The professional audio system is composed of many different functional devices. After designing the sound system according to the requirements of use and selecting the equipment used, these discrete devices should be connected according to the design requirements to form a complete sound system that can meet the design requirements. For a fixed installation system, install the equipment in the cabinet, and install all the systems in a fixed manner according to certain standards and specifications. For mobile systems, such as concerts, outdoor performances and other temporary devices, effective temporary fixing measures should be taken for equipment and cables to ensure their safety.
The connection and installation of the audio system involves many engineering problems, including the design and construction of the audio control room, the pipeline engineering of the audio system cable, and the power supply of the system. Here are some questions to be aware of in the sound system signal transmission.
1. The impedance matching signal input port is also the load of the signal output port. The impedance matching between them must be within the range to meet its requirements, generally depending on the design requirements of its signal output device. In order to optimize the transmission state of the audio signal, the impedance of the signal input interface must meet the impedance matching requirements of the signal source output interface for its load. Otherwise, it will affect the working state of the audio device and cause distortion of its output signal. In severe cases, there is even a danger of damage to the source device.
In theory, 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, the signal power will be mostly lost on the signal output circuit, which is obviously not conducive to the signal transmission. Therefore, audio equipment is usually designed with input impedance greater than output impedance.
Generally, the connection of the audio equipment can make it work normally as long as the load impedance is greater than the impedance of the signal output end. However, the input impedance of the audio equipment cannot be designed 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.
2. Signal transmission level The purpose of the audio system connection is to transmit signals. The optimal state of audio signal transmission requires that the level of the signal source output must be greater than or equal to the sensitivity of the input interface. Otherwise, the signal-to-noise ratio of the signal will be caused. The indicator has deteriorated. The gain of the line input and output circuits on professional audio equipment is generally set at 0 dB, that is, the level of the input or output signal of the device is neither amplified nor attenuated, so that it can be maintained during transmission. The level value is unchanged, which is mainly to ensure that the adjustment of the level control unit can be numerically guaranteed.
In the system connection, attention should be paid to the matching of the output and input levels. Otherwise, clipping distortion may occur, or the excitation signal may be insufficient, resulting in a decrease in the overall system signal-to-noise ratio. Usually the connection between audio equipment (mixers, peripherals, amplifiers) is transmitted at the line level. There are generally two line standards, one is +4dB (1.228V) and the other is 0dB (0.775V). The line level of the equipment used in the system is preferably uniform, so that it is convenient to adjust and use. In addition, there are some sound processing equipment, especially effects, in order to balance the needs of electro-acoustic instruments and professional audio systems, the interface level conversion function is set, the switch is generally set behind the device, can be divided into +4 dB, -10 dB, -20 dB Several gears, please pay attention to adjust it to +4dB when using the sound reinforcement system.
3. Weak signal connection mode The input and output terminals of professional audio equipment have several methods such as unbalance, transformer balance and differential balance. Balanced and balanced, unbalanced and unbalanced ports can be directly fed with signals; in higher-demand situations, between balanced and unbalanced ports, special converters must be connected to each other. Converters generally have three types: passive transformer converters, half voltage converters, and active differential amplifiers.
In some occasions where the requirements are not high, the unbalanced terminal of the signal and the balanced terminal can be directly fed. The wiring method is as follows: the thermal 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 end, and the ground end of the balanced end is connected to the shielding layer of the signal feeder.
In addition to the power transmission between the amplifier and the speaker, in order to improve the anti-interference ability of the system and ensure 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 also provides balanced input and output functions.
The balanced mode signal transmission uses a three-wire system. Connected with a two-core shielded cable, the shielded mesh layer acts as a grounding wire, and the remaining two core wires are connected to the signal hot end (reference positive end) and cold end (reference negative end). Since the signal currents flowing through the two signal cores are the same size and opposite directions, the external electromagnetic interference induced on the transmission line will be offset by the subtraction at the input.
Some professional audio equipment are sometimes used in professional audio systems, and their output is unbalanced. In addition, electric guitars, electric basses, electric keyboards, synthesizers, etc. in electroacoustic instruments also use unbalanced output methods, so the connection of the sound system will inevitably use some unbalanced connection. Special attention should be paid here to two points. First, when the unbalanced mode is used, especially when the transmission level is low, the length of the connecting cable should be shortened as much as possible. If necessary, the amplifier can be set locally in the vicinity of the unbalanced output device, and the level can be increased and converted into a balanced output for long-distance transmission. The transformer can also be used to convert the signal into a balanced mode before long-distance transmission. Due to the existence of devices with unbalanced signal transmission in the system, balanced/unbalanced, unbalanced/balanced conversion problems are proposed. Sometimes such conversion is not difficult, but sometimes it is necessary to use a transformer to solve the problem better.
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