The natural sound source (such as speech, singing, playing musical instruments, audio reproducing, etc.) issued by the sound energy is limited, the decay of the sound pressure level decreases with the increase of the propagation distance and, and influence of environment noise, propagation distance of a sound source is limited, therefore in the field of public activities to electronic technology, audio sound system for sound reinforcement, amplify the sound source signal, improve the sound pressure level of the audience area, to ensure that everyone in the audience can get proper sound pressure level.

Public broadcasting system (public address system (PA) belongs to the amplifying sound system in a branch of, from the factories, schools, hotels, railway stations, docks, square to the venue, theatre, gymnasium, residential quarters, such as all without exception and has close relationship.

Public broadcasting system is a systems engineering, it requires close coordination of electronic technology, electronic technology, built acoustic and acoustic art etc. a variety of disciplines, the sound effect of the public broadcasting system not only with the comprehensive performance of the electro acoustic system related, also with the propagation of sound acoustic environment science and on-site tuning is closely related to the usage, so public broadcasting system is the final effect need the best with the correct and reasonable acoustic system design and debugging, good sound propagation conditions and correct on-site tuning technology, three complementary and indispensable. Considering the above problems, in the choice of the good performance of the electro acoustic device on the basis, through careful system design, careful system debugging and good built acoustic conditions, achieve electroacoustic sweet, natural sound effects must be in the system design.

(a) the characteristics of the public broadcasting system

Background music (back ground music (BGM), its main role is masking noise and create a relaxed and harmonious atmosphere, listening to people if not listening to, cannot tell the difference between the position of the sound source, with less volume is a relaxed and pleasant atmosphere of music creation. Therefore, the effect of the background music has two, one is to cover up the psychological environment noise, the two is to create an atmosphere of adaptation with the indoor environment.

Public broadcasting system includes background music and emergency broadcast function, usually combine together, its object is the public place, in the corridor, elevator door, elevator car, halls, shopping malls, restaurants, bars, banquet hall, residential garden installed combined sound column or distributed loudspeaker box, usually playing background music, when an emergency occurs, strong cutting to the emergency broadcast, use it to command to disperse the crowd.

The control function of the public broadcasting system is more, such as the selective broadcast and the full call broadcast function, the mandatory switch function and the priority broadcast function. Public broadcasting system speaker load is more and more dispersed, transmission line length. In order to reduce the transmission line losses, the general use of 70V or 100V constant voltage high impedance transmission. The pressure is not high, quality to midrange and treble. Broadcast in this system with the microphone to the public broadcast of the speaker is generally not in the same room, so the problem of silent feedback.

(two) the classification of public broadcasting system

1 public broadcasting system can be divided into the following categories according to the transmission and signal processing:

1.1 traditional public broadcasting system: traditional public broadcasting system is through an audio cable to power transmission of analog signals to the speaker terminals, to room of the power amplifier to the speaker is using high level transmission, usually for 100V or 70V. Its advantages are line loss, load connection convenient, just with a step-down transformer speaker is connected on the line can be.

Figure 1.2 addressing public broadcasting system: the true meaning of digital can be addressing public broadcasting system which the audio signal, control signal and communication in the digital domain were, with longer transmission distance and better transmission effect, especially the audio signal and control signal has focused on the two cores of the twisted pair transmission, not only greatly saves the cost of installation and wiring, high reliability but also for future system maintenance and system provides prerequisite condition is superior.

1.3 streaming media (IP) intelligent digital broadcasting: streaming media (IP) intelligent digital broadcasting is pure digital network audio broadcasting system based on LAN and the Internet, broadcasting system, different from the traditional analog audio and radio FM, addressing NC, streaming media (IP) intelligent digital broadcasting system is fully compatible with the Internet and now LAN, TCP/IP protocol, no additional wiring, use of the existing campus network or intranet; embedded terminal hardware, to set up an independent IP address, through multilayer switches, through the host can control each terminal settings and play more in the terminal can select the program content required by computer technology, system can be almost compatible with all audio format, using the IP technology, content playback can not have any restrictions , each terminal can be free to play the needs of the program content. Can completely replace the analog or digital traditional intelligent broadcast system based on and realize based on the Internet or LAN IP audio broadcasting function, support broadcast, broadcast and video on demand, and supports a wide range of popularization and application of.

2 the public broadcasting system can be divided into the following categories according to the function and nature of the use:

2.1 business broadcast system: This is dominated by business and administrative language broadcast, for office buildings, commercial buildings, offices, schools, railway stations, wharves, airports and other places, business of broadcasting are usually by departments in charge of management;

2.2 service broadcasting system: This is to enjoy the background music with a service nature of the broadcasting system