Most professional audio systems are composed of unit equipment. After designing the audio system and selecting the equipment to be used according to the requirements of use, these discrete devices should be connected according to the design requirements to form a complete audio system that can meet the design requirements. For a fixed installation system, the equipment should be installed in the cabinet, and all system connections should be fixedly installed in accordance with certain standards and specifications (relevant regulations for building weak current). For mobile systems, such as concerts, outdoor performances and other temporary installations, effective temporary fixation measures should be taken for equipment and cables to ensure their safety. The connection and installation of the sound system involves many engineering issues, including the design and construction of the sound control room, the pipeline engineering of the sound system cable, and the power supply of the system. This section will focus on these engineering issues. The connection of the sound system can generally be divided into three aspects: signal transmission, grounding network and power supply system.
One, impedance and transmission level
1. Impedance matching
The signal input port is the load of the signal output port. What range of impedance matching between them can meet its requirements, generally depends on the design requirements of the signal output device. To achieve the transmission state of the audio electrical signal, the impedance of the signal input interface must meet the impedance matching requirements of the signal source output interface to its load, otherwise, it will affect the working state of the audio equipment and cause the distortion of its output signal. In severe cases, there is even a risk of damaging the sound source equipment. Theoretically, when the output impedance is equal to its load impedance, the signal transmission efficiency is the highest. If the output impedance is greater than the load impedance, most of the signal power will be lost in the signal output circuit, which is obviously not conducive to signal transmission. Therefore, audio equipment is usually designed according to the input impedance greater than the output impedance.
The connection of general audio equipment, as long as the load impedance is greater than the impedance of the signal output terminal, can make it work normally. However, the input impedance of audio equipment cannot be designed to be too high or too low, too high will reduce the anti-interference of its feeder, and too low will cause its frequency response index to drop. At present, most of the output and input ports of professional audio equipment use the IEC268-15 standard, and all audio equipment using this standard can be connected arbitrarily. The IEC268-15 standard uses voltage matching technology (VMT), which is designed to enable the load to obtain the level value from the signal source to achieve lossless signal transmission. This requires that the impedance of the load should be much greater than the signal source impedance. The IEC268-15 standard stipulates that the impedance of the line output of all audio equipment should be below 50Ω, and the impedance of the line input as a load should be above 10kΩ. In addition, the signal feed line of the microphone is generally long and requires strong anti-interference, so its input interface impedance is generally about 1kΩ.
2. Signal transmission level
The purpose of the audio system connection is to transmit signals. The state of audio signal transmission requires that the level value of the signal source output must be greater than or equal to the sensitivity of the input interface, otherwise, the signal-to-noise ratio indicator will deteriorate. The gain of the line input and output circuits on professional audio equipment is generally set at 0dB, that is, the equipment does not amplify or attenuate the level of the input or output signal, so that it can be maintained during transmission. The level value remains unchanged, which is mainly to enable the adjustment of the level control unit to have a numerical characterization. The signal transmitted through the external wire connection in the audio system can be divided into the following categories:
(1) Micro signal: Microphone output signal (mV level)
LP Phono output signal (mV level)
Sound source output (-10dB, 250mV)
(2) Line level: mixer output (+ 4dB, l.22V)
Peripheral equipment input/output (+ 4dB, 1.28V)
line transmission (0dB, 0.775V)
(3) Power transmission category Z: power amplifier output (high level, high current)
Obviously, in the system connection, attention should be paid to the matching of output and input levels. Otherwise, either the device will be over-excited, causing clipping distortion, or the excitation signal will be insufficient, resulting in a decrease in the signal-to-noise ratio of the entire system. For some signal processing equipment, the input level will not match and the desired effect will not be achieved. Usually the connections between audio equipment (mixers, peripherals, power amplifiers) transmit signals at line level. There are generally two line standards, one is +4dB (1.228V), this standard is the most common and most common. The other is that 0dB (0.775V) is less common than the above +4dB. The line level of the equipment used in the system can be unified, so that it will be easier to adjust and use. However, as long as all levels of equipment have level adjust functions, 0odB and +4dB equipment can generally coexist in the same system without any problems. In addition, some sound processing equipment, especially effectors, are equipped with an interface level conversion function in order to take into account the needs of electro-acoustic musical instruments and professional audio systems. The conversion switch is generally set at the back of the equipment and can be divided into +4 dB, -10 dB, -20 dB several gears, when using the sound reinforcement system, you should pay attention to adjust them to the +4dB gear.
Generally, the level control knobs of the line input and output circuits are marked with a scale in decibels. If all the signal feeders of the audio system are from line output to line input, each level control knob on this system The sum of the upper scale values is the gain in decibels of the entire system; if there is a level attenuation switch on the signal circuit, the gain in decibels of the system should also be added to the number shown on the switch. Since the output level of the microphone is very small, the gain of the input interface used to pick up the microphone signal is usually above 60dB. In other words, the magnification of this input interface circuit is more than 1000 times.
3. Signal connection method
The input and output terminals of professional audio equipment have several methods such as unbalanced, transformer balanced, differential balanced and so on. Signals can be directly fed between balanced and balanced, unbalanced and unbalanced ports; in high-demand situations, the balanced and unbalanced ports must be connected to each other through a special converter. There are generally three types of converters: passive transformer converters, half-voltage converters, and active differential amplifier converters.
In some less demanding occasions, the unbalanced terminal and the balanced terminal of the signal can still be directly fed. The connection method is: the hot end of the balanced end is connected to the signal end of the unbalanced end, and the cold end of the balanced end is connected to the unbalanced end. The ground terminal, and the ground terminal of the balanced terminal is connected to the shielding layer of the signal feeder. In addition to the power transmission between the power amplifier and the speakers, in order to improve the anti-interference ability of the system and the signal-to-noise ratio, the signal connection in the professional audio system should be transmitted in a balanced manner as much as possible. Professional audio equipment generally also provides balanced input and output functions. The balanced signal transmission adopts a three-wire system. Use a two-core shielded wire to connect, the shielded mesh layer is used as a ground wire, and the remaining two core wires are respectively connected to the hot end (refer to the positive end) and the cold end (refer to the negative end) of the signal. Since the signal currents flowing on the two signal cores are the same in magnitude and opposite in direction, the external electromagnetic interference induced on the transmission line will be subtracted and cancelled at the input end. Some home audio equipment is sometimes used in professional audio systems, and their output is unbalanced. In addition, electric guitars, electric basses, electric keyboards, synthesizers, etc. in electro-acoustic musical instruments also use unbalanced output, so the connection of the sound system will inevitably adopt some unbalanced connections. Two points should be paid special attention to here: First, when the unbalanced method is used, especially when the transmission level is low, the length of the connecting cable should be shortened as much as possible. If necessary, an amplifier can be set up near the unbalanced output device, and the level can be raised and converted into a balanced output before long-distance transmission. A transformer can also be used to convert the signal into a balanced mode before long-distance transmission. Due to the existence of equipment that uses unbalanced signal transmission in the system, the problem of balanced/unbalanced and unbalanced/balanced conversion is proposed. Sometimes this conversion is not difficult, but sometimes the problem can be solved with the help of a transformer.