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FIVE Monitor Installation 2.5
l Connect all microphone lines.
By following this sequence much time and future trouble will be saved, and the result
will be a quiet, stable system.
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Audio equipment is supplied with a variety of input and output configurations, which
must be taken into consideration when deciding where the screen connections should
be made. There are three sources of unwanted signal being impressed on the screen,
which are as follows:
l Extraneous electrostatic or electromagnetic fields.
l Noise and interference on the earth line.
l Capacitive coupling between the screen and signal wires.
To minimise the adverse affects of the unwanted coupling to the signal wires, it is
important that the screen is connected at one end only, i.e. the screen must not carry
any signal current. Any signal on the wires within the screen will be capacitively cou-
pled to the screen. This current will ultimately be returned to the source of the sig-
nal, either directly, if the screen is connected at the signal source end, or indirectly via
the earthing system, if the signal is connected at the signal destination end. The indi-
rect connection will cause an increase in high frequency cross-talk, and should be
avoided wherever possible.
Therefore, in general, always connect the shield only at the signal source end. In high
RF areas, the screen can also be connected to earth via a 0.01
µF capacitor. This will
present a short circuit at RF frequencies, thus lowering the effective shield impedance
to ground. However, at low audio frequencies the reactance of the capacitor will be
sufficiently high not to cause an earth loop problem.
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l In all cases, use good quality twin screened audio cable. Check for instability at
the output.
l Always connect both conductors at both ends, and ensure that the screen is
only connected at one end.
l Do not disconnect the mains earth from each piece of equipment. This is
needed to provide both safety and screen returns to the system star point.
l Equipment which has balanced inputs and outputs may need to be electrically
isolated from the equipment rack and/or other equipment, to avoid earth
loops.
It is important to remember that all equipment which is connected to the mains is a
potential source of hum and interference and may radiate both electrostatic or elec-
tromagnetic radiation. In addition, the mains will also act as a carrier for many forms
of RF interference generated by electric motors, airconditioning units, thyristor light
dimmers etc. Unless the earth system is clean, all attempts to improve hum noise lev-
els will be futile. In extreme cases there will be no alternative but to provide a com-
pletely separate and independant technical earth to replace the incoming noisy
earth. However, always consult your local electricity supply authority to ensure that
safety regulations are not being infringed.