Compared with civil (home) audio, public broadcasting (PUBLICADDRESS) is also a professional audio. But we usually say that the professional audio (PROFESSIONALAUDIO) is a particular stage (performance) type of sound. Public radio is different from this kind of sound.

Public broadcasting is a broadcasting service to the public in a limited (but rather large) area. Under normal circumstances, a public broadcast signal is transmitted through a broadcast line arranged in a broadcast service area, usually a one-way (down) broadcast.

Public broadcasting is usually set within the organs, enterprises, troops, schools, communities, buildings, supermarkets and various venues for publishing news and internal information, publish schedule signal, and provides the background music for paging (Paging- for broadcasting) and forced into disaster emergency broadcast and so on. Public radio is also useful for wireless transmission, but not mainstream.

1, the most simple public broadcasting system and its characteristics

A public broadcasting system must be configured at least the following links: radio speaker, radio amplifier, microphone.

The following is a small power amplifier for 360W, which is usually provided with a front level and no need to configure a preamplifier. Radio amplifiers with large power, such as a few hundred watts or more, usually have to be broadcast with a pre amplifier. But whether it is a pre amplifier in a combined broadcast or a separate broadcast preamplifier, the input port should have priority ordering. Usually at least one microphone at the priority signal of the microphone can automatically suppress the other input signal (the so-called automatic "mute" function), in order to have an emergency in the paging and, can be forced into the emergency broadcast. This is also different from the general HiFi (or professional) preamplifier and mixer.

The terminology of the system: microphone, line, power amplifier, speakers, etc., are familiar with the general electro acoustic workers or audio enthusiasts. But one of the radio lines, radio amplifier and radio speakers and ordinary speaker line, power amplifier, speakers are slightly different.

The key is that the broadcast line is usually quite long (hundreds of meters or even kilometers), and the general professional audio speaker line will be more than 100 meters. In order to reduce the transmission loss, the broadcast signal is in principle "high voltage / low current" transmission. Therefore, the general use of radio lines do not need to use expensive speaker wire and the use of ordinary twisted pair; if configured outdoors, it should be equipped with lightning protection equipment.

Because the system is used for high voltage transmission, the broadcasting amplifier must provide high voltage signal. Usually a radio amplifier is built with an output transformer to enhance (or regulate) its output voltage, known as the "constant pressure" power amplifier. The output terminal of the constant voltage power amplifier is marked with the nominal output voltage (NormalOutputVoltage) of the 70V/100V/120V/200V, rather than the rated load impedance value (ohms), as indicated by the professional power amplifier. The latter is also known as the "fixed resistance" amplifier.

Speaker terminal broadcast system, because the broadcast line is provided by high voltage / low current signal, accordingly, broadcasting speakers should be of high resistance, they have built and used for impedance transformation "transformer", its input terminals also marked 70V/100V /120V/200V nominal voltage specifications (numerical and the rated power), and not the value of impedance. Connected with the speaker do not have to calculate their impedance, but its applicable nominal transmission voltage specifications with the broadcasting line match can hang them up, but hung (parallel) rated power output total power can not be on the same line of the speaker is greater than the drive line radio power amplifier. Otherwise the broadcast amplifier will overload. The minimum number of broadcast speakers is not limited, radio amplifier will not cause light or no-load performance deterioration, but will not be damaged.

The public broadcasting system can only broadcast voice broadcasts, such as notices, paging, speeches, etc.. If you want to broadcast background music, broadcast news and release recordings, shall purchase CD, card holder, tuner (radio) and other equipment. A broadcast amplifier is usually provided with a plurality of priority sorted input interfaces, which can be easily connected with these devices.

Incidentally, some people think that the output voltage of the power amplifier at any time should be equal to the nominal voltage, they use the voltage meter (or level meter) monitoring the running of constant voltage power amplifier, the output voltage is always found, so that power failure. In fact, this is a misunderstanding. The nominal output voltage of the "constant pressure" broadcast amplifier is the output voltage corresponding to the rated output, not the output voltage in any case. In fact, because the sound signal is a kind of strong and weak signal, so the actual output voltage of the audio power amplifier in normal operation is also changing. In the condition of no overload, only the peak signal will reach the nominal value, and its average output is always less than the nominal value. This is the use of the level meter to monitor the "always under pressure" reasons. The so-called "power" refers to the output voltage will not change with the load weight (in overload conditions) that has the output characteristics of the voltage source, but it does not mean that the output voltage as the power supply voltage that is fixed in a quantity. The vast majority of modern audio power amplifiers use a large loop voltage negative feedback technology, therefore, even the so-called "fixed resistance" power amplifier, in fact, the vast majority of the "constant pressure"