Sound system signal transmission, need to pay attention to what issues?

2018-06-11

Professional audio systems are made up of many different functional devices. After designing the audio system according to the requirements and selecting the equipment to be used, these discrete devices should be connected according to 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 the cables of all systems according to certain standards and specifications. For mobile systems, such as concerts, outdoor performances and other temporary devices, take effective temporary fixation measures for equipment and cables to ensure their safety.

The connection and installation of audio systems involves many engineering problems, including the design and construction of audio control rooms, the pipeline engineering of audio system cables, and the power supply of the system. Here are some of the issues that need to be addressed in the audio system signal transmission.

1, impedance matching

The signal input port is also the load of the signal output port, and the impedance matching between them needs to be within the range to meet its requirements. Generally, it depends on the design requirements of the signal output device. To achieve the best transmission state of the audio signal, the impedance of the signal input interface must meet the impedance matching requirements of the output interface of the signal source to its load. Otherwise, it will affect the working status of the audio equipment and cause distortion of its output signal. In severe cases, there is even a danger of damaging the audio device.

In theory, the signal transmission efficiency is highest when the output impedance is equal to its load impedance. 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 based on the input impedance is greater than the output impedance.

The connection of general audio equipment can make it work as long as the impedance of the load is greater than the impedance of the signal output. However, the input impedance of audio equipment can not 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 indicator to drop.

2, signal transmission level

The purpose of the audio system connection is to transmit signals. The optimal state of the audio signal transmission requires that the level value of the signal source output be greater than or equal to the sensitivity of the input interface; otherwise, the signal to noise ratio index of the signal will be deteriorated. The gain of 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 equipment is neither amplified nor attenuated so that it can be maintained during transmission. The level value does not change. This is mainly to ensure that the level control unit can be adjusted numerically.

In the system connection, the output and input level should be matched. Otherwise, it may cause clipping distortion or insufficient excitation signal, resulting in a decrease in the signal-to-noise ratio of the entire system. Usually the connection between audio equipment (mixer, peripherals, power amplifiers) is signaled at the line level. There are two types of 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, which is convenient when adjusting and using. In addition, there are some sound processing devices, especially effect processors, which are equipped with an interface level conversion function in order to take into account the needs of electro-acoustic musical instruments and professional sound systems. The switch is generally set behind the device and can be divided into +4 dB, -10 dB, -20dB several files, use the sound reinforcement system should pay attention to adjust it to +4dB block.

3, weak signal connection

Professional audio equipment input and output terminals are unbalanced, transformer balance, differential balance and other ways. Balanced and balanced, unbalanced and unbalanced ports can feed signals directly; in the higher requirements, balanced and unbalanced ports must be connected through a special converter. Converters typically include passive transformer converters, half-voltage converters, and active differential amplifiers.

In some undemanding applications, the unbalanced terminal and the balanced terminal of the signal can be directly fed. The wiring method is as follows: 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. Ground, and the balanced end of the ground is connected to the signal feeder shield.

In addition to the power transmission between the amplifier and the speaker, in order to improve the system's anti-jamming capability and guarantee the signal-to-noise ratio, the signal connection in the professional audio system should be transmitted in a balanced manner as far as possible. Professional audio equipment also generally provides balanced input and output functions.

Balanced signal transmission adopts three-wire system. With two-core shielded wire connection, shielding mesh layer as a grounding wire, the other 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 lines are the same size and opposite in direction, the external electromagnetic interference induced on the transmission line will be canceled out at the input.

Professional audio systems sometimes use home audio equipment, and their output is unbalanced. In addition, electric guitars, electric basses, electric keyboards, synthesizers, etc. in electro-acoustic instruments also use unbalanced output methods, so the connection of the audio system inevitably uses some unbalanced connections. In this regard, two special considerations should be taken: First, when using an unbalanced method, especially when the transmission level is low, the length of the connecting cable should be as short as possible. If necessary, an amplifier can be placed near the unbalanced output device in place to raise the level and convert it into a balanced output before 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 equipment with unbalanced signal transmission in the system, the issues of balance/unbalance, unbalance/balance conversion were proposed. Sometimes this conversion is not difficult, but sometimes it is necessary to solve the problem with the help of a transformer.