Digital Sound & Music: Concepts, Applications, & Science, Chapter 1, last updated 6/25/2013
The XLR connector shown in Figure 1.26 is widely used in professional audio systems. It
is a typically round connector that has three pins. Pin 1 is for the audio signal ground, Pin 2
carries the positive polarity version of the signal, and Pin 3 carries the inverted polarity version
of the signal. The inverted polarity signal is the negative of the original. Informally, this means
that a single-frequency sine wave that goes “up and down” is inverted by turning it into a sine
wave of the same frequency and amplitude going “down and up,” as shown in Figure 1.27.
XLR female 3-pin cable
XLR male 3-pin cable
XLR Female 3-pin panel
XLR male 3-pin panel
Figure 1.26 XLR connectors
Sending both the original signal and the inverted original in the XLR connection results
in what is called a balanced or differential signal. The idea is that any interference that is
collected on the cable is introduced equally to both signal lines. Thus, it's possible to get rid of
the interference at the receiving end of the cable, by subtracting the inverted signal from the
original one (both now containing the interference as well). Let's call S the original signal and
call I the interference collected when the signal is transmitted. Then
is the received signal plus interference
is the received inverted signal plus interference
If –S + I is subtracted from S + I at the receiving end, we get
( )
That is, we erase the interference at the receiving end and end up with double the amplitude of
the original signal, which is the same as giving the signal 6 dB boost (explained in Chapter 4).
This is illustrated in Figure 1.27. For the reasons just described, balanced audio signals run on
two-conductor cables with XLR connectors tend to be higher voltage and lower noise than
unbalanced signals run on single-conductor or coaxial cables.
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