In audio applications the installer usually encounters two types of signal
wiring schemes. One, using shielded cable is unbalanced where the center
conductor of the cable carries the actual audio signal and an outside braided
shield acts as the signal return as well as a means to block extraneous signals
from interfering with the audio. This scheme is simple but often suffers the
problem of hum inducement if the shields are not terminated properly. The second
scheme is to use a balanced configuration where the audio is transmitted as a
differential signal through two wires. Any extraneous signal coupled to this
type of arrangement winds up being present on both conductors and tends to
cancel itself out at the terminated end. While this scheme is more interference
and hum resistant than the unbalanced mode, it requires that both signal leads
float from any shield. If a shield is present, it is not connected to any of
the signal leads.
The AT/AR-1001 fiber optic audio transmission system is designed to transmit
either balanced or unbalanced line-level audio signals from point-to-point.
Since the input and output port of each unit is transformer coupled, the use of
balanced or unbalanced audio signals is simple as this application note will
Figure 1 shows the unbalanced input configuration. In this example position 5 of
the terminal block is used for the audio, position 4 is jumped to position 3 and
used for the audio return and/or shield. Note that position 3 is also common to
the housing thereby making the entire installation part of the shield.
Figure 2 shows the balanced configuration. Here positions 5 and 4 are used for
the audio and position 3 is used for a shield if present. In this case there is
no signal connection to the housing and the audio leads truly ?float?.
In both cases the interface is a 600-ohm transformer winding so it is important
that only audio without any DC component be used.