18
CONTROL
CIRCUITS
Audio In
Audio Out
Control Information
Gain Element
•
F EEDFORWARD CONTROL
Figure - Feedforward control circuitry
In feedforward dynamics control, the input audio signal to the gain control circuits is monitored, and
adjusted if necessary,
before
the gain element. The resulting control information is then “fed forward
”
to the element. This produces
dB linear
gain control at a consistent ratio that remains constant
regardless of the amount of processing is employed.
For example, if the low band leveler is operating with a compression ratio of 4:1, any amount of
control, whether it is 3dB or 23dB, will operate the 4:1 ratio. Because the developed control
information is derived from the
input
side of the gain element, all corresponding control reaches the
gain element
as it happens
in real time. As the incoming input audio changes, the gain element is able
to make the same corresponding level adjustment change at the same time.
The gain element in a feedforward control scheme is a “dumb” device. It only does what it is told to
do by the control information and nothing more. If the control circuits are able to develop the correct
information, then the gain element is capable of providing a very natural and smooth effect to the
audio that is available over a wide operating range. If the control circuits develop the wrong
information, i.e. incorrect time or ratios, the gain element will produce an unnatural audio effect. The
Unity AM utilizes feed forward control which aids in creating its natural unprocessed sound.
As you can see from the figure below, when using a feedback gain processing technique the control
circuits develop the control information after the audio has passed through the gain element. The
information is then “fed back”
to the element to accomplish gain control. Gain control operates in a
dB log/linear
fashion. This changes and increases the processing ratio and continues to do so as more
processing is employed. For example, when 3dB of audio crosses the processing threshold the ratio
may be 3:1, but at 23dB the ratio may have increased to 20:1.
