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Public Address System

A public address system (PA system) is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a sound source, e.g., a person giving a speech, a DJ playing prerecorded music, and distributing the sound throughout a venue or building.

Simple PA systems are often used in small venues such as auditoriums, churches etc. PA systems with a larger number of speakers are widely used in institutional and commercial buildings, to read announcements or declare states of emergency. Intercom systems, which are often used in educational institution, also have microphones in each room so that the occupants can reply to the central office.

Sound reinforcement systems and PA systems may use some similar components, but with differing application. Sound reinforcement systems are for live music or performance, whereas PA systems are for reproduction of speech and recorded music in buildings and institutions. In colloquial British English, a PA system installed for public address in a building is sometimes referred to as a tanoy system.

Small system
The simplest PA systems consist of a microphone, a modestly powered mixer amplifier and one or more loudspeakers. Simple PA systems of this type, often providing 50 to 200 watts of power, are often used in small venues such as auditoriums, churches etc. A sound source such as a CD player or radio may be connected to a PA system so that music can be played through the system.

Public address systems typically consist of input sources, preamplifiers and/or signal routers, amplifiers, control and monitoring equipment, and loudspeakers. Input sources refer to the microphones and CD Players that provide a sound input for the system. These input sources are fed into the preamplifiers and signal routers that determine the zones to which the audio signal is fed. The preamplifier signals are then passed into the amplifiers. Depending on a country's regulations these amplifiers will amplify the audio signals to 50V, 70V or 100V speaker line level. Control equipment monitors the amplifiers and speaker lines for faults before it reaches the loudspeakers. This control equipment is also used for separating zones in a PA system. The loudspeaker is used to transducer electrical signals into analog sound signals.

Large system
Public Address System consisting of amplifiers, mixers and routers for a major international airport
Some PA systems have speakers that cover an entire campus of a college or industrial site, or an entire outdoor complex (e.g., an athletic stadium). More than often this PA system will be used as voice alarm system that make announcement during emergency to evacuate the occupants in a building.

Telephone paging system
Some analog or IP private branch exchange (PBX) telephone systems use a paging facility that acts as a liaison between the telephone and a PA amplifier. In other systems, paging equipment is not built into the telephone system. Instead the system includes a separate paging controller connected to a trunk port of the telephone system. The paging controller is accessed as either a designated directory number or central office line. In many modern systems, the paging function is integrated into the telephone system, and allows announcements to be played over the phone speakers.
Many retailers and offices choose to use the telephone system as the sole access point for the paging system, because the features are integrated. Many schools and other larger institutions are no longer using the large, bulky microphone PA systems and have switched to telephone system paging, as it can be accessed from many different points in the school.

PA over IP
PA over IP refers to PA paging and intercom systems that use an Ethernet or GSM-R network instead of a centralized amplifier to distribute the audio signal to all paging locations in a building or campus. Network-attached amplifiers and intercom units are used to provide the communication function. At the transmission end, a computer application transmits a digital audio stream via the local area network, using audio from the computer's sound card inputs or from stored audio recordings. At the receiving end, specialized intercom modules receive these network transmissions and reproduce the analog audio signal. These are small specialized network appliances addressable by an IP address just like any other computer on the network.

Such systems are inter-connected by the networking infrastructure and thus allow loss less transmission to remote locations across the Internet or a local area or campus network. It is also possible to provide for multiple or re locatable transmission control stations on such a network.