How to distinguish the quality of video cables

Visual appearance identification of video cables Transmission performance test of video cables
Visual appearance identification:

1. PVC sheath: The surface can be seen to have regular "unevenness" of the compressed inner braid, indicating that the processing technology is good and there will be no relative sliding, which is a good cable. The appearance is smooth, and the "unevenness" of the compressed braid cannot be seen. The sheath feels loose when pinched by hand, which is a poor cable;

2. Check the braiding of the shielding layer: Is the number of braids enough? 1. Check the copper mesh for solderability, scrape the tinned copper wire to see if there is copper wire inside, the hardness of the aluminum-magnesium alloy wire is obviously greater than that of the copper wire; the mesh is sparse, unevenly distributed, and not tightly wrapped with the insulation layer, which is a poor cable;

2. Check the core wire: diameter - SYV cable is 0.78-0.8mm, SYWV cable is 1.0mm; recently there has been a SYV75-5 cable with a core wire diameter of 1.0mm. The characteristic impedance of this cable is definitely not 75 ohms and should not be used in 75 ohm transmission systems;

3. Check the adhesion between the core wire and the insulation layer: cut the insulation layer obliquely, pull the core wire in the stripping direction, and see if the core wire and the insulation layer are bonded with the process material; good cables have greater adhesion, and poor cables have no adhesion;

4. Longitudinal tensile test: take a one-meter cable, peel off the core wire, insulation layer, shielding layer, and outer sheath in layers, leaving 10 cm long on each. The method is: hold the two adjacent layers of the cable with both hands and pull them in opposite directions; good cables cannot be pulled with normal force, but poor cables can be easily pulled out without much effort - this is very important for elevator cables, and many so-called "special cables for elevators" have problems in this regard;

transmission performance test:

video cables, as the name suggests, are transmission cables used to transmit video signals. Since it is transmitting video signals, at least we need to understand the transmission characteristics of the transmission line in the 0-6M frequency band, or in other words, the transmission performance. Here we mainly talk about the "oscilloscope measurement method" for reference, because the oscilloscope is a necessary "weapon" for engineering companies and one of the must-check equipment for qualification review; the following description is based on the ability to use the oscilloscope proficiently.

1. The video signal of a color camera can be used as a "standard video signal source": the video output of the camera used in the test project should have an amplitude of 1Vp-p on a 75-ohm load, that is, the peak-to-peak value from the bottom of the horizontal sync header to the highest white level of the video signal; note that the amplitude of the horizontal sync header is "-0.3V", and the amplitude of the color sync header (4.43M sine wave pulse) is 0.3Vp-p; select the sensitivity of the oscilloscope and set it to the amplitude calibration state. Select a camera with better indicators as the "video source";

2. The test cable should be as long as possible to reduce the measurement error, such as 1000 meters. The middle joint of the cable must use an "F-type connector" and a coaxial double pass (cable TV equipment). Do not use welding methods, because welding methods destroy the coaxiality of the cable and the continuity of the characteristic impedance.

3. Measure the DC resistance data of the cable: For example, for SYV75-5 cable of 1000 meters, the DC resistance core wire is 35-40 ohms, and the outer shielding layer resistance of 1000 meters is 24-36 ohms (the number of shielding layers is different, and the resistance is very different); for SYWV75-5 cable of 1000 meters, the DC resistance core wire is 18-22 ohms, and the outer shielding layer resistance of 1000 meters is 24-36 ohms; it is very useful to accumulate data in this regard, not only to judge the quality of cable materials, but also to check the quality of engineering wiring and pipe penetration. For example, when the customer is wearing the pipe, the wire is broken, the resistance value becomes larger, the video signal becomes weaker, and interference that should not appear also appears. The probability of such "accidents" is very high, but they are often ignored;

4. Measure the high and low frequency attenuation characteristics of the cable: measure the amplitude of the line head and the color synchronization head at the end, and calculate the attenuation with 0.3V as the 0db benchmark. The line head represents low frequency attenuation, and the color synchronization head represents 4.43M high frequency attenuation. For example, the line synchronization head of 1000 meters is measured to be 0.15V, and the db number of the attenuation multiple is calculated according to 20log as "-6db/1000m". The amplitude of the 4.43M color synchronization head after 1000 meters of attenuation is 30mv, that is, 1/10 times, and the attenuation is -20db/1000m. This method 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 more accurately measure the difference and performance of SYV and SYWV cables of the same model and structure, compare the difference and performance of products from different manufacturers, and compare the changes of different batches of products from the same manufacturer.

5. The above method can also detect the performance of video transmission systems and equipment: such as the transmission characteristics of each coaxial video cable in the project, the transmission characteristics of the optical terminal (you can measure the quality, don't think they are all so ideal), RF transmission, microwave transmission characteristics, twisted pair transmission characteristics, video distributor distribution characteristics, matrix host switching characteristics, special attention should be paid to when multiple outputs switch the same input signal at the same time, if you find that the more switching paths, the greater the attenuation, it is not right, it should not change, after testing you will master a lot of unqualified products;

6. Observe the field signal to see if the field synchronization position distortion is large (flat) - it should be very flat;

7. At the same time, you can also use an oscilloscope to check the low-frequency interference: if the field signal has slow fluctuations, it is 50/100 cycle interference, there are many "thatched" jumps, most of which are variable frequency harmonic interference, disconnect the remote camera, short-circuit the inner and outer conductors of the remote cable, and use an oscilloscope to directly observe the interference waveform and intensity at the end; this method can also check and test the true performance of anti-interference equipment.