What Is Television Interference?

There is wood grain interference. The appearance of this interference will not drown the normal image when it is slight, and the image will not be viewed when it is severe (or even destroy the synchronization).

Interference resolution

Right!

There is wood grain interference. The appearance of this interference will not drown the normal image when it is slight, and the image will not be viewed when it is severe (or even destroy the synchronization).

Chinese name

Interference resolution

influences

Images cannot be viewed

Features

Wood grain interference

the reason

The quality of the video transmission line is not good

Wood grain interference

The appearance of such interference will not overwhelm the normal image when it is slight, and the image cannot be viewed when it is severe (even destroying synchronization). There are many and complicated reasons for this failure phenomenon. There are roughly the following reasons:

(1) The quality of the video transmission line is not good, especially the shielding performance is poor (the shielding network is not a good quality copper wire network, or the shielding network is too thin to serve the shielding function). At the same time, the line resistance of this type of video cable is too large, which causes a large attenuation of the signal and also aggravates the failure. In addition, the characteristic impedance of this type of video cable is not 75 and the parameters are beyond the specifications. Because the above-mentioned interference phenomenon is not necessarily a failure caused by a bad video cable, the cause of this failure must be accurate and cautious when judging. Only after excluding other possibilities can we consider it from the perspective of poor video lines. If it is really a cable quality problem, the best way is of course to replace all such cables with cables that meet the requirements. This is the best way to completely solve the problem.

(2) Caused by the power supply system is not "clean". The power supply is not "clean", which means that the interference signal is superimposed on the normal power supply (50-cycle sine wave). And most of the interference signals on this power source come from the silicon controlled equipment in this power grid. In particular, high-current, high-voltage thyristor equipment has a very serious pollution to the power grid, which has caused the power supply in the same power grid to be clean. For example, there are high-power thyristor frequency and speed regulating devices, thyristor rectifiers, thyristor AC-DC converters, etc. in this power grid, which will pollute the power supply. The solution to this situation is relatively simple, as long as the entire system is powered by purified power or online UPS.

(3) There are strong interference sources near the system. This can be judged through investigation and understanding. If this is the reason, the solution is to strengthen the shielding of the camera and ground the pipeline of the video cable.

2. Deep and chaotic large-area moire interference

In severe cases, all the images are destroyed, and no image or synchronization signal can be formed. This kind of failure is caused by the short-circuit or disconnection of the video cable's core wire and the shielding network. This situation often occurs on BNC connectors or other types of video connectors. That is to say, when this kind of failure phenomenon occurs, it is often not that all the signals of the entire system are out of order, but only appear on those that have bad connections. Just check these connectors carefully one by one and you can solve it.

3. Several vertical bars with equal spacing

The frequency of the interference signal is basically an integer multiple of the horizontal frequency. This is due to impedance mismatch caused by the characteristic impedance of the video transmission line is not 75. It can also be said that the occurrence of this interference phenomenon is caused by the combination of the characteristic impedance and distribution parameters of the video cable. The solution usually depends on the method of "starting series resistance" or "terminal parallel resistance". In addition, it is worth noting that when the video transmission distance is short (usually within 150 meters), using the video cable with the above impedance mismatch and excessive distribution parameters does not necessarily cause the above-mentioned interference phenomenon. The fundamental solution to the above problems is to ensure quality when purchasing video cables. If necessary, the cables should be sampled.

4. Space radiation interference introduced by transmission lines

Most of this interference is caused by strong, high-frequency spatial radiation sources near the transmission system, the front end of the system, or near the central control room. One solution to this situation is to understand the surrounding environment when the system is established, and try to avoid or stay away from the radiation source; the other method is to strengthen the shielding of the front-end and center equipment when the radiation source cannot be avoided. The pipeline of the transmission line is made of steel pipe and well grounded.

How surveillance cameras can improve anti-interference ability

Because surveillance cameras generally use the most common transmission method is video baseband transmission (baseband transmission refers to the method of directly transmitting television signals without any processing such as frequency conversion). The advantage of this transmission method is the simple structure of the transmission system. In a certain range, the distortion is low and the noise is low (the system signal-to-noise ratio is high). The disadvantage is that the transmission distance cannot be too far, and it must be transmitted within the range required by the cable characteristics. A video coaxial cable can only transmit one TV signal at a time.

