Audio and Video Cable Installation Ideas and Technical Considerations

Going to install an audio or video cable? In this article we take some time to go over some common issues that can influence the performance of audio or video cables.

COMMON ISSUES

System performance can be seriously reduced due to any unwanted and interfering voltage, which is developed inside or external to your system, which presents itself as “electrical noise”. In the past this noise was ordinarily reduced by filtering it as soon as it has gotten into your system. Present generation systems; due to their intricacy and greater usage of digital signals, makes this method expensive and impractical. The solution is to prevent the noise before it invades your system. Installing noise-rejecting cables, integrated with good grounding and isolation techniques, is a foremost portion of the solution.

Audio/video cables are exceedingly vulnerable to electromagnetic induction (EMI), electrostatic coupling radio frequency interference (RFI) and changes in capacitance. Cables need to be planned and manufactured to exacting standards to eliminate or reduce these factors which contribute to unwanted line noise, electrostatic hum, cross talk and noise induced by cable handling.

EMI

Stirring electrons generate magnetic fields. Sources of these fields may be from power cable, motors, energy transformers, incandescent light ballasts in addition to SCR’s as dimmers. These magnetic fields as well as the resultant changes from regularly changing voltages can induce a voltage in neighboring conductors which is heard as a buzz or hum. Twisting a cable’s interior conductor and where there are several conductor pairs twisting them with changing lays so the fields do not align, minimizes vulnerability to this possibility.

HANDLING NOISE

This is another cause of noise, which is induced as a result of changes in capacitance. When cables are bent, subjected to pulsation, impacted or trampled, the space between conductors or between conductors and shields is altered. This results in a alteration of capacitance, which localizes changes in impedance, exhibiting reduced structural return loss characteristics (SRL). Cables are made with fillers where necessary and exacting dimensional tolerances to give the cable strength, mechanical firmness and hardiness. Noise can also be induced by the fixed build up of electrons as a result of cables rubbing against each other or as they are being pulled along carpets. Proprietary vinyl and carbon insulating material within the shields scatter static noise, increase shield thickness and further decrease noise intrinsic in natural handling. This results in video cable that could stand up to the most rigorous demands of the business.

ELECTROSTATIC HUM

In an AC circuit, hum results because of voltage being electrostatically joined into the cable due to the capacitive reactance inherent between power and signal lines. The higher the capacitive reactance and the higher the impedance, the higher the induced hum. In addition, the longer the run, the more prone the cable might be. A properly grounded screen can eliminate this hum and the effectivity is proportional to the shield’s coverage. To maintain wire flexibility it is chosen to employ up to 95% coverage braid shields, depending on the application. When the utmost in elasticity is required a spirally served screen with many ends of fine wire is offered. Tape shields do not possess the suppleness and bend life compared to braid and spiral shields and as well possess a tendency to unwrap under rigorous or numerous bending cycles. It is for this rationale that they are not normally chosen for microphone or instrument uses, which need portability and lots of movement. When 100% coverage is called for, as in digital broadcasting uses, double braids or a mixture of tape and braid can be utilized. Also as mentioned previously, the longer the wire, the more important the shielding becomes.

GROUND LOOPS

From time to time, buzz or noise is produced as a result of regular mode hum. This results when a wire linking two pieces of gear is grounded by both ends. What this does is create additional parallel ground paths with hard-wired Alternating current power line grounds and the resultant electrical potential differences are called ground loops. Plus in difficult cases this grounding gives a return path for internal gear fluctuation at very high frequencies. If regular mode noise presents itself, cut the shield at the mixer end of the microphone cable. Multiple channel cables feature an overall drain wire so a ground can be returned to the mixer starting at a remote junction box.

CROSS TALK

Occurs when either by capacitive coupling or electromagnetic induction, signals go into adjacent channels of multi-conductor cables. Shielding every channel and inserting twists into the conductors, as talked about under “electrostatic hum” and “EMI”, minimizes cross talk. Within higher impedance uses, e.g. digital audio uses, a construction using fillers exactly spaced within a cable’s bunched construction significantly minimizes induced noise levels. Plus where the maximum flexibility is necessary select the higher strand count center conductor cables.

RFI

Composite sound systems employ a considerable array of gear, all operating at different voltage levels and frequencies. Plus all of this gear is giving off stray radio waves. Cables, which are in effect antennas spread this radiation as well as become great receivers, all of which contribute to the “noise” problem again evidenced by a buzz or a hum. Well grounded shielding gets rid of or significantly minimizes this trouble. For this reason, there are a collection of cables, which consist of tape shields, braid shields or combinations of tape/braid and braid/braid. Each one have their specific advantages and price considerations so should be specified and chosen based on application.