How to distinguish the quality of the video cable
Central issues:
Visual appearance identification of the video cable Visual performance identification of the video line:
1. PVC sheath: The surface can see the regular "unevenness" of the braided mesh inside, indicating that the processing technology is good, and it will not produce relative sliding, which is a good cable. The appearance is smooth, the "unevenness" of the compressed braid is not visible, and the jacket has a loose feeling with the hand, it is a poor cable;
2. Check the shield layer network: is the number enough? Copper braided wire mesh, check solderability, tinned copper wire scrape to see if it is copper wire, the hardness of aluminum-magnesium alloy wire is significantly greater than copper wire; braided wire mesh is sparse, unevenly distributed, not tightly wrapped with insulating layer, etc. is poor cable;
3. Check the core wire: diameter-SYV cable is 0.78-0.8mm, SYWV cable is 1.0mm; recently a SYV75-5 core wire diameter is 1.0mm cable, the characteristic impedance of this cable is definitely not 75 Ohm, not applied to 75 ohm transmission system;
4. Check the adhesion force of the core wire and the insulation layer: cut the insulation layer obliquely and pull the core wire away in the stripping direction to see if the core wire and the insulation layer are bonded with process materials; good cables have a large adhesion force, poor The cable is not bonded;
5. Longitudinal tensile test: take one meter of cable, peel off the core wire in layers, insulation layer, shielding layer, outer house, and leave 10 cm long each. The method is: two hands hold the adjacent two layers of the cable and pull in the opposite direction; the good cable generally cannot be pulled by force, and the poor cable can be easily pulled out without much effort-the elevator cable is very important, many so-called "Elevator special cables" have problems in this regard;
Transmission performance test:
Video cable, as the name suggests, is a transmission line used to transmit video signals. Since it is a video signal transmission, at least we must understand the transmission characteristics of the transmission line in the 0-6M band range, or rather, the transmission performance. Here we mainly talk about the "oscilloscope measurement method" for reference, because the oscilloscope is one of the necessary "weapons" of the engineering business and one of the necessary equipment for qualification review; the following description is based on the ability to use the oscilloscope proficiently.
1. The color camera video signal can be used as the "standard video signal source": the video output of the camera used in the test project, the amplitude on the 75 ohm load should be 1Vp-p, that is, the bottom of the line sync head to the highest white level of the video signal "Peak-peak value"; note that the amplitude of the line sync head is "-0.3V" and the amplitude of the color burst head (4.43M sine wave pulse) is 0.3Vp-p; select the sensitivity of the oscilloscope and hit the amplitude calibration state. Select a camera with a good index as the "video source";
2. The test cable should be as long as possible to reduce the measurement error, such as 1000 meters, the cable middle connector must use "F-type connector" and coaxial double-pass (cable TV equipment), do not use the welding method, because the welding method damages the cable Coaxiality and continuity of characteristic impedance.
3. Measure the DC resistance data of the cable: such as SYV75-5 cable 1000 meters, DC resistance core wire is 35-40 ohms, outer shielding layer resistance is 1000 meters is 24-36 ohms (different number of shielding layers, the resistance varies greatly) ; SYWV75-5 cable 1000 meters, DC resistance core wire is 18-22 ohms, outer shield resistance is 1000 meters is 24-36 ohms; accumulating information in this area is useful, not only can judge the quality of cable materials, but also used for engineering Check the quality of wiring and pipe penetration.For example, when Gu Ren wears a pipe, the wire is broken, the resistance value becomes larger, the video signal becomes weaker, and interference that should not occur also appears. The probability of this type of "accident" is very high, but Often overlooked;
4. Measure the high and low frequency attenuation characteristics of the cable: measure the line head and color burst amplitude at the end, calculate the attenuation with 0.3V as the 0db reference, the line head represents the low frequency attenuation, and the color burst represents the 4.43M high frequency attenuation ,-For example: the measured 1000-meter line sync head is 0.15V, the db number of the attenuation factor calculated according to 20log is "-6db / 1000m", and the measured amplitude of the 4.43M color sync head after 1000 meters attenuation is 30mv, which is 1 / 10 times, attenuation is -20db / 1000m; with this method, you can accurately grasp the transmission quality of different cables, and have an intuitive concept of "frequency distortion (high and low frequency attenuation difference)", you can measure it more accurately The difference and performance of SYV and SYWV cables of the same model and structure are compared, and the difference and performance of different manufacturers' products are compared, and the changes of different batches of products from the same manufacturer can also be compared;
5. The above method can also detect the performance of the video transmission system and equipment: such as the transmission characteristics of each coaxial video cable in the project, the transmission characteristics of the optical transceiver (you can measure the quality, do not think it is so ideal), RF transmission, microwave Transmission characteristics, the transmission characteristics of twisted pair, the distribution characteristics of the video distributor, and the switching characteristics of the matrix host. Pay special attention to when multiple outputs simultaneously switch the same input signal. No, it should be the same. After testing, you will have a lot of unqualified products;
6. Observe the field signal and see if the distortion of the field synchronization position is not large (uneven)-it should be very flat;
7. At the same time, you can also use the oscilloscope to observe the low-frequency interference situation: if the field signal has slow fluctuations, it is 50/100 cycle interference, there are many "thatch" jumps, mostly frequency conversion harmonic interference, disconnect the remote camera, the cable The inner and outer conductors of the far end are short-circuited, and the oscilloscope can be used to directly observe the interference waveform and intensity at the end; this method can also check and test the real performance of anti-interference equipment.
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