In recent years, for the reasons of comfort and energy saving, the air-conditioning system and lighting system have increasingly applied electronic digital control technology. The wiring of these numerical control systems will cause the weak signals in the microphone circuit to generate electromagnetic pulse noise.
The signal amplitude range carried by the microphone cable is between 1/100 volts (10mV) & #12316; 1/1,000,000 volts. A small error in the wiring process or cable selection may turn the entire microphone system into a large noise collecting system. antenna.
The following chapter summarizes some questions about configuring the microphone system by means of question and answer.
Q1. Under what conditions should a two-core microphone cable be used?
The two-core microphone cable is suitable for environments where noise is not the main consideration and the audio signal level is high, which can reach -20dB-0dB. At this time, the two-core cable shows its advantages of small diameter and low cost. Of course, if it is not the line level but the microphone level, a four-core star-quad cable should be used.
Q2. Under what conditions should a four-core star quad microphone cable be used?
A four-core star-quad microphone cable should be used in an environment where the noise factor is more important, or the signal level is only -50dB or even lower. Compared with the two-core shielded cable, this cable can shield 90% of the noise (as shown in the figure)
Twisted pair noise (MVVS)
However, if this kind of cable is to be routed together with a large-capacity power cable, it should be inserted into a metal tube for safety.
<Test conditions>
The 20-meter microphone cord is placed parallel to the 20-meter power cord.
Connect the power cord to the dimming device and set it at the 50% output position of 1KW;
The noise introduced into the microphone line is amplified by a 50dB preamplifier and detected by a spectrum analyzer.
Q3. Is the four-core star-quad cable not expensive?
The cost of this kind of cable has dropped drastically in recent years. Decades ago, only large concert halls and professional radio stations could afford them. In 1981, Qituo Electronics developed a cost four-core star-quad cable with aluminum foil shielding. In addition to traditional professional equipment, this kind of cable is also widely used in many non-traditional fields such as wedding halls and school classrooms.
Q4. If metal conduit is not used, how far should the microphone cable be from the power cable?
If you do not use metal conduits, you can check the distance between the cables from Figure 2. Note that if the spacing is not considered when laying the cables, it is easy to cause very troublesome noise problems in the future. Since the signal in the microphone cable is very weak, metal conduit should be used as much as possible.
<Prerequisites>
1. The microphone cable is a star twisted cable;
2. The power cord has a round rubber sheath.
Q5. What issues should be considered when using a cabinet with a very large current?
The same as the answer to question 4 (question to be considered when not using metal conduit).
Q6. What are the disadvantages of laying cables with soft metal conduits?
Metal hoses can reduce noise, but they are not as effective as metal hard pipes. Figure 3 recommends using metal hoses to keep the cable spacing when threading.
The distance between the microphone cable and the power cable when the metal hose is used
<Prerequisites>
1. The 4-core star-quad microphone cable runs through the metal hose;
2. The metal hose is grounded;
3. The power cord uses a round rubber insulation sheath.
Q7. What is "corrugated shielding"?
The corrugated shield <picture below> is a shielding layer that is spirally wound around the cable with a metal tape, which is mainly used for underground cables. The shielding effect is between the bare cloth cable and the metal pipe threading. The disadvantage is poor flexibility, so be careful when bending.
Q8. There are many types of microphone cables. What is the selection standard?
The shielding effects of various types are basically the same, and they are only different in some specific applications. The cable type should be selected according to the specific use requirements.
Braided copper tape shield
No matter how many times it is bent, twisted, twisted or retracted, the copper mesh shield can maintain a good shielding ability. This type is ideal when used as a handheld cable or extension cable. In order to ensure a high shielding ability, this kind of cable is woven particularly finely, so it is more expensive than other types of cables and has a better effect. It requires professional experience to deal with the end.
Spiral shield
Spiral shielding is to wrap the copper wire tightly around the cable in a spiral shape. Winding twice, each winding in the opposite direction is also called "double helix shielding", the shielding effect is better. The price of spiral shielding is roughly between the prices of copper mesh shielding and aluminum foil shielding. Although the end processing is relatively simple, frequent activities will not reduce the performance of the spiral shield. The spiral shield is designed for fixed installation.
Aluminum foil shield
Aluminum foil shielding is to melt aluminum foil on a polyester film to make aluminum foil tape, and use the aluminum foil tape to wrap the cable. The end of this kind of cable is easy to handle, and the price is relatively cheap, and it is suitable for fixed installation. For the aluminum foil shielded cable that needs to be routed through the metal pipe, the type with filler rope must be used. When threading, the filling rope can protect the cable from twisting or retracting due to excessive force. Aluminum foil cables are now widely used in multi-function halls, stadiums and other environments.
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