Although China's wire and cable industry has achieved significant development over the past two decades, it has not yet escaped the vicious cycle of being large but not strong, and still lags far behind world-class enterprises. It suffers from numerous problems, including low industry concentration, a large number of small-scale enterprises, insufficient economies of scale, inadequate independent innovation capabilities, and prominent structural contradictions in its products. With the continuous advancement of market competition, my country's wire and cable enterprises face a market environment of survival of the fittest.
Cable Selection and Quality Identification
Identifying the quality of cables requires specialized equipment and instruments, which are often unavailable to design and engineering firms. So, in practical engineering applications, how do we determine the quality of cables?
1. PVC sheath: A good cable will have a visible, regularly uneven surface indicating the compressed braided mesh. This suggests good manufacturing quality and prevents slippage. A poor cable will have a smooth surface, lack the visible unevenness, and feel loose when squeezed.
2. Check the shielding mesh: Is the braid count sufficient? For copper braided wire, check its solderability. For tinned copper computer wire, scrape to see if the inside is copper wire. Aluminum-magnesium alloy wire is significantly harder than copper wire. Sparse, unevenly distributed braided mesh, or poor wrapping with the insulation layer indicate a poor-quality cable.
3. Check the core wire: Diameter—SYV cables are 0.78-0.8mm, SYWV cables are 1.0mm; recently, a type of SYV75-5 cable with a core wire diameter of 1.0mm has appeared. The characteristic impedance of this cable is definitely not 75 ohms and should not be used in 75-ohm transmission systems.
4. Check the adhesion between the core wire and the insulation layer: Cut the insulation layer diagonally, pull the core wire apart in the peeling direction, and check if there is any adhesive material between the core wire and the insulation layer; good cables have strong adhesion, while poor cables have no adhesion.
5. Longitudinal tensile test: Take one meter of cable and peel off the core wire, insulation layer, shielding layer, and outer sheath layer, leaving 10 cm of each. The method is: hold two adjacent layers of the cable with both hands and pull in opposite directions; a good cable is generally difficult to pull with force, while a poor cable can be easily pulled out without much effort.
Cable installation applications
Video signal transmission typically employs direct modulation technology at baseband frequency (approximately 8MHz bandwidth), with coaxial cable being the most commonly used transmission medium. Coaxial cable is specifically designed for transmitting video signals, exhibiting minimal frequency loss, image distortion, and image attenuation, thus effectively fulfilling the task of transmitting video signals. Generally, dedicated SYV75 ohm series coaxial cables are used, with the common model being SYV75-5 (its unrepeatable transmission distance for video signals is typically 300-500m). For longer distances, SYV75-7, SYV75-9, or even SYV75-12 coaxial cables are required (in practical engineering, the unrepeatable transmission distance of thicker cables can reach over 1km). Alternatively, video amplifiers can be added to enhance video brightness, chroma, and synchronization signals; however, interference signals in the line will also be amplified, so too many video amplifiers cannot be connected in series in the loop, otherwise saturation will occur, leading to image distortion. For even longer distances, fiber optic transmission is used. Fiber optic transmission has a series of advantages such as low attenuation, wide bandwidth, immunity to electromagnetic interference, light weight, and good security, and is mainly used in national and provincial backbone communication networks, cable television networks, and high-speed broadband computer networks. In closed-circuit television (CCTV) monitoring systems, fiber optic transmission has also become the preferred method for long-distance audio, video, and control signal transmission.
Video signals can also be transmitted using twisted-pair cables, which requires twisted-pair transmission equipment. In certain special applications, twisted-pair transmission equipment is indispensable. For example, when a building already has a large amount of twisted-pair cabling (referred to as Category 3 or Category 5 cabling in the standard) installed according to the structured cabling standard, and corresponding information interfaces (RJ45 or RJ11) are provided in each relevant room, then adding new closed-circuit television monitoring equipment does not require additional cabling. Audio and video signals, as well as control signals, can all be transmitted through twisted-pair cables, with video signal transmission requiring twisted-pair transmission equipment.
In addition, twisted-pair transmission equipment is also required for applications where twisted-pair cables (or two-core sheathed cables) have already been laid and the images from the front-end cameras need to be transmitted to the central control room equipment. The function of twisted-pair video transmission equipment is to convert video signals suitable for unbalanced transmission (i.e., suitable for 75Ω coaxial cable transmission) to video signals suitable for balanced transmission (i.e., suitable for twisted-pair transmission) at the front end; and at the receiving end, the reverse processing is performed, converting the video signal transmitted through the twisted-pair cable back into an unbalanced video signal. Twisted-pair transmission equipment itself has video amplification capabilities, thus it is also suitable for long-distance signal transmission. The transmission components and transmission lines used for these different transmission methods differ significantly.
Communication cables are generally used in camera systems equipped with motorized pan/tilt units and lenses, requiring on-site installation of a remote control decoder. The communication transmission cable between the on-site decoder and the video matrix switching host in the control center typically uses 2-core shielded communication cable (RVVP) or Category 3 twisted-pair UTP, with each core having a cross-sectional area of 0.3mm² to 0.5mm². The basic principle for selecting communication cables is that the longer the distance, the larger the wire diameter. For example, the basic communication distance specified for RS-485 communication is 1200m, but in actual projects, using RVV2-1.5 sheathed cable can extend the communication length to over 2000m. When the communication distance is too long, an RS-485 communication repeater is required.
Control cables typically refer to multi-core cables used to control pan/tilt units (PTZs) and motorized zoom lenses. One end connects to the PTP/magnetic lens control terminals on the controller or decoder, while the other end connects directly to the corresponding terminals on the PTP/magnetic lens. Since control cables provide DC or AC voltage and are generally used over short distances (sometimes less than 1 meter), interference is minimal, thus shielding is unnecessary. Common control cables are mostly 6-core or 10-core cables, such as RVV6-0.2 and RVV10-0.12. The 6-core cable connects to the six terminals on the PTP: up, down, left, right, auto, and common. The 10-core cable, in addition to connecting to the six PTP terminals, also includes the four terminals on the motorized lens: zoom, focus, aperture, and common.
In closed-circuit television monitoring systems, the control cable between the decoder and the pan-tilt unit and lens is generally not subject to special requirements due to its relatively short distance; however, the distance from the controller in the central control room to the pan-tilt unit and motorized lens can be anywhere from tens of meters to hundreds of meters, which requires certain requirements for the control cable, namely, the wire diameter must be thicker, such as RVV10-0.5 or RVV10-0.75.
Audio monitoring cables typically use 4-core shielded communication cable (RVVP) or Category 3 twisted-pair UTP, with each core having a cross-sectional area of 0.5 mm². In interference-free environments, unshielded twisted-pair cables can also be used, such as Category 5 twisted-pair cables (4 pairs, 8 cores) commonly used in structured cabling. Since the audio signals from the monitoring heads to the control room in the surveillance system are transmitted via point-to-point cabling using high voltage and low current, unshielded 2-core cables, such as RVV2-0.5, are sufficient.
