**Picture Distortion and Interference**
In a video surveillance system, whether using coaxial cables or digital network cables for video signal transmission, external interference can significantly impact the image quality, reducing the effectiveness of monitoring. There are numerous causes of video noise and interference, and it is challenging to fully list them all. However, it is clear that these issues often originate from both the equipment itself and the surrounding environment. The solutions vary depending on the source of the problem. If the issue comes from faulty equipment, replacing the defective component usually resolves it. However, dealing with interference caused by external lines or environmental factors is more complex. In the following sections, we will analyze each of the twelve common phenomena in detail to better understand their root causes.
**Color distortion and twisting:** This issue typically occurs when there is a short circuit between the positive and negative terminals after an analog coaxial cable is connected to the video input. It may also happen if the coaxial core wire is bent, leading to micro-contact issues in the connector housing. Additionally, if the polarity of the transmitter and receiver is reversed, or if the network line’s signal pins are incorrectly connected, color distortion can occur. These problems do not damage the equipment but can be fixed by simply reversing the polarity.
**Horizontal white or black ripples moving upward:** This phenomenon is often due to insufficient voltage supply. For example, if the AC power supply has low voltage or the voltage drop exceeds 10% of the camera or DVR’s rated operating voltage, it can cause screen movement. Similarly, a DC power supply failure that drops below 9V from 12V can lead to this issue. Other possible causes include current limitations that prevent the load device from functioning properly or harmonic currents generated by the power supply.
**Multiple black horizontal lines on the screen:** This is commonly caused by using thin signal cables over long distances, which weakens the signal. It can also occur when a signal distributor without amplification or compensation is used.
**Overlapping images on the screen (ghosting):** This happens when signals reflect back due to incorrect cable impedance or short circuit lengths. It can also result from too many signal assignments, causing delayed reflections and overlapping images.
**Twill or web-like stripes on the video:** These patterns are usually caused by poor-quality transmission lines, especially those with inadequate shielding or isolation. Poorly grounded networks, capacitive reactance from aluminum foil and copper wire, or imbalanced ground impedance (not 75Ω) can introduce floor wave noise into the cable. In severe cases, this imbalance can create a capacitor effect, accumulating current in the cable. Touching the BNC connector can lead to electric shocks or sparks, potentially damaging the equipment. Other causes include unequal ground potentials in the power supply, incorrect phase connections, or nearby high-frequency interference sources like radio towers.
**Color and sound deviation:**
**Lighter colors and unusual hues:** This issue may arise from improper color saturation adjustments, but it can also be caused by damaged cable insulation or poor grounding.
**Screen flickering up and down:** If the signal strength is not the issue, it could be due to the camera losing synchronization because of incorrect grayscale signal adjustment. Poor soldering during installation or improperly wired connections can also cause out-of-sync display. When multiple signals are involved, uneven vertical sync signals may cause screen jitter.
**Water ripple and bright spot noise:** This is often due to improper lens aperture settings. However, it can also result from environmental interference affecting the field signal lines. External noise can cause background subtraction, creating fragmented spots that distort the image and lead to system misjudgment. Adjusting the aperture can often resolve simple bright spot noise issues.
**Color video turns black and white:** This can happen if the camera is set to the wrong video standard (NTSC vs. PAL), a common oversight among technicians. While checking the cables, monitors, and DVRs, the camera's own system settings are often overlooked. However, other causes, such as a short circuit in the wiring, should also be considered.
**Regular white highlights in the image:** This typically occurs when the video transmitter’s line is poor or moisture enters the fiber connector. High-frequency devices, especially near FM/AM radio stations, can also cause such interference. Poor grounding of the monitoring equipment may also contribute to this issue.
**Bright colored patches in dark areas:** This issue appears only on digital optical transceivers and is usually caused by improper signal demodulation. The quality of the manufacturer’s design can also affect the outcome.
**Second video interference in video or audio:** Crosstalk occurs when signals interfere with each other, either through electromagnetic radiation or from the power and ground planes. It can also result from adjacent equipment or close proximity between different types of cables.
As you can see, many of these issues are closely related to cable quality and installation. Before addressing video noise and interference, it's essential to understand how the video frequency spectrum and coaxial cables influence signal integrity. Proper cable selection and installation play a critical role in ensuring reliable and clear video transmission.
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