Video Interference & Solutions

1. During video transmission, the most common breakdown is that there is a black whiffletree or white whiffletree with upward or downward rolling slowly. So when analyzing such breakdown, we should distinguish the two reasons which both can cause this breakdown.

In order to determine reason lies in whether power supply or ground loop, one method is that putting in control server a signal from a closer camera with normal power supply. If there is no interfering appearance, control server is normal. Then it is operable to connect a portable monitor with closer video-out interface of front-end camera, checking cameras one by one. If there are problems in camera, we solve them. If there is no problem, interference may be caused by other reasons such as ground loop.

2. Monitor appears grainy interference: such interference with slightness can not cover image, but with severity cover image thoroughly (even destroy synchrony). Such breakdown can be caused by many reasons and complex. They usually involve below factors:

• Poor quality transmission cable, especially with poor screen quality (screened net is not composed by good copper wires, or too sparse to screen). Moreover, breakdown becomes more serious caused by another reason that such cable with high resistance leads to reducing signal greatly. Besides, such cable is not with 75Ω resistance and parameter over specification are the other two reasons causing breakdown. Due to defective video cable is not the sole reason causing interference, so we should carefully and accurately judge reason for such kind of breakdown. After excluding other reasons, we analyze from video cable defective. If problem lies in cable quality, we had better replace all poor cables by those meeting requirements, which is the best way to solve interference.
  
• Due to “impurity” of power supply:  here power “impurity” means there is interference signal in normal power (50Hz sine wave). Such interference signal mainly comes from equipments with controlled silicon, especially large currency, high voltage silicon which pollutes power seriously, leads to “impurity” in same power network. Such as in this power network, there are big power silicon, frequency adjusting, speed adjusting device, controlled silicon rectifier, and controlled silicon AC-DC converter. This situation is easy to solve by adopting pure power or online UPS power supply.
  
• System nearby having intensive interference: it can be judged by investigation. If this reason is affirmed, the best way is strengthening camera screen and setting grounding for tube of video cable.

3. Breakdown caused by short circuit or open circuit between video cable core and screened network: such breakdown shows that there is big area, deep, messy moire interference on monitor, even image is destroyed thoroughly and fail to form image and synchronous signal. Such situation usually appears on BNC joint or other video joints. In another word, this breakdown is not caused by all channels signal problem but just those channels signal with poor joint. The best solution is checking these joints carefully.

4. Breakdown caused by resistance unsuitable for transmission cable: such appearance shows that there are some vertical interferent lines on monitor image with same space between each other, and interference frequency is integral multiple of line frequency. This is caused by that transmission cable resistance is not 75Ω, which causes resistance unsuitable. By other words, this breakdown is caused by both video cable resistance and distribution parameter which do not meet requirement. Solution is usually solved by “initial end serial resistance” or “terminal end merge resistance”. Besides, what should be noticed is that in short distance transmission (within150m), adopting video cable with unsuitable resistance and overlarge distribution parameter do not always causes the above problem.

5. Interference coming from space radiation of transmission cable: this interference is usually caused by intensive, high frequent space radiation source nearby transmission system, system front-end or control center. Solution for this problem is that when system setup, we are familiar to surrounding environment, in order to keep away radiation source; the other solution is that when radiation source can not be kept away, we screen front-end and central equipments, covering transmission cable with steel tube and adopting good grounding.

The best solution for above problems is that when buying video cable, we must ensure its quality, even check sample of them.