27
Discrete Left/Right Audio Outputs and De-emphasis Selection
Individual Left and Right channel outputs are available on the male XLR jacks. This signal is the
output of the final processing function of the system, the Clipper/Low Pass Filter. The output circuitry
is active balanced, pin #2 high, pin #3 low, and pin #1 ground. Make sure that pin #2 is connected to
the “
+
” terminal of the audio input on the transmitter. If not, incorrect modulation polarity can result.
It is important in AM broadcasting to observe correct output polarity. Since the Unity AM is capable
of modulating asymmetrically , incorrect polarity, coupled with asymmetrical operation, could
actually cost modulation. This would occur by modulating the asymmetrical waveform in the negative
direction. Thus yielding positive modulation of about 80%, having an adverse affect on signal
coverage.
Under normal circumstances the output signal is pre-emphasized, but through jumpers, located on the
motherboard, the signal can be de-emphasized for installations that require a “flat” response as would
be the case when feeding land lines.
It should be noted that through the system jumpers, two different “flat” response audio conditions
could be created. The first, is when no pre-emphasis or de-emphasis is used. Passing the audio signal
through the system without any specified boost and complimentary cut to the high frequencies. The
second, is when a specific amount of pre-emphasis is utilized to boost the high frequencies , and then
the corresponding de-emphasis is used to reduce to the high frequencies to an “overall” flat response.
If there are two options, what are the differences, and why use them? Good question! In the first
instance, there is never any boost to the high frequencies. Because of this, any subsequent pre-
emphasis boost employed AFTER the Unity AM will cause unwanted overshoots in the signal. This
occurs because there is not any final limiting control that follows the pre-emphasis curve that is used
AFTER the processing of the UNITY. These overshoots cause “lost” modulation because their
amplitude must be accounted for in the total modulation of the system.
The second option, where pre-emphasis and de-emphasis is used, makes use of the final limiting
system within the Unity’s processing structure. Since this follows whatever pre-emphasis has been
selected, tight control of the high frequencies is accomplished. Even though the signal is subsequently
de-emphasized, any post pre-emphasis that will then be employed will only boost the high
frequencies to the already previously controlled level. This will allow the system, as a whole, to
modulate with relatively low, or no overshoots.
When, or where is either of these options applied? The answer is determined by what type of system
follows the Unity AM. If any portion of the following system does not employ ANY form of pre-
emphasis, then the first option can be used. Applications might be: final limiting before power
amplifiers, final mixing, and final limiting for mastering purposes.
