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OPTIMIZING “WINDOWED” AGC ACTION
Access the AGC screen and its adjustments with:
Processing /
AGC
:
Before setting the DAVID IV input gain under the previous sub-
heading, the AGC was confirmed at the factory default setting;
that is, AGC enabled over its entire ±18dB correction range with
mid-values for correction rate and ‘windowing.’ Actually, these
settings should work pretty well for nearly any broadcast for-
mat, but there may be instances where they might want to be
changed.
Defeating
the AGC
Turning the AGC off:
Bypass
is never a good idea in everyday
operation. The AGC presents downstream processing stages
with an input signal that is optimized for further processing.
AGC has been carefully engineered for intelligent response to
program material, taking into account both the peak and aver-
age values of the audio signal. The only justification for turn-
ing AGC off is for test purposes, or if the user is 100% confi-
dent that the board operator is diligent in riding the gain man-
ually (fat chance!). Even in the latter instance, trust the AGC to
know best what the downstream processing stages want to see.
AGC
Maximum Gain
Classical music and jazz are two genres that often have wide
level variations that beg to be preserved.
Pianissimo
classical
passages and a standup-bass solo are two examples. In these
instances, a normal configuration of the AGC stage would slow-
ly bring up these passages to the 100%-modulation point, which
would not sound right at all.
The
Gain
slider is generally kept at
+18.0
(dB) for full AGC ac-
tion. If you want to limit the amount of
positive
gain that the
AGC can impart to the program input, this slider can be adjust-
ed anywhere between
+18.0
and
0.0
. This does not change the
static, ‘resting’ gain of the AGC section, nor does it limit the
amount of negative gain that the AGC can introduce for inputs
that are too loud.
Gain
only limits how far the AGC can bring
up low-level material. Experiment with
Gain
settings for for-
mats that call for a wider dynamic range.
‘Windowing’
and the AGC
Correction Rate
The DAVID IV has a ‘windowed’ AGC with one correction rate
when AGC gain is close to the target value and a faster,
‘makeup’ rate when the incoming program level shifts abruptly.
The AGC
Window
may be set to any dB figure between
0
(no
window) and
12
, which in actuality is ±6dB. This means, for ex-
ample, with the window set for
8
, if the input signal wanders
only ±4dB or less from where it is right now, correction is a
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