Summary：Analysis and Solutions for Transmission System Faults: Video surveillance transmission systems primarily rely on video transmission. Due to space constraints, the following discussion focuses on analyzing fault phenomena associated with video transmission and proposing corresponding solutions.
A. The most common fault encountered in video transmission is 50 Hz power-line interference. This manifests as the appearance of... on the monitor screen.
Analysis and Solutions for Transmission System Faults: Video surveillance transmission systems primarily rely on video signal transmission. Due to space constraints, the following discussion focuses on analyzing faults associated with video transmission methods and proposes corresponding solutions.
A. The most common fault in video transmission is 50Hz power frequency interference. This manifests as a black or white horizontal bar appearing on the monitor screen and slowly scrolling upwards or downwards. Such phenomena are usually caused by a ground loop within the system, which introduces 50Hz power frequency interference (AC interference). It is worth noting that this fault—sometimes appearing as two black or white bars—can also result from poor power supply performance (or partial damage) in the camera or the control unit (matrix switcher); therefore, when analyzing this type of fault, it is essential to distinguish between these two potential causes. A simple method to differentiate between a power supply issue and a ground loop issue is to connect the output signal of a single camera with a known-good power supply directly to the control unit. If the aforementioned interference does not appear on the monitor, the control unit is functioning correctly. Next, use a portable monitor to check the video output of each camera at the source one by one to identify any cameras causing interference due to power supply problems. If a faulty camera is found, address the issue accordingly; if not, the interference is likely caused by a ground loop or other factors.
B. "Wood-grain" pattern interference appears on the monitor. In mild cases, this interference does not obscure the normal image, but in severe cases, the image becomes unviewable (and synchronization may even be disrupted). The causes of this fault are numerous and complex. They generally include the following: poor quality of the video transmission cable, particularly regarding shielding performance (e.g., the shielding mesh is not made of high-quality copper wire or is too sparse to provide effective shielding). Additionally, excessive line resistance in such cables causes significant signal attenuation, which exacerbates the fault. Other contributing factors include characteristic impedance deviating from the standard 75Ω and distributed parameters exceeding specified limits. Diagnosing this type of fault is difficult, and since the installation (cabling) is already complete, resolving it by replacing the cable is often impractical. Therefore, it is essential to ensure from the outset that the video cables selected meet all standards and requirements. One must never purchase low-quality video cables simply to save money, as this can lead to endless future problems. Since the aforementioned interference phenomena are not necessarily caused by defective video cables, diagnosis must be accurate and cautious; one should only consider cable defects as the cause after ruling out other possibilities. The diagnostic procedure involves—after eliminating other potential causes—sending a sample of the cable in question (or a section cut from the system if no spare is available) to a testing facility for inspection. If the cable fails the test, the issue is confirmed to be a quality problem. In such cases, the best solution is to replace all cables of that type with ones that meet the requirements; this is the most effective way to resolve the issue completely. If the interference is not severe, powering the entire system via a purified power supply or an online UPS can often mitigate or virtually eliminate the interference. However, the effectiveness of this method can vary—sometimes working and sometimes not—depending on the ambient signal environment surrounding the system. Such issues often stem from "unclean" power supply conditions. "Unclean" power refers to a standard power supply (50 Hz sine wave) that has interference signals superimposed upon it. These interference signals typically originate from equipment within the same power grid that utilizes thyristors (SCRs). High-current, high-voltage thyristor equipment, in particular, causes significant grid pollution, resulting in "unclean" power for other devices on the same network. Examples include high-power thyristor-based variable-frequency drives, rectifiers, and AC/DC converters, all of which can pollute the power supply. This type of problem is relatively easy to resolve: simply powering the entire system with a purified power supply or an online UPS usually provides a solution.
C. Presence of strong interference sources near the system. This can be determined through investigation and assessment. If this is the cause, solutions include enhancing camera shielding and grounding the conduits used for the video cables. Faults caused by short circuits or open circuits between the video cable's core conductor and its shielding mesh. This type of fault manifests as extensive, dark, and chaotic moiré-like interference patterns on the monitor, often rendering the image completely unrecognizable and preventing the formation of a coherent picture or synchronization signal. Such issues typically occur at BNC connectors or other types of video connectors; the problem can usually be resolved by carefully inspecting each connector individually. A key characteristic that aids in diagnosing this fault is that it rarely affects every signal channel in the system simultaneously; instead, it appears only on the specific channels where the connectors are faulty.
D. Faults caused by a mismatch in the transmission line's characteristic impedance. This phenomenon manifests as several evenly spaced vertical interference bars on the monitor screen, with the interference signal frequency essentially being an integer multiple of the line frequency. This occurs because the video transmission line's characteristic impedance deviates from the standard 75Ω, resulting in an impedance mismatch. Observing the waveform of the interfered image on an oscilloscope reveals high-amplitude line-frequency harmonic oscillations superimposed on the "back porch" of the line synchronization pulse; this is the source of the interference. Waveform analysis and quantitative measurements of the video cable reveal that cables failing to meet impedance requirements also fail to meet specifications regarding distributed parameters—a factor that actually contributes to the impedance mismatch. Consequently, this interference arises from a combination of the video cable's characteristic impedance and distributed parameters failing to meet requirements. Such issues are generally resolved by connecting a resistor in series at the source end or in parallel at the terminal end. It is worth noting that over very short transmission distances (typically under 150 meters), cables with impedance mismatches or excessive distributed parameters may not necessarily produce the aforementioned interference. Therefore, when analyzing this type of fault in a system with varying transmission distances, one should not be misled by the absence of interference on short-distance runs. The fundamental solution is to ensure high quality when purchasing video cables, conducting sample testing if necessary.
E. Interference caused by radiated signals picked up by the transmission line. This type of interference usually arises when there is a strong, high-frequency source of radiated signals in the vicinity of the transmission system, the system's front-end equipment, or the central control room. One solution to this situation is to assess the surrounding environment during system setup and make every effort to avoid or stay clear of radiation sources. Another approach, when avoiding such sources is not possible, is to enhance shielding for the front-end and central equipment, and to use steel conduits—properly grounded—for transmission lines.
ServiceService support
Hot Products
Solution
Contact information
