Public broadcasts are broadcasts that serve the public to a limited (but would be quite large) scope. Under normal circumstances, public broadcast signals are transmitted through broadcast lines located in broadcast service areas, and are usually a one-way (downstream) cable broadcast.
Public broadcasting is usually set up in institutions, troops, enterprises, schools, communities, buildings, supermarkets, and various venues. It is used to release news and internal information, issue work schedules, provide background music, and be used for paging. Tracing) and forced emergency broadcasts of catastrophic accidents, etc.
Public broadcasting is also useful for wireless transmission, which is also a trend. I won't introduce it here.
A public address system must be equipped with at least the following links: radio speakers, radio amplifiers, microphones, etc.
The difference between public address system and professional audio system
Low-power broadcast amplifiers below 100W usually have their own pre-stages, and there is no need to configure a pre-amplifier, commonly known as a combined broadcaster. Broadcast power amplifiers with higher power (for example, hundreds of watts or more) usually need to be equipped with a separate broadcast preamplifier. However, whether it is a preamplifier in a combined broadcaster or a discrete broadcast preamplifier, its input port should be prioritized. Usually at least one microphone has the highest priority, and the signal of the microphone can automatically suppress other input signals (so-called with automatic "silence" function), so that it can be forced to insert emergency broadcast when paging and emergency. This is also a little different from general HiFi (or professional) preamps and mixers.
The terms in the system: microphone, line, amplifier, speaker, etc. are familiar to ordinary electroacoustic workers or electroacoustic enthusiasts. However, the broadcasting lines, broadcasting power amplifiers and broadcasting speakers are slightly different from ordinary speaker wires, power amplifiers and speakers. The key point is that the broadcast line is usually quite long (several hundred meters or even more than a kilometer), and the speaker cable of general professional audio rarely exceeds 100 meters. In order to reduce transmission loss, the broadcast signal is transmitted in principle by "high voltage / low current". Therefore, broadcast lines generally do not need expensive speaker wires, but only ordinary twisted pair cables; if deployed outdoors, lightning protection equipment should be added.
Because the system uses high voltage transmission, the broadcast power amplifier must provide high voltage signals. Generally, broadcast power amplifiers have built-in output transformers to boost (or regulate) their output voltage, which is called "constant voltage" power amplifier. The output terminals of the constant voltage power amplifier are marked with nominal output voltage (NormalOutputVoltage) specifications such as 70V / 100V / 120V / 200V, instead of the rated load impedance value (ohm- number) like professional power amplifiers. The latter is also known as "constant resistance" power amplifier.
Broadcast loudspeakers are the terminals of broadcast systems. Since the broadcast lines provide "high voltage / low current" signals, accordingly, broadcast loudspeakers should be high impedance. They all have built-in "line-to-line transformers" for impedance conversion. , Its input terminals are also marked with nominal applicable voltage specifications (and rated power values) such as 70V / 100V / 120V / 200V, rather than its impedance value. It is not necessary to calculate their impedance when they are connected to broadcast speakers. They can be connected only if their applicable voltage specifications are consistent with the nominal transmission voltage of the broadcast line, but the total of broadcast speakers connected (parallel) on the same line The power cannot be greater than the rated output power of the broadcast power amplifier driving the line. Otherwise the radio amplifier will be overloaded. There is no limit to the minimum number of broadcast speakers, and the broadcast amplifier will not cause performance degradation due to light or no-load, let alone be damaged.
Simple system can only publish voice broadcast, such as notification, paging, speech, etc. If you want to broadcast background music, broadcast news, and release recordings, you must add CDs, decks, tuners (radio) and other equipment. Broadcast power amplifiers are usually equipped with multiple priority input interfaces, which can be easily connected to these devices.