Because this transmission method has the advantages of high stability, simple equipment used in the system, and convenient wiring, it has been widely used in real life. However, the high bandwidth of the cable and the use of actual low frequency cause the signal to be transmitted in the cable. The amplitude and phase of the cable will be significantly different between the low frequency band and the high frequency band, especially when the phase distortion is too large. Circuit for compensation. And the low-frequency part of baseband transmission is easily susceptible to interference from interference sources such as strong electricity, transmission towers, base stations, motors, and frequency converters. For example, common interference sources in engineering:

1. Broadcast interference:

When the cable is erected in the air, the cable itself is equivalent to a long antenna. As a result of the antenna effect, a considerable broadcast interference voltage will be generated in the cable, and an interference current will be generated on the cable sheath. This current forms a loop through the ground points at both ends of the cable and ground, so broadcast interference will occur on the terminal load The voltage of the signal makes the interference signal mixed into the video signal. This interference signal appears as dense horizontal lines, vertical lines, diagonal lines, etc. on the image, and even the entire video image will be drowned when it is serious.

2. High-frequency interference:

The shielding effect of the cable shielding layer for signals with lower frequencies is worse. The high-frequency interference signals introduced due to this reason are carrier telephones, radio signals, and so on. They cause horizontal streaks in the image.

3. Power interference:

When the system needs to be grounded at the beginning and the end at the same time, due to the difference in ground potential at both ends and the existence of the cable sheath resistance, a 50Hz ground potential difference is caused between the two grounds, thereby generating an interference signal voltage. When the interference signal is superimposed on the video signal, a wide horizontal dark band and the like appear on the normal image.

4. Harmonic interference:

Harmonic interference is mainly manifested around high-current or high-voltage power lines. It is the radiation signal of power cables to the surroundings. Its frequency is 2500Hz and 125000Hz, which mainly interferes with the low frequency band of video signals.

5. Transmission line interference:

The quality of the video cable is poor, the shielding performance is poor (sparse shielding layer or non-copper dielectric shielding layer, etc.), the cable resistance is too large, and the video signal is seriously attenuated.

6. Unclean power interference:

For example, there are high-power thyristor frequency and speed regulation devices, thyristor rectifiers, and thyristor AC / DC converters in the power grid, which will pollute the power supply. An unclean power source makes the camera and other active equipment unstable and causes interference.

The above parts make the video image on the scene strongly interfered. We use SHWIT brand video anti-jammers to solve the above interferences. The specific solution is as follows:

1. Frequency shift: frequency shift technology is used to shift the video signal (0-6MHZ) to a range of 49-300MHZ; the video signal is transmitted through the anti-jammer transmitter for long distance transmission, and the signal is restored to normal video at the receiver Signal output. Due to the increased frequency, multiple interference signals are effectively avoided while transmitting over long distances;

2. Encoding: The video signal is digitally encoded after entering the anti-jammer, and then decoded before reaching the monitoring center equipment. In order to avoid the interference of the interference source on the video signal and the crosstalk and attenuation of the video signal itself;

3. Enhancement: As all signal transmission distances increase, the signal attenuation increases, so we must strengthen the signal carrier capability at the transmitting end so that it can transmit longer distances;

4. Self-adaptation: The device contains the function of automatic adjustment and adaptation to the signal strength, so the device has the ability to automatically adapt to transmission from 0 to its longest distance

5. Free debugging: A good device should not have a lot of debugging parts, so our device already has 90% market adaptability when it leaves the factory, basically it can be used when connected, without unnecessary debugging;

6. Safety: The design concept of wide voltage (12V-24V), to avoid the sudden increase or sudden drop of power supply voltage to burn out the equipment, to protect the safety of equipment and operators;

7. Stability: The long-term aging and test tests before the equipment leaves the factory, as well as the statistics of the monitoring results of the project, this product can achieve continuous use around the clock and can meet long-term and stable working requirements.