19
CONTROL
CIRCUITS
FEEDBACK CONTROL
Audio In
Audio Out
Control Information
Gain Element
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Figure - Feedback control circuit
Notice that the gain element is operating within the closed loop of the control circuits. By the time the
control information reaches the gain element, the level that required adjustment has already passed
the point of control. This is especially problematic when transient peak information occurs. The gain
element never really adjusts the leading edge of the peak, leaving the peak to be processed by a
subsequent stage. If a dynamic limiter follows, the limiter should take care of the peak, although the
resultant sound may be perceived as “dense” due to high limiting ratios. If a feedback limiter follows,
that limiter will not be able to control the leading edge of the waveform resulting in added, and
sometimes excessive, clipping of the waveform.
If the audio levels to be adjusted and the control information generated to process that audio vary over
a wide range, the gain element will generate increased intermodulation distortion. This is a key reason
why feedback gain control circuitry yields a “denser” and more fatiguing sound the harder it is driven
with audio level. This explains why these systems always will have a narrow “sweet spot” of
operation that is usually ranges only a few dB.
This discussion may lead you to ask why
feedforward control is not more prevalent. The reason is, in
part, to the difficulty in designing the needed control circuits within the true analog domain. In order
for this style of circuit to operate, true dB linear adjustment is necessary and, for this to take place in
analog, log circuits are needed to create the dB linear control. In the past, such analog circuitry
usually was very unstable, cumbersome, and costly and could cause inconsistency within the
processing circuits. Since this dB linear function is performed digitally in the Unity AM, simple,
consistent, and reliable operation is achieved.
Quality versus loudness
The tradeoff between quality and loudness is primarily affected by the use of the limiting and clipping
sections. While either function will generate more “dial presence,” they both offer differing artifacts
and side-effects. With additional use of limiting, intermodulation distortion is increased. The added
dynamic activity of the limiters causes the audio to sound as if it is “overly controlled.” This can be
perceived to the ear as “pumping,” “breathing,” “dense,” or “mushy.” When the clipping is raised,
