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adt-audio
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There is only one possible way to deal with these problems. After the
installation is ready, tested and clean, check if there is any degradation
when one of these devices is connected to the system. If this happens,
install audio transformers on the inputs and outputs of the device and use
separate power lines to avoid pollution of the audio protection ground. If
there is no degration, you are lucky. Leave it as it is and make sure that
you have a list of these devices ready. If at any time in the future you
have noise in the system, disconnect all devices on the list before you
start to disassemble the entire studio installation. There is always a very
good chance that something that is not under your control has changed
with one or more of these devices.
2.3 Cables
The selection of the cables that are used for the audio installation is very
important for the proper operation of the entire system, for the sound
performance and, last but least, for the cost of the installation. Multicore
cables are commonly used for most of the necessary lines. There is an
overwhelming offer of multicore cables from many brands in a price range
from almost nothing to amounts that are not reasonable anymore.
As far as the sound performance of a cable is concerned, be aware of the
fact that a cable does not ‘sound’. A cable is a passive electric component
that has a resistance, an inductance and a capacity, nothing else. If a
cable alters anything, it depends not only on the cable itself but also on
the qualities of the audio output and input that is connected with this
cable. Since the electric properties of a cable are proportional to its length,
longer cables are more critical than shorter cables. Nothing else but
resistance, inductance and capacity is important for audio signals. The
skin effect and the impedance have no influence on audio signals, because
these effects are not relevant in the audio frequency band. The skin effect
for instance is the influence of the magnetic field that is caused by any
current in a conductor that displaces the electrons to the boundaries of
the cable. It is obvious that this effect is proportional to the frequency of
the signal, since it is caused by the inductive reactance that increases with
the frequency. The skin depth is the distance from the boundary of the
cable to that point inside the cable where the current is reduced by 1/e,
which is equivalent to 36.8 %. For audio signals at a frequency of 20 kHz,
the skin depth is approx. 0.3 mm. With audio cables that usually have a
diameter in the range of 0.2 to 0.4 mm this means, that there is simply
no skin effect. As far as the impedance or wave resistance of a cable is
concerned, there is also no effect in the audio band. The meaning of the
wave resistance is not so easy to understand. The speed of an electrical
signal is in principal the speed of light. However, in a cable, the real speed
of an electron is impeded. The real speed is in the range of two thirds of
the speed of light. If we imagine a generator that is connected to one end
of a cable and an output signal that is a pulse that is as small as possible,
a so called Dirac pulse, the pulse runs thru the cable at approx. 200000
km/sec. For a certain period of time the pulse is only existing in the cable
until is arrives at the input that is connected to the other end of the cable.
Depending on the qualities of the cable, the pulse will be send back to the
output of the generator and produce reflexions of the original signal that
run along the cable from one end to the other if the wave resistance of the
cable does not match the impedance of the connected input and output. If
we use a cable of 100 meters, and assume that the speed is 200000 km/s
we can calculate, that the time that is needed for the cable length of 100
meters is 500 nano seconds. The wavelength of an audio signal of 20 kHz
is 0.050 ms, which is 100 times longer than the delay that is caused by
the 100 m cable. This means, that even if there would be a reflection on
the cable, it will not alter the signal, since the amplitude of the signal has
not changed during the time between the original signal and the reflection.
Actually, the phase angle difference of the 20 kHz single during the 500 ns
period is only 3 degrees.
The most important quality of a cable is the capacity. Since both lines
and the screen are close together, the capacity of any cable cannot be
neglected. We need to make a difference between the capacity core to
core and core to screen. These two values have a different influence,
depending on the type of audio connection. This means, that the core-to-
core capacity is not that important with unbalanced signals. However, the
lower the total capacity, the lower the possible influence. The most critical
source is a microphone, since its source impedance is usually 200 ohms
while the source impedance of professional audio line level outputs is
below 60 ohms. Let’ make some calculations to determine a critical value
for the cable capacity. If you assume that the source resistance of the
microphone is not higher than 200 ohms up to frequencies of 20 kHz, and
