- With audio and video systems becoming increasingly more complex and diverse, an understanding of proper cable selection is paramount. Cables are the arteries of your system, providing the critical path for signal flow and interfacing. With proper cable specification, designers and installers can significantly reduce the occurrence of cable related errors, loss, or failure.
For each type of signal transmission, there is a cable that correctly corresponds to it. Just like you wouldn't use a sledgehammer to hammer in a finishing nail, you want to make sure you're not selecting a cable that's wrong for the interconnect application.
(Top) Typically, the higher the analog resolution or digital data rate, the shorter the cable can be run. (Bottom) High definition cable has the tightest electrical tolerances, lower attenuation and return loss, and a wider bandwidth.
There are multiple types of cables for audio, digital audio, networking, camera, and lighting applications. Each cable has specific electrical and mechanical specifications for its intended application and differs by the number and types of conductors, loss, capacitance, and impedance, among other attributes. In addition, many cables must meet specific standards established by organizations such as TIA, SMPTE, or AES.
Even within a particular general application, there are multiple cable designs and options. For example, when selecting video cables, you need to not only consider RG types which designate size, and ultimately maximum signal distance, but the correct type of cable for the application.
Each type, whether it be high definition, broadband, or CATV, has its own unique construction, precision and quality. High definition cable has the tightest electrical tolerances, lower attenuation and return loss, and a wider bandwidth. This is necessary to ensure accurate transmission of uncompressed video that operates at 1.485 Gbs. CCTV coax, on the other hand, operates at base band frequencies and does not require the same performance or precision, and as a result, is a more cost-effective cable design for this application. Broadband coax carries both video and data on a modulated carrier, making the low-frequency attenuation of the cable not as critical. In addition, broadband coax is typically terminated with F-type connectors that use the center conductor as a pin. As a result, copper-clad conductors are used in broadband coax because it is a more cost-effective solution for this application and can withstand the repeated mating cycles of F-type connectors.
Proper connector selection is another aspect in the prevention of signal loss. For instance, when interfacing an HD/SDI signal, it is important to use a BNC connector that meets the SMPTE 292M standards for uncompressed HD video interconnects. Since the BNC format has been around for many years, there are many different brand types, but not all meet the standards for HD/SDI video.
Older connector styles are often 50ohm designs. Others may be 75ohm design but are limited to a 1GHz bandwidth. Improper cables or connectors that do not meet or exceed the standard can result in excessive attenuation or structural return loss (back reflection) that can result in bit-errors in the data signal.
In addition to appropriate connectors, proper termination methods are another safeguard against signal loss. Termination should always follow the manufacturer's standards. This includes strip dimensions and proper crimping methods to ensure that both the electrical and mechanical specifications are met.
Along with cable and connector selection, improper installation is another factor that can contribute to cable failure. When installing, it is important not to damage the cable, as electrical performance is directly related to the mechanical integrity of the cable.
The two major factors that affect performance are the bend radius and pulling tension. The minimum bend radius should not exceed 10 times the diameter of the cable (or 20 times under load). Pulling tension should not exceed the specifications for that cable. If a cable is excessively bent or deformed, conductors can break, resulting in electrical faults. The dielectric can also be deformed, changing the electrical characteristics of the cable such as impedance or return loss, which can also result in signal loss or errors.
Maximum Distance Considerations
When selecting the right wire for each situation, the distance of the run must also be considered. All signals have a distance limitation. Typically, the higher the analog resolution or digital data rate, the shorter the cable can be run. In an analog cable, as the length increases, the signal is progressively reduced and the quality gradually degrades. As the cable length carrying a digital signal increases, the quality of the signal remains the same until the cable loss is great enough that the data is no longer recognizable and the signal is dropped.
Cable manufacturers can make maximum distance recommendations based on industry standards, however, the actual maximum distance is also dependent upon the quality of the signal, error correction and loss compensation of the hardware that is included in the set-up. Because there are so many factors involved when considering the maximum distance of a cable run, it is important to also consult the hardware manufacturer and perform an individual system evaluation prior to a full installation.
Proper UL Ratings
In any installation, the foremost factor above performance is safety. For the most part, cables installed in permanent locations such as walls, air returns, shafts, etc., must be UL rated. To receive a UL rating, a cable must pass specific safety criteria for the release of smoke particles and flame retardency when ignited. Using a non-UL rated cable or the wrong type of rated cable can be expensive and dangerous. If a cable does not pass UL inspection, the cable may be required to be removed and reinstalled with the properly rated cable. This can cause delays and often add unexpected costs to a project.
Common UL ratings include (in order from lowest to highest rating) CL2, CM, riser CMR and plenum CL2P or CMP. A plenum-rated cable has the highest UL rating, but can have electrical limitations and is often more costly. A UL riser CMR-rated cable has one of the highest non-plenum ratings and in most cases can be substituted for any other UL requirement other than plenum. Individual requirements vary by local ordinances, so it is recommended to contact your local fire marshal for local requirements.
Just as you perform research to select the best and latest hardware for your AV installation, the same consideration should be given to the system's interconnects. Without quality transmission, even the top-of-the line hardware can seem to function like it's second-rate. Selecting the proper cables, connector and installation process will ensure that your system is at peak performance.