The simplest public broadcasting system and its characteristics

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

100W below the small power amplifier is usually equipped with a pre class, without the need for pre amplifier, commonly known as the merger of the transmitter. A power amplifier that has a large power (e.g., a few hundred watts or more) is usually accompanied by a pre amplifier. However, whether the pre amplifier in the combined radio transmitter is still a separate broadcast preamplifier, the input of the transmitter should have priority ordering. Usually at least one microphone at the priority signal of the microphone can restrain the other input signal (the so-called "mute" function is active), in order to have trouble in paging and pressing, to forcibly stab into urgent broadcast. This is somewhat different from the usual (or professional) preamplifier and the 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 a nominal output voltage specification such as 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-in, for change "impedance transformer, the input terminals also indicated that 70V/100V /120V/200V is the nominal voltage standard (numerical and the extra power), and not the value of impedance. Connected with the speakers do not broadcast their impedance accounting, only the applicable voltage standard with nominal voltage transmission line transmission line can hang them up, but hung (parallel) extra power output power can not be on the same line broadcast speaker is larger than the line transmission power amplifier driver. Otherwise the broadcast amplifier will overload. The minimum number of speakers is not restricted, and the transmission power will not deteriorate due to light load or no load, and will not be damaged.

Simple system can only broadcast voice broadcast, such as notification, paging, speech, 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, due to the sound signal is a strong and weak signal, so the audio power amplifier in normal operation of the output voltage is also changing. In the condition of no overload, as long as the peak signal will reach the nominal value, and its uniform output is always less than the nominal value. This is the use of the level meter monitoring always under pressure reason. 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. Most modern audio power amplifiers are used high voltage negative feedback loop skills, therefore, even the so-called "resistance" amplifier, practice most is "constant pressure", but the voltage at the output is not marked additional output value of the load impedance, but correspond with the value.

In addition, the public address system is not a stereo system. The stereo system is a kind of electro acoustic system that can reproduce the original sound source to some extent. We know that in order to build the image of the characteristics of the orientation, the stereo system requires at least two channels, that is, at least with two rules will be equipped with the speaker out of the two channels are associated with different voices. The public broadcasting system, in principle, only one channel. Although the public broadcasting system can have many speakers, but they only show the same sound; a plurality of speakers are not in order to build image, but to replay the sound field as uniformly as in broadcast service area construction. Of course, if it is necessary to broadcast a stereo program in the public broadcasting system, it is not impossible, but only in a limited broadcasting area, and shall be subject to a nominal investment.