CHAPTER ONE
INTRODUCTION
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.
46
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.
46
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
46
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.