In the field of modern communication and audio-visual transmission, coaxial cable is a very common cable, which is widely used in home theater, radio and television, data network and other scenarios. However, there are very few people who really understand it, and buying an inappropriate coaxial cable may also affect the transmission efficiency.
Today, let's take a closer look at coaxial lines.
1. The construction and transmission principle of coaxial cable
Users like coaxial cable, of course, because it has excellent signal transmission performance, high transmission frequency, good anti-interference ability and stable signal quality. Especially in audio systems, coaxial cables can ensure that the signal transmission process is clear and accurate.
All coaxial wires are inseparable from the structure of "central conductor-insulating-shielding-outer sheath".
The center conductor is the core part of the coaxial wire, which is mainly responsible for the transmission of electrical signals and is usually made of materials such as copper, silver-plated copper, etc. The center conductor is surrounded by an insulating layer, which is mainly used to separate the center conductor from the outer shield and maintain the characteristic impedance of the cable.
The outside of the insulation layer is a metal shielding layer, which plays a shielding role in electromagnetic interference and radio frequency interference, which can not only prevent the signal leakage of cable transmission, but also prevent external signals from interfering with the signal transmitted coaxially. The materials of the shielding layer are relatively rich, such as metal braided aluminum foil, copper foil, copper wire, and so on. Finally, the outer layer of the coaxial cable is protected, which is in direct contact with the external environment to protect the inside of the cable from external interference and damage. Like other types of wires, the outer layer of protection is generally made of PVC, PE and other materials.
So, how does coaxial wire work transmission?
The high-frequency current transmitted by the central conductor of the coaxial wire can be seen as an antenna that emits radio outwards, which is essentially a kind of "alternating current". Recall that in junior high school physics, "the direction of alternating current changes periodically, and there are many reversals in a second. In order to reduce the impact of uncertainty in the direction of the current, the design of the coaxial cable incorporates a "mesh conductive layer isolation", that is, a shielding layer, which can control the radio emitted by the central conductor by grounding.
However, the current transmission process is not so smooth, if a certain section of coaxial cable encounters a relatively large extrusion or twist deformation, it is likely to cause the internal radio waves to be reflected back to the signal transmission source, affecting the signal quality at the receiving end. Therefore, in order to overcome this problem, it is necessary to add an insulator between the central conductor and the shield layer to ensure that the distance between them is consistent and to maintain the stability of current signal transmission.
Second, the type of coaxial line
In the 40s of the last century, coaxial cables began to be widely used in the field of military communications and radar systems. With the popularization of TV broadcasting, it is applied to the transmission of TV signals to provide users with more stable reception of TV programs. Since then, coaxial cable has experienced the development of cable television networks, computer networks, broadband Internet, home theaters and other fields. According to the purpose of the cable, coaxial cable can be basically divided into two types:
(1) Baseband coaxial cable
Baseband coaxial cables can be used for both analog and digital signals, transmitting a single signal at a very high speed at a time, and the entire baseband bandwidth is used to transmit a single signal. The shielding layer of the baseband coaxial cable is generally a mesh structure made of copper, and the characteristic impedance is 50Ω, and the common types such as RG-8 and RG-58 are common.
(2) Broadband coaxial cable
Broadband coaxial cables can transmit multiple signals simultaneously using different frequencies. Its shielding layer is usually aluminum foil, copper foil, and the characteristic impedance is 75 ohms, such as RG-59. The coaxial cable used in audio and video transmission is usually a broadband coaxial cable, which can provide low signal loss and high signal quality during transmission.
In order to compare the performance difference between the two more intuitively, the editor made a comparison chart:
3. Briefly talk about the advantages and disadvantages of coaxial cables
There are several advantages: first, strong anti-interference ability. The coaxial cable has a good shielding effect and can effectively prevent electromagnetic interference. Second, the transmission is stable and can maintain high signal quality. Third, it has a wide range of applications. With these advantages, coaxial cable is widely used in various audio and video transmission scenarios.
However, there are also some shortcomings of coaxial cables. In terms of physical properties, the coaxial cable is less flexible and not easy to bend. In audio transmission, coaxial cables are limited in the transmission of high-specification surround tones such as 5.1.2 and 7.1.4. In video transmission, coaxial cables (RG-6, RG11) with sufficient bandwidth can only transmit high-definition video signals over medium distances, but the transmission performance of coaxial cables is not as good as that of HDMI cables over longer distances or higher resolutions.
4. How to choose a high-quality coaxial cable?
Knowing some basic knowledge of coaxial cables, we can choose the right coaxial cable for home theater transmission:
First, choose the right impedance. At present, there are two main impedances of coaxial cables: 75Ω and 50Ω, and the specific choice of which type depends on the type of interconnection equipment and devices. 50Ω is more commonly used for data and wireless purposes, and for audio and video signal transmission in home theater equipment such as TVs, DVD players, Blu-ray players, etc., a coaxial cable with a 75Ω impedance is usually chosen.
Second, you can look at the conductor material and thickness. High-quality conductors generally choose high-purity copper or silver-plated copper as the material, and single crystal copper will also be selected for occasions with higher conductivity requirements. These materials have excellent electrical conductivity, which reduces signal loss and transmission delay. What is the importance of conductor thickness? The thicker the conductor, the stronger the signal transmission capacity and the lower the loss. For long-distance transmission, a thicker conductor is more suitable. The home audio and video transmission belongs to the short-distance transmission of "< 100 meters", you can choose 20AWG or 22AWG specification coaxial cable, if there is a higher demand, consider 18AWG.
Third, pay attention to the use of insulating materials. The performance is better than polyethylene (PE), polytetrafluoroethylene (PTFE), foamed PVC and so on. The uniform thickness of the insulating material can avoid instability and reflection in signal transmission, effectively isolate conductors and shields, and maintain the characteristic impedance of the cable.
Fourth, the quality of the shielding layer. Some coaxial cables may have multiple layers of shielding to further reduce interference from coaxial signal transmission. A high-quality shield is usually made of a densely woven copper mesh or aluminum foil, and the denser the shield, the stronger the anti-interference ability. Some higher-quality cables may also add an outer conductor to provide stronger extrusion resistance and higher rigidity.
The CX200 is a high-quality coaxial cable, with a center conductor of 18AWG, strong signal transmission capacity, and also meets the specific impedance of 75Ω of digital coaxial cable, which is easy to use in home theater systems and professional audio equipment connection applications. The CX200 coaxial cable is made of high-purity single crystal copper conductor and silver-plated to make the transmission performance more stable. In the selection of insulator materials, physical foam is selected, which is better than PCV and PE insulation. It also attaches great importance to the shielding layer, using three layers of shielding of "tinned copper mesh + graphene + aluminum foil", which can achieve 99.99% shielding interference and easily provide stable cinema-level audio signal transmission.
GIEC CX200 monocrystalline copper silver-plated RCA audio coaxial digital audio cable coaxial sound bar TV set-top box power amplifier audio Blu-ray DVD cable woven mesh 2m [picture price brand quotation] - JD.com
Through the full description, I believe you also have an in-depth understanding of the coaxial line and learn how to choose a high-quality coaxial cable. In short, choosing a good coaxial cable can help us improve the quality of audio experience.
(Part of the picture comes from the Internet)