Avoid noise in audio system.

First, the cause of noise.

1. The cause of noise.

(1) inappropriate grounding.

In the audio system, it is necessary to require the whole system to have outstanding grounding, grounding resistance should be less than 4 ohms, otherwise the equipment in the audio system because of various radiation and electromagnetic induction induced charge will not be able to flow to the ground, and then constitute a noise voltage superimposed on the audio signal.

Ground potential difference exists between the wires of different equipments because of the different grounding resistance, or the grounding noise is caused when there is a loop grounding in the system. When two different audio systems are connected to each other, noise may also occur because the ground wires of the two systems are directly connected.

(2) noise generated by electromagnetic radiation disturbance.

Environmental stray electromagnetic wave radiation disturbance, such as mobile phones, walkie-talkies and other communication equipment, high-frequency electromagnetic wave radiation disturbance, elevator, air conditioning and other electrical pulse disturbance, studio lighting control with silicon controlled rectifier control equipment radiation, will be directly mixed into the transmission signal through the transmission line to form noise. Because a large number of SCR dimming devices exist in a studio, there is always noise in the sound reinforcement.

(3) circuit noise inside equipment

When a device works alone, the electrical noise generated by the internal electronic components can reach the required specifications, but after cascade of multiple devices, the noise will accumulate and add up, constituting a severe noise in the system.

(4) noise generated by power disturbance.

Urban power grid has become a serious disturbance source because of all kinds of lighting equipment, power equipment and control equipment connected together. If connected to the same power grid lighting control equipment, air conditioning, motor, etc. will occur on the power line peak pulse, surge current, different frequency of ripple voltage. The power supply is transferred through the power cord into the device. There will always be a part of the disturbing noise voltage can not be effectively filtered out by the power circuit of the audio equipment, will certainly constitute noise inside the equipment.

Second. Plan to deal with noise.

(1) outstanding grounding treatment

In order to shield the cable with shielding layer from stray electromagnetic disturbance, it is necessary to make the shielding layer have correct connection and outstanding grounding. Equipment suspension is a method without special ground wire conditions, which is an unstable operating condition. Random noise often occurs. Therefore, there should be special ground wires. The simplest way to temporarily bury a wire in an outdoor place is to insert a steel or aluminum tube into the ground.

The general system is a link system with several devices connected by cables. It is very simple to form a chain grounding method by its shielding system. When electromagnetic radiation or electrostatic induced noise occurs on a certain equipment, the induced voltage will occur in the whole system because of the grounding system composed of the shielding layer of transmission line and the shell of ferrous equipment, and then the noise level of the system will occur. Therefore, the system should try to prevent the use of chain grounding, and the use of star grounding method. This requires that the shielding layer of the audio cable connecting all the equipment be grounded at one end (connecting the shielding layer), and the ground wire of each equipment is connected to a grounding point through a special wire.

To ensure that there is no ground loop structure, only one grounding conductor can be interconnected among the devices. In the occasion of less stringent requirements, unbalanced equipment can be levitated, through the audio signal line to share the ground of the next level of equipment, that is, choose the chain grounding. This chain type earthing level can not be too much, generally does not exceed two levels, otherwise the noise will be severely added.

(2) obstruction of the system

In some large audio systems. It often consists of many subsystems. Most of these systems are far from each other and have independent grounding systems. Once the 2 systems are connected to the ground, they must become earthing noise. On the other hand, because the transmission interval is long, the grounding resistance of the transmission line shield layer is added, a large number of external electromagnetic field radiation noise is simply introduced.

In practice, if each system operates independently, the noise can be reasonably wired and grounded within the allowed level. However, when the two sub-systems are interconnected, the single-end shielded grounding and long-line sectional grounding are used, and the radiated noise caused by long-interval transmission can not be dealt with. The best way to do this is to install an audio barrier transformer. The two systems are separated by an audio barrier transformer, and the ground wires of the two systems are not connected.

Nowadays, many systems are connected by optical fiber, which is the best. Fiber-optic long-interval audio transmission will not cause the SCR dimming equipment and other stray electromagnetic waves to disturb the audio system due to the skin effect of the cable, and can form a resistance barrier to the audio system at each transmission end, and then prevent the disturbance between the various audio systems.

(3) the correct link of the system.

There are many devices used in audio system. The device has different interface modes, including balanced and unbalanced input and output mode. In order to shield the electromagnetic radiation and disturbance from the outside, it is necessary to use the shielded cable consistently and choose the correct connection method.

When the audio signal transmission is balanced. The common mode disturbance level of the external disturbance power supply is almost equal to the ground loop of the two signal lines in the cable. Therefore, we should try to choose a balanced way of convergence as far as possible.

When connecting with unbalanced output devices. Direct use of a single-core shielded cable will balance the port of the equipment and the port of the unbalanced equipment to connect, instead of using the balance - unbalanced converter. The noise induced by the shield layer is mixed into the audio signal, and then added to the noise, which is a primary way to introduce noise. Therefore, whether the transmission is balanced or unbalanced, double-core shielded cable should be selected, and the shielding layer is only grounded at one end of the balanced output or input.

When the two ends are unbalanced equipment, if the transmission interval is far, the best use of balanced, unbalanced converter or audio barrier transformer conversion to balanced transmission.

Third, power purification

In order to obstruct the disturbing noise caused by the public power grid. The best choice is to separate the purified power supply or to block the transformer. The grounding terminal of the barrier transformer or purifying power source must have outstanding grounding, otherwise the function of the barrier is not good. It should be separated from some high-power electrical appliances which are disturbed. Power alone. The filter can also be added to the input of the audio device power supply to filter the disturbance noise.

In practice, by changing the orientation of the front and zero line of the single-phase power supply audio equipment, an interpolation method with the least noise can be found, and some noise disturbance can be reduced.

These are the reasons for noise in audio systems such as broadcasting, television, KTV, home theater and the corresponding treatment plan. In practice, if the cause of noise can not be found out in a short period of time, we can also adopt useful methods such as noise gate, noise canceller, filter to reduce noise.