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Cable princples

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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
can’t 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 don’t 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.

[diagrams not available in website]

David Mellor

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David Mellor