By the way, some people think that the output voltage of a constant voltage power amplifier should be equal to its nominal voltage at any time. When they use a voltmeter (or level meter) to monitor a running constant voltage power amplifier, they always find that the output is undervoltage, and Think the power amplifier is faulty. This is actually a kind of misunderstanding. The "nominal output voltage" of the "constant voltage" broadcast power amplifier refers to the output voltage corresponding to the rated output, not the output voltage in any case. In fact, since the sound signal is a kind of signal that is not constant in strength, the actual output voltage of the audio power amplifier during normal operation also changes at any time. Under overload conditions, only the peak signal will reach the nominal value, and its average output will always be less than the nominal value. This is why “always under voltage” is used when monitoring with a level meter. The so-called "constant voltage" means that its output voltage does not change with the load (under no-load conditions)-that is, it has the output characteristics of a voltage source, not that its output voltage is fixed at the same level as the power supply voltage. A magnitude. The vast majority of modern audio power amplifiers use large loop voltage negative feedback technology. Therefore, even the so-called "constant-resistance" power amplifiers, in fact, most of them are "constant-voltage", except that the output end is not The voltage value at rated output is the corresponding load impedance value.
In addition, the public address system is not a stereo system in principle. The stereo system is an electroacoustic system that can reproduce the orientation of the original sound source to a certain extent. As you know, in order to create a sound image with azimuth properties, a stereo system needs at least two channels, that is, at least two speakers configured according to certain rules are used to emit two sounds that are both related and different. The public address system has only one channel in principle. Although there may be many speakers in a public address system, they only play the same sound; multiple speakers are not intended to create a sound image, but to create a sound field that can be reproduced as evenly as possible in the broadcast service area. Of course, if it is necessary to broadcast stereo programs in the public broadcasting system, it is not impossible, but it can only be in a limited broadcasting area, and it requires additional investment.
2. Interchange of public broadcasting equipment and professional audio equipment
Is it possible to build a public address system with professional audio equipment or vice versa? The answer is yes, but not necessarily appropriate.
If you use a professional power amplifier instead of a broadcasting power amplifier, you need to find a way to increase (or regulate) its output voltage. The methods are:
· Select a professional amplifier or a combination of amplifiers (bridged or cascaded) with the appropriate rated output power to obtain a constant voltage output (usually 100V) compatible with broadcast speakers.
Match a suitable output transformer at the output of a professional amplifier.
The output voltage of a professional power amplifier has the following relationship with its rated output power (Po) and rated load resistance (Rz):
U = (Po · Rz) 1/2
According to this, 2500W / 4Ω professional power amplifier has exactly 100V output voltage and can be used directly. However, the total power of the broadcasting speaker cannot exceed 2500W (only considering the load capacity of the power amplifier and not considering the requirements of the relevant regulations); on the other hand, no matter how small the total power of the broadcasting speaker, you cannot choose a power amplifier with low power, otherwise the broadcasting cannot be guaranteed The transmission voltage that the line should have. With two 650W / 4Ω professional power amplifier bridges, you can also get an output voltage close to 100V. Obviously, when the total power of the broadcasting speaker is far less than 2500W, it is not cost-effective to use the above method. If you want to use an amplifier that is equivalent to the total power of the broadcast speaker, you should calculate its output voltage according to the above formula, and then connect an appropriate output transformer. However, connecting an output transformer to a professional power amplifier can easily cause damage to the amplifier, and appropriate measures need to be taken (omitted here).
Professional speakers can also be used as broadcast speakers, but they must be equipped with a "line-to-line transformer". As mentioned earlier, the "line-to-line transformer" plays the role of impedance conversion, so that "constant-impedance" professional speakers can draw power equivalent to their rated power in the "constant-voltage" broadcast transmission line. Several “professional loudspeakers” can be used without proper “line-to-line transformers”. They can also be used, but they need to undergo tedious calculations. Moreover, if one of the loudspeakers is damaged during operation, the system may be subverted (all without Ring or burn other speakers or burn the amplifier).
Using a mixer as a broadcast preamp is barely feasible. However, mixers usually do not have signal priority sequencing, which is not convenient for implementing the "forced insertion" function necessary for broadcasting systems.
In turn, radio amplifiers and radio speakers can also be used in professional audio systems. But because they all have built-in output transformers or line-to-line transformers, their frequency response characteristics are not as good as professional-grade power amplifiers. Broadcast preamps are also reluctant to use in professional audio systems, because they do not have functions such as grouping, equalization, and effects; and their prioritization functions are mostly redundant for professional audio systems.
So, although it is feasible to exchange equipment for public address systems and professional sound systems, it is not necessarily appropriate. Often it will definitely lead to worse performance-to-price ratio.