Cable knowledge that audio and video design must know

In the audio and video system, the signal transmission between the devices needs to be connected, and the connection cable (audio cable) and interface are required, which occupies a very important role in the entire audio and video system.

First, the requirements of wire material

Both audio lines and video lines require good conductivity, that is, the resistivity of the wire material itself should be as small as possible. As can be seen from Table 1, the best conductivity under normal conditions is silver, copper, gold, and aluminum. Gold and silver are expensive, so copper is the most widely used, and almost all application wires are made of copper (aluminum is often used as a high-voltage wire because it is less dense than copper). Plastics and rubbers have a large resistivity and are often used as insulation materials. The most common plugs are silver-white and copper-yellow, silver is copper silver-plated, and copper-yellow is copper. Therefore, the surface of some plugs on the market is gold-plated but worse than the conductivity of copper, and only the conductivity of silver-plated plugs will be improved.

                                     Table 1 Table 1 List of resistivity of several commonly used materials at 20ºC

                                                                          Figure 1 Gold-plated lotus plug × 

                                                    Figure 2 The √ of a large two-pin plug with a silver plating surface

Second, the method of reducing noise interference

Microphone and audio signal lines are very sensitive to interference from electromagnetic fields because of the low level of these sources. During the transmission process, some unwanted interference signals will be picked up, which will affect the sound quality effect. For example, passing near a magnetic field with fluctuations (such as AC power leads), a noise signal will be induced and an AC sound will be heard in the speaker. The main measures to reduce interference signals are:

1. Increase the shielding layer

Typically, the central part of the audio wire is a wire (usually copper wire), the outside of which is covered with non-conductive plastic or rubber, and the outside is a shield made of conductors to isolate external interference signals (sometimes as ground wires). The national standard GB/T8734.5-2006 stipulates that the braiding density of the shield should not be less than 80%, and the diameter of the copper wire should not be less than 0.15mm. The shielding practices are shown in Table 2:

                                                              Table 2 List of common practices for shielding layers

2. Adopt balanced cable

Adding a shield can isolate most of the outside noise interference, but it is not foolproof. Balanced transmission is a very widely used audio signal transmission method, which uses the principle of phase cancellation to minimize the interference during audio signal transmission. Balanced transmission is a transmission method that combines the original signal and the copy signal by transmitting a copy signal in reverse phase with the original signal in the same cable, and then inverting the copy signal. The two wires are twisted together to ensure that any noise encountered along the way is picked up evenly by both cables, and also contributes to the overall stability of the cable. At the input of the second device, the reverse copy signal on the negative terminal is reversed back to the original positive terminal, and the phase difference between the noise present in the original two signals is 180 degrees. After superposition, the noise is eliminated due to the opposite phase, and the original signal remains pure. The balanced cable uses three wires, hot (Hot, which transmits the original signal), Cold (a copy signal that transmits the original signal), and ground (Gnd).

3. Adopt 4-core stranded wire

The 4-core stranded wire is named after its double-balanced pair structure, which minimizes the annular electromagnetic interference zone of the cable and greatly reduces the noise caused by electromagnetic induction of adjacent cables. The double balance pair adopts 5 wires, respectively, 2 hot end wires, 2 cold end wires and 1 ground wire, of which 2 hot end lines and 2 cold end wires need to be arranged cross-arranged, and cannot be arranged next to each other.

                          Figure 3 Schematic diagram of the connection between the 4-core stranded wire and the XLN head

                              Figure 4 Schematic diagram of magnetic field distribution of 4-core and 2-core stranded wires

Third, the requirements for cables and interfaces for mobile performances

Mobile performance cables require easy retraction and anti-pulling (it is easy to be accidentally pulled), so the requirements are:

1. The wire flexibility should be relatively good, easy to bend, and it is made of multiple strands of fine copper wire (diameter about 0.1mm) twisted.

2. The insulation material adopts rubber with good elasticity, which is tensile and torsion-resistant.

3. The inside of the cable is filled with rope, fiber or steel wire to improve the tensile resistance of the cable.

4. It is recommended to use a woven shielding layer for shielding, which has strong anti-deformation ability.

5. The interface must have a locking device to prevent the plug from falling off.

4. Types of audio and video cables

The cables used to transmit the audio signal generated by the sound source (signal source) are collectively referred to as audio cables. It can be roughly divided into 3 categories: microphone cable (cable connecting microphone and other equipment), audio signal line, audio cable (cable connecting power amplifier and speaker), musical instrument line and coaxial cable. Audio signals are mainly divided into analog signals and digital signals, the cables used to transmit analog signals have analog audio signal lines, and the cables used to transmit digital signals include digital audio signal lines, network cables, and optical fibers.

Microphone lines, audio signal lines and coaxial lines transmit signals with low power and low current, and have high impedance and are susceptible to interference. Therefore, in order to match the impedance, such wires are generally thin (the resistance will be correspondingly larger); At the same time, it is necessary to set up a shielding layer to avoid interference from external signals.

The audio line mainly transmits high-power, high-current signals, so the audio line is required to have excellent conductivity, that is, the resistance is small enough.

                                                                Table 3 Common types of audio and video cables

5. Requirements and types of interfaces

The interface requires that the contact surface of the plug and socket is relatively large and tight, and the plug and socket can be silver-plated to improve conductivity.

                                                              Table 4 API requirements and common types