An acoustician's Night at the Opera
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Each signal conductor is made of a number of fine strands of copper. Around each signal conductor is a layer of plastic insulation - often one is coloured red and the other black. Around the two signal conductors are more fine strands of copper - known as the screen. Keeping the inside in and the outside world out is another layer of plastic insulation. This type of cable is known as a lapped screen twin cable. Get out your microscope and look at the fine detail...
The signal conductors each consist of between twenty and sixty copper strands, each approximately 0.1mm in diameter. The material is copper because it is a good electrical conductor, and also easy for the manufacturer to form into a wire. There is a large number of thin strands because this is a more physically flexible arrangement than having just one thick strand. If there were, say, thirty strands 0.1 mm in diameter, the conductor would be simply described as 30/0.1 mm.
The insulation around the signal conductors is typically PVC, although other plastic materials may be used. The screen consists of fifty or sixty strands of copper wire, each once again about 0.1mm in diameter. When it is wound round and round the signal conductors, it is called a lapped screen. When the strands are woven together it is known as a braided screen. The screen is normally connected to earth and keeps electrical interference away from the signal conductors. The outer insulator is PVC once more, and the overall diameter is about 6mm. This is a standard audio cable and is colloquially known simply as mic cable, although it can equally well be used for line level sources. Fig. 5.1c shows Foil Screen Twin or FST, an installation cable which is described later. Fig. 5.1d is a conductive plastic screen mic cable which uses electrically conducting plastic for the screen rather than copper wire. Since you cant solder to plastic there is a drain wire running along the wire in contact with the screen which should be connected to earth. Fig. 5.1e is quad cable which is used for the ultimate in interference rejecting cables, and Fig. 5.1e is good old fashioned 13 amp mains cable which is often used for connecting up speakers, although even thicker conductors would be preferable.
Now we know about the construction, what else is there to know about cables? One significant feature of audio cables is capacitance. Capacitance in a cable can be compared to holes in a hose pipe. When you water the lawn, most of the liquid comes out of the end of the hose as it should, but some leaks out of the hose, along its length, into the ground. In the cable, some of the electricity in the signal conductor can leak through to the screen, which is of course connected to electrical ground. It is always the high frequencies that are first to get through, so if you had a long cable of high conductor to screen capacitance, you could expect to get a dull sound. There is also capacitance from conductor to conductor which has precisely the same effect. These two characteristics are measurable and must be included in the specification of a cable. Reasonably low capacitance is obviously desirable. Cables also possess electrical resistance and inductance, but these dont really have too much significance in the lengths of cable typically used in a home studio. If you were laying a transatlantic telephone cable the situation would be different.
This is probably as good a time as any to examine why there are two conductors in this cable instead of just one conductor and a screen. In professional audio, it is normal to use balanced connections between equipment. Balanced wiring reduces hum and interference and makes it simple to connect any piece of equipment to any other without the likelihood of problems arising. It costs more, but pros are prepared to make that initial investment because it saves time and money in the long run. The balanced system works by having the same signal in both conductors, but one inverted in polarity - a bit like a battery connected the other way round. Balanced equipment has transformers, or the electronic equivalent, which sort everything out at both input and output. The advantage is that any interference that gets into the cable is cancelled out in the balancing/unbalancing process.
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