaker). This will be achieved with adequate tone controls and mixer stages so as to enable communication or reproduction of speech and recorded music in buildings and institutions.
Over the ages, science and technology has been developing with new inventions in various fields; including the increase in modifications on existing technology all gearing towards improving effectiveness and reliability of equipment and achieving to a great degree miniaturization and optimal cost.
This project is backed by my interest in the area of electronics and communication, and having been groomed to a great extent with introductions into the various fields of electrical and electronics engineering, with the knowledge in principles of telecommunications, telecommunication engineering,
basic electronics and lab practice. I chose to combine basic electronics and lab practice. I chose to combine all these ideas in embarking on this project to develop a special system that utilizes all the above stated knowledge in one system; “the public address system”.
This system makes use of radio communication system, it utilizes radio frequency to achieve all its set objectives, like data communication in the operation of wireless communication between the transmitter and the receiver.
This system is intended to provide a fast and reliable means of communication in small venues such as school auditoriums, churches, and small bars.
1.4 SCOPE OF THE PROJECT
This 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 and loudspeaker units. It has a power On/Off button with volume control.
1.5 SYSTEM BLOCK DIAGRAM
Fig 3: Block diagram of a public address system
The system is made up of the transmitter which is incorporated in the microphone; and the receiver unit, the tone control and mixer unit, the audio amplifier unit and the loudspeaker.
TONE CONTROL AND MIXER CIRCUIT
AMPLIFIER UNIT CIRCUIT
1.6 PROJECT WORK ORGANISATION
The various stages involved in the development of this project have been properly put into six chapters to enhance comprehensive and concise reading. In this project thesis, the project is organized sequentially as follows: