• Introduction • Criteria • Design • Insulation • Stranding • Conductor • Screen • Grounding • Connectors • Results • BACK TO SITE
		Cable Design: To screen or not to screen
		The technology behind Abbey Road Cable
		A 'screen', sometimes called a 'shield' is a layer of conducting material wrapped around a cable that has a connection to ground. It literally acts as a screen between the internal wires of the cable and the outside, so blocking any electrical interference going into, or coming from the cable. Interference picked up by the screen is conducted away on the ground connection to earth via a wire, called the 'drain' wire. Ideally, but unusually, interference is conducted away via a dedicated earth circuit called a 'dirty earth'. However, more usually the electronics earth is used for this;- not ideal as this is the shared audio reference ground and noise can be increased. A screen inevitably changes the way a cable works as it is in very close proximity to the conductors and therefore electro-magnetically coupled to them. Whether a cable should be screened or not depends on how much protection is needed against interference balanced against how much signal distortion and deterioration is tolerable.
				Hi Fidelity line and speaker cables perform with vastly enhanced clarity without the use of a screen. And, usually, if there is an audible noise or hum, this normally suggests an earth or ground loop problem. In most domestic applications there is little electrical interference, and what there is can be combated by designing the system using some basic guidelines (See link from 15 May). Balanced line level audio cable can be used very successfully without a screen giving far more natural and phase accurate audio- give it a try -it really does sound a lot better!
		UNDER NO CIRCUMSTANCE should a mains safety earth be removed to reduce noise or hum or for any other reason. If in doubt, seek suitable qualified advice or email us if you have an enquiry (cable@movingair.info). Some practitioners connect the screen/drain wire (the third wire) of a balanced cable to the ground at both ends of the cable, so as to connect the earth between two pieces of equipment. This is not good practice! Having understood that a screen is there to shunt away interference, we can see that in an ideal system, signal or zero reference should not be passed through the screen as it is subject to interference. A screen is to provide protection from interference, not to act as an earth link between equipment. Ground reference earth between units should be provide by other means. As good earthing practice is crucial in getting the best out of a system, we have an additional feature on this. (Link available from 15th May). Screens are used extensively in industrial applications, but it is fair to say that in hi-fidelity screens can be beneficial for some cables, such as mains or ones in close proximity to interference sources, and they are essential in very sensitive applications such as for microphones, record deck cables and some unbalanced circuits. There are some specialist screens that do not influence the single cable so much as others that have specific properties. An example of a specialised screen is on Moving Air's 'Abbey Road' Power Chord [link to Power Chord active 15th May] with its special low capacitance carbon screen. [To next page]