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Home>> Technical Service >> Common problem
How to identify video cable is good or bad?
Source: | Author:Haiwei | Published time: 2016-04-12 | 975 Views | Share:
The quality of the video line identification, detection is a real need for specialized equipment and instruments. These apparatus and equipment design and engineering unit is not available. Engineering practice, how to identify it good or bad video cable:　　
一、 the visual appearance of identification: 
PVC sheathed:
1. The surface can see pressure inside the net regularly "roughness" and shows good processing technology, won't produce the relative sliding, is a good cable.Appearance is smooth, can not see compaction net "roughness", knead sheathed have loose feeling with the hand, is a bad cable; 
2. Check the shield net: make up several is enough?Copper net, check the weldability, copper wire, tin copper scrape see full aluminum magnesium alloy wire is bigger than the hardness of the copper wire;Fabric is thin, uneven distribution, and poor insulation package is not tight and cable; 
3. Check the conductor: - SYV cable diameter 0.78 0.8 mm, SYWV cable 1.0 mm;Recently there is a SYV75-5 conductor cable is 1.0 mm in diameter, the characteristic impedance of the cable, certainly not 75 ohms, not applied to 75 ohms transmission system; 
4. Check the conductor and insulation layer of touch force: oblique incision insulating layer, according to the direction of stripping away conductor, the conductor and insulation ever touch us craft materials;Good cables have larger touch together, bad cable untouched; 
5. The longitudinal tensile experiment: take a meter cable, layered peeling conductor, insulation, shielding layer, outside the door, each 10 cm long.The method is: two hands grip the adjacent two layers of the cable respectively, pull to the opposite direction.Good cable power without moving, generally poor cable at no cost to you can easily pull out - this one is very important, the elevator cable many so-called "elevator cable have this aspect of the problem;
       二、Transmission performance test: 
     Video line, just as its name implies, is used to transmit video signal transmission line.Since be transmission video signal, at least to understand the transmission line in the range 0 to 6 m band transmission characteristics, in other words, the transmission performance.Here mainly talk about some "oscilloscope measurement method", a total of reference, because the oscilloscope is essential "weapons" engineering contractor, one of qualification examination shall be checked;The following narrative is based on has been able to skillfully use the oscilloscope.  
1. The color video camera video signal can be used as a standard video signal source: test engineering use cameras, video output in the range should be 1 of the 75 ohm load vp - p, that is, the bottom of the line synchronization head to video signal with the highest white "peak - peak";Note row synchronization head by "0.3 V", color synchronization head (4.43 M) sine wave pulse amplitude is 0.3 the Vp - p;Choose good oscilloscope, sensitivity to amplitude calibration status.Choose a measurement indicators better camera as "video source"; 
2. The test cable to take a bit longer as far as possible, to reduce the error of measurement, such as 1000 meters, the cable intermediate joint must use "F type joint" and coaxial two-way (cable equipment), do not use welding method, because the welding way destroys the continuity of the coaxial and characteristic impedance of the cable. 
3. Measure the dc resistance of the cable data: such as SYV75-5 cable is 1000 meters, dc resistance wire of 35-40 ohms, outside the shielding layer resistance, 1000 meters to 24-36 ohm (shielding layer number is different, the electric resistance difference is very big);SYWV75-5 cable is 1000 meters, the dc resistance wire for 18 to 22 ohm, outside the shielding layer resistance, 1000 meters to 24-36 ohm;Accumulation of this information is useful, not only can determine the cable material quality, and is used to check wiring, pipe quality of the project, such as when people wear tube, line to break, resistance, video signal is weak, wrong interference appeared, and this kind of "accidents" probability is very high, but often overlooked; 
4. Measurement cable high frequency attenuation characteristics: in the end of the measurement gear process and synchronization head range of color, with 0.3 V 0 db benchmark, calculate the attenuation, line with process on behalf of the low frequency attenuation, color synchronization head on behalf of the 4.43 M high frequency failure, - such as: measured 1000 MiHang synchronization head is 0.15 V, according to the calculated the log number of db attenuation multiples of 20 "1000-6 db/M", measured 4.43 M color synchronization head after the 1000 - meter failure decreased amplitude is 30 mv, namely 1/10 times, attenuation for - 20 db / 1000 M;Using this method can accurately grasp the transmission quality of different cables, and the "frequency distortion (high frequency attenuation bad)" is the concept of intuitive, you can more accurately measure the same type and structure of SYV and SYWV cable and the difference between a good or bad performance, compare the difference between different manufacturers and the performance is good or bad, manufacturer can compare the same changes to various batches; 
5. The method can detect the performance of video transmission systems and equipment, such as engineering of each video coaxial cable transmission characteristics, all the way the transmission properties of optical transceiver (ideal) to measure good or bad, don't think so, rf transmission lose, microwave transmission characteristics, the transmission properties of twisted pair, and distribution characteristics of video distributor, matrix switching characteristics of host, pay special attention to when the multiplexed output switch the same road input signal at the same time, if it is found that switch because the more the greater the attenuation, is wrong, should be the same, measuring the later you will master a lot of unqualified products; 
6. Observe the field signals, look at synchronization position distortion big (flat) - should be flat; 
At the same time can also use the oscilloscope at low frequency interference, such as the slowly varying rolling field signals, and the interference is 50/100 weeks, there are a lot of "thatch" beating, mostly frequency harmonic interference, the far end camera disconnect, distal cable conductor short-circuit inside and outside, in the end the oscilloscope can be used directly observed interference waveforms and strength;This method can also check and test anti-interference the real performance of the equipment.