Country Home Products Wireless Microphone Systems - Manual (Page 39)

A udio Interface
Here we will discuss the sound system interface.
Remember that the basic function of a wireless
microphone system is to replace the connecting cable
between the source and the sound system. In the typical
case, the output of the wireless receiver will resemble the
output of the original source both electrically and physically.
That is, most wireless microphone receivers have a
balanced, low impedance, microphone level output,
usually on a standard 3-pin XLR-type audio connector.
This can be connected to a standard microphone input
of a sound system using an ordinary balanced
microphone cable. (See Figure 4-5.)
Some receivers, particularly those designed for use with
electric instruments, may be equipped with a quarter-inch
phone jack instead of (or in addition to) an XLR connector.
Normally, this output is an unbalanced, low or medium
impedance, microphone or instrument level signal. It can be
directly connected to the input of an instrument amplifier
using a standard shielded instrument cable.
In addition, a few receivers have line level outputs
available. These can be connected to line or aux level
inputs of sound systems equipped with similar types of
input connectors.
If it is desired (or necessary) to connect one type of
output to a different type of input a few possibilities
should be
considered.
For a
balanced
XLR output
to an
unbalanced
quarter-
inch input
an adapter which connects Pin 2 of the XLR to the tip of the
phone plug and connects Pin 1 and Pin 3 of the XLR to the
sleeve of the phone plug may be used. A similar adapter
(with appropriate XLR connector) may be used to connect
an unbalanced quarter-inch output to a balanced XLR
input. Simple adapters of this type will usually work if the
levels and impedances of the outputs and inputs are
compatible.
In some cases simple adapters cannot be used due to
significant impedance or level differences. In addition, the
quarter-inch phone-to-XLR hookups just described (which
cause the circuit to be unbalanced) can occasionally
create audible hum problems due to ground loops
between the receiver and the sound system. In either
case, the use of a transformer may offer a solution. It can
provide the proper transition between different
impedances and between balanced and unbalanced
circuits. The transformer also allows ground loops to be
eliminated by lifting the shield connection at the source
end of the balanced cable.
Finally, the presence of phantom power at the
balanced microphone input of the sound system must be
considered. If the receiver output is unbalanced, phantom
power may cause noise or distortion in the signal. Phantom
power should be turned off at that input if possible. If not, a
suitable transformer or an adapter with capacitors will block the
voltage in the connecting path. However, if the receiver output
is balanced, phantom power is usually not a problem though
a manufacturer may specify the maximum voltage that the
receiver can tolerate. A few receivers present a significant load
to the phantom source. This can result in a lowering of the
phantom voltage at other inputs on a mixer that has insufficient
isolation of the phantom supply between inputs.
Once the receiver has been properly connected, then
the sound system controls may be set. The first control
adjustment on a receiver is the output level. This usually
consists of a rotary pot and possibly a switch to select
microphone or line level. The general procedure is to set the
output level so that it is approximately the same as that of a
wired source of the same type. This will provide
normal gain structure in the rest of the sound system.
Though microphone level is most common, line level can be
appropriate for long cable runs or for driving line level
devices
such as
equalizers,
crossovers
or power
amplifiers.
On most
receivers,
the audio
level indicators are pre-volume control and are unaffected
by receiver volume control settings. Use the indicators on
subsequent equipment to gauge the actual output level.
Squelch Adjustment
Another receiver adjustment is the squelch control.
Recall from the previous discussion that the function of the
squelch circuit is to mute the audio output of the receiver
when the transmitted signal is lost or becomes
unacceptably noisy. Depending on the type of squelch
system used (threshold squelch, noise squelch, tone-key
squelch) the adjustment procedure will vary:
If a simple threshold squelch is used, adjustment may
be required if the radio background noise level changes
substantially. This would be indicated by loud "white
noise" from the receiver output when the transmitter is
turned off or drops out.
Selection
and Operation
of W
ireless Microphone Systems
40
C
H A P T E R
